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LITE and SAGE II measurements of aerosols in the southern hemisphere upper troposphere
Abstract
Two global satellite data sets have been used to characterize the behavior of aerosols in the upper troposphere of the southern hemisphere during the spring season. The first data set was obtained by the Lidar-In-Space Technology Experiment (LITE) during 10 days in September 1994 and provides high-resolution information about aerosol layering and optical characteristics. The second data set was obtained by the Stratospheric Aerosol and Gas Experiment (SAGE) II over the time period 1984-1996 and provides information on the aerosol distribution and long-term climatology. During September, elevated aerosol layers are found to occur within a latitude band between 20°S and 40°S that extends to almost all longitudes. The latitude and altitude distribution and the optical characteristics of the aerosol suggest that a major source is smoke from biomass burning within the southern hemisphere. This conclusion is supported by the results of back-trajectory analyses that show airmasses originating in the region of southern Africa and traveling longitudinally across the Indian Ocean and Australia into the western Pacific Ocean. The dominant source of the smoke is uncertain, but quite possibly some of it may have originated from Brazil, with additions from southern Africa. The aerosol distribution shows strong similarities to published distributions for ozone and carbon monoxide, also believed to have originated from biomass burning.
... The origins of the air mass can be traced back from between days 3 and 5 to southern South America, where it appears these aerosols must have originated. Previous researchers have described aerosol transport events of South American biomass burning matter into the southern Atlantic basin [Anderson et al., 1996;Browell et al., 1996;Kent et al., 1998]. Further advection to the African continent is plausible. ...
... The first is based on the existence of two deserts in southern South America, the Atacama (along coastal Chile between 18°and 30°S) and Patagonia (between roughly 73°-64°W and 55°-40°S shared between Chile and Argentina). Grousset et al. [1992] and Iriondo [2000] previously found evidence for intercontinental transport of Patagonia dust matter to the Antarctic Peninsula. Current global dust transport modeling efforts, such as the Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model consider both of these deserts as source generating regions [Ginoux et al., 2001]. ...
... Upper level winds were found to be more favorable for a sustained particulate transport event in ARREX than during SAFARI 2000. Should it be proven in subsequent analyses that these aerosols were the result of dust transport, previous observations of intercontinental transport from these deserts exists only to the Antarctic Peninsula [e.g., Grousset et al., 1992;Iriondo, 2000]. Our findings, though ultimately not wholly conclusive, are potentially very noteworthy. ...
During the Aerosol Recirculation and Rainfall Experiment (ARREX 1999) and Southern African Regional Science Initiative (SAFARI 2000) dry season experiments, a micropulse lidar (523 nm) instrument was operated at the Skukuza Airport in northeastern South Africa. The lidar was colocated with a diverse array of passive radiometric equipment. For SAFARI 2000, a daytime time series of layer mean aerosol optical properties, including layer mean extinction-to-backscatter ratios and vertical extinction cross-section profiles are derived from the synthesis of the lidar data and aerosol optical depths from available AERONET Sun photometer data. Combined with derived spectral Angstrom exponents, normalized broadband flux measurements, and calculated air mass back-trajectories, the temporal evolution of the surface aerosol layer optical properties is analyzed for climatological trends. For dense biomass smoke events the extinction-to-backscatter ratio is between 50 and 90 sr, and corresponding spectral Angstrom exponent values are between 1.50 and 2.00. Observations of an advecting smoke event during SAFARI 2000 are shown. The smoke was embedded within two distinct stratified thermodynamic layers causing the particulate mass to advect over the instrument array in an incoherent manner on the afternoon of 1 September 2000. Significant surface broadband flux forcing of over -50 W/m 2 was measured in this event. The evolution of the vertical aerosol extinction profile is profiled using the lidar data. Finally, observations of persistent elevated aerosol layers during ARREX 1999 are presented and discussed. Back-trajectory analyses combined with lidar and Sun photometer measurements indicate the likelihood for these aerosols being the result of long-range particulate transport from the southern and central South America.
... The global mean undetected aerosol extinction is ~0.002 km −1 at 5 km and ~0.001 km −1 at 10 km, which is consistent with previous reports for "background" aerosol extinction. Kent et al. [1998] found the mean extinction for typical background aerosols of 0.0034 km −1 at 532 nm from LITE measurements in the southern hemisphere (between 5°S and 45°S) from 6 km to the tropopause. Winker et al. [2013] report that the average lower limit on aerosol extinction between 6 km and 9 km is about 0.001 km −1 at 525 nm using Stratospheric Aerosol and Gas Experiment (SAGE II) satellite data. ...
... Moreover, optical properties of background aerosols in the upper troposphere and stratosphere can be different from those in the low troposphere. Kent et al. [1998] shows that the lidar ratio in the stratosphere and upper stratosphere lies in the relatively broad range 20 -50 sr at 532 nm from Mie calculations for aerosols from biogenic, anthropogenic or volcanic sources. An error of 10 sr in the lidar ratio leads to ~50% in ULA when the lidar ratio for undetected layer is about 30 sr. ...
The Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) data processing scheme only retrieves extinction profiles in those portions of the return signal where cloud or aerosol layers have been identified by the CALIOP layer detection scheme. In this study we use 2 years of CALIOP and Moderate Resolution Imaging Spectroradiometer (MODIS) data to quantify the aerosol optical depth of undetected weakly backscattering layers. Aerosol extinction and column-averaged lidar ratio is retrieved from CALIOP level 1B (version 4) profile using MODIS aerosol optical depth (AOD) as a constraint over oceans from March 2013 to February 2015. To quantify the undetected layer AOD (ULA), an unconstrained retrieval is applied globally using a lidar ratio of 28.75 sr estimated from constrained retrievals during the daytime over the ocean. We find a global mean ULA of 0.031 ± 0.052. There is no significant difference in ULA between land and ocean. However, the fraction of undetected aerosol layers rises considerably during daytime, when the large amount of solar background noise lowers the signal-to-noise ratio. For this reason, there is a difference in ULA between day (0.036 ± 0.066) and night (0.025 ± 0.021). ULA is larger in the northern hemisphere and relatively larger at high latitudes. Large ULA for the polar regions is strongly related to the cases where the CALIOP level 2 product reports zero AOD. This study provides an estimate of the complement of AOD that is not detected by lidar and bounds the CALIOP AOD uncertainty to provide corrections for science studies that employ the CALIOP level 2 AOD.
... A global climatology of aerosol extinction in the upper troposphere for the period 1989-1996 was developed by Kent et al. (1998) using solar occultation data from the SAGE II satellite sensor. These measurements are derived from observations along a tangent path through the atmosphere, typically representing averages over paths of 300 km. ...
... We expect the free troposphere to be dominated by fine mode aerosol with extinction near background levels: 0.0001 km −1 to 0.001 km −1 , as reported in Clarke and Kapustin (2002). Kent et al. (1998) estimated lidar ratios of 50 to 80 sr at altitudes above 6 km in the South-ern Hemisphere, consistent with expectations for fine mode aerosol. ...
The CALIOP lidar, carried on the CALIPSO satellite, has been acquiring global atmospheric profiles since June 2006. This dataset now offers the opportunity to characterize the global 3-D distribution of aerosol as well as seasonal and interannual variations, and confront aerosol models with observations in a way that has not been possible before. With that goal in mind, a monthly global gridded dataset of daytime and nighttime aerosol extinction profiles has been constructed, available as a Level 3 aerosol product. Averaged aerosol profiles for cloud-free and all-sky conditions are reported separately. This 6-yr dataset characterizes the global 3-dimensional distribution of tropospheric aerosol. Vertical distributions are seen to vary with season, as both source strengths and transport mechanisms vary. In most regions, clear-sky and all-sky mean aerosol profiles are found to be quite similar, implying a lack of correlation between high semi-transparent cloud and aerosol in the lower troposphere. An initial evaluation of the accuracy of the aerosol extinction profiles is presented. Detection limitations and the representivity of aerosol profiles in the upper troposphere are of particular concern. While results are preliminary, we present evidence that the monthly-mean CALIOP aerosol profiles provide quantitative characterization of elevated aerosol layers in major transport pathways. Aerosol extinction in the free troposphere in clean conditions, where the true aerosol extinction is typically 0.001 km−1 or less, is generally underestimated, however. The work described here forms an initial global 3-D aerosol climatology which we plan to extend and improve over time.
... A global climatology of aerosol extinction in the upper troposphere for the period 1989-1996 was developed by Kent et al. (1998) using solar occultation data from the SAGE II satellite sensor. These measurements are derived from observations along a tangent path through the atmosphere, typically representing averages over paths of 300 km. ...
... We expect the free troposphere to be dominated by fine mode aerosol with extinction near background levels: 0.0001 km −1 to 0.001 km −1 , as reported in Clarke and Kapustin (2002). Kent et al. (1998) estimated lidar ratios of 50 to 80 sr at altitudes above 6 km in the South-ern Hemisphere, consistent with expectations for fine mode aerosol. ...
The CALIOP lidar, carried on the CALIPSO satellite, has been acquiring
global atmospheric profiles since June 2006. This dataset now offers the
opportunity to characterize the global 3-D distribution of aerosol as
well as seasonal and interannual variations, and confront aerosol models
with observations in a way that has not been possible before. With that
goal in mind, a monthly global gridded dataset of daytime and nighttime
aerosol extinction profiles has been constructed. Averaged aerosol
profiles for cloud-free and all-sky conditions are reported separately.
This 6-yr dataset characterizes the global 3-dimensional distribution of
tropospheric aerosol. Vertical distributions are seen to vary with
season, as both source strengths and transport mechanisms vary. In most
regions, clear-sky and all-sky mean aerosol profiles are found to be
quite similar, implying a lack of correlation between high
semi-transparent cloud and aerosol in the lower troposphere. An initial
evaluation of the accuracy of the aerosol extinction profiles is
presented. Detection limitations and the representivity of aerosol
profiles in the upper troposphere are of particular concern. While
results are preliminary, we present evidence that the monthly-mean
gridded CALIOP aerosol profiles are representative for aerosol
extinction greater than about 0.001 km-1 and up to an
altitude of 4-6 km in most cases. The work described here forms an
initial global 3-D aerosol climatology which hopefully will be extended
and improved over time.
... These data, in conjunction with data from sister instruments SAM II and SAGE I, can be used to estimate long-term constituent trends and to identify responses to episodic events. SAGE II has a horizontal resolution of the order of 200 km and a vertical resolution of 1 km [Kent et al., 1998]. A global climatology of stratospheric aerosol radiative properties has been compiled from SAGE II multiwavelength extinction measurements [Thomason et al., 1997]. ...
... These low-level SAGE II data afford a rough but potential way to understand the vertical distributions of tropospheric aerosols. In this study, aerosol extinction coefficient data (km À1 ) at 1.02 mm was used because of its better quality [Kent et al., 1998]. ...
Multiple satellite data sets in conjunction with the Monte Carlo Aerosol-Cloud-Radiation (MACR) model are employed to determine climatological distributions and radiative impacts of dust plumes over the Pacific, Indian, and Atlantic oceans. Three target regions, namely the Yellow Sea (YS), Arabian Sea (AS), and Saharan Coast (SC), are examined for quantitative comparisons of dust properties and their impacts on climate. Twenty year averaged Advanced Very High Resolution Radiometer (AVHRR) aerosol optical depth (AOD) data clearly show the peak dust season for the three target regions, March–April–May for YS and June–July–August for AS and SC. Georgia Institute of Technology–Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) modeled dust AOD fraction and Moderate Imaging Spectroradiometer (MODIS) large-mode AOD ratio are adopted to evaluate the dust fraction estimate. Stratospheric Aerosol and Gas Experiment (SAGE) II aerosol extinction coefficient data are used to define the vertical distribution of dust. The elevated dust plumes are detected by subtracting the non-dust-season values from dust season values of SAGE II data, showing extinction peak around ∼4 km over AS and SC. Over YS, dust plumes are found presenting multilayered structure. The shortwave (SW) forcing of dust, although moderated by the longwave (LW) effect, dominates the net effects (SW + LW) of dust plumes. Under clear-sky (i.e., cloudless) conditions, dust plumes reduce about 5.9 W m−2, 17.8 W m−2, and 14.2 W m−2 regional and seasonal mean radiative flux reaching the surface over YS, AS, and SC, respectively. Of the three regions, dust plumes over AS have the largest effect on atmospheric heating owing to a lower single-scattering albedo and the relatively large dust loading. The maximum SW heating occurs over AS with the value around +0.5 K/day inside the dust layer at ∼4 km. The LW effect results in strong cooling throughout the dust layer and moderate heating below the dust layer, and dust plumes over SC exert the maximum LW effect on heating rates, with up to −0.5 K/day LW cooling in the free troposphere and about +0.3 K/day warming in the boundary layer. As the sum of the SW and the LW heating rates, net heating rate shows a more complex pattern. Over SC, large LW cooling inside the dust layer offsets up to 80% SW heating and results in about −0.1 K/day net heating rate change at the height ∼5 km over SC. Over AS the net heating rate change is dominated by SW heating because the maximum LW cooling is less than 60% of the SW heating, which leads to +0.3 K/day net heating inside the dust layer and moderate heating below the dust base. The net heating rate change over YS is the smallest among the three regions, with magnitude within 0.1 K/day.
... constraints of physically plausible lidar retrievals and assuming S a was constant over the upper troposphere [Kent et al., 1998] . The combination of backscattering nephelom- eter [Anderson et al., 1996] and measurements of integrated scattering and absorption has afforded directly measured S a values at two polluted continental sites in Washington and Illinois and aerosols of clean continental and marine origin [Anderson et al., , 2000 Doherty et al., 1999; Masonis et al., 2003]. ...
... [33] It is possible that lidar itself can help constrain this problem. Presently, we are working on processing data from the special case of a lofted dust layer collected by the Lidar In Space Technology Experiment, LITE [Kent et al., 1998], where the only necessary assumption to reduce data at both wavelengths is the color ratio. Preliminary comparisons of the retrieved lidar, backscattering and extinction ratios all show very good agreement with those computed using the present shape distribution of spheroids, lending support to our choice of shape distribution to describe nonspherical particles. ...
The lidar (extinction-to-backscatter) ratios at 0.55 and 1.02 μm and the spectral lidar, extinction, and backscatter ratios of climatically relevant aerosol species are computed on the basis of selected retrievals of aerosol properties from 26 Aerosol Robotic Network (AERONET) sites across the globe. The values, obtained indirectly from sky radiance and solar transmittance measurements, agree very well with values from direct observations. Low mean values of the lidar ratio, Sa, at 0.55 μm for maritime (27 sr) aerosols and desert dust (42 sr) are clearly distinguishable from biomass burning (60 sr) and urban/industrial pollution (71 sr). The effects of nonsphericity of mineral dust are shown, demonstrating that particle shape must be taken into account in any spaceborne lidar inversion scheme. A new aerosol model representing pollution over Southeast Asia is introduced since lidar (58 sr), color lidar, and extinction ratios in this region are distinct from those over other urban/industrial centers, owing to a greater number of large particles relative to fine particles. This discrimination promises improved estimates of regional climate forcing by aerosols containing black carbon and is expected to be of utility to climate modeling and remote sensing communities. The observed variability of the lidar parameters, combined with current validated aerosol data products from Moderate Resolution Imaging Spectroradiometer (MODIS), will afford improved accuracy in the inversion of spaceborne lidar data over both land and ocean.
... Equation (2) shows that the R 2 obtained in the comparison between MODIS/MERRA-2 and AERONET AOD is correlated to atmospheric RH. These results are compatible with previous studies [65][66][67]. However, when investigating other statistical parameters in the present study, such as slope, offset, RMSE, and RMB, a relation with RH does not occur when all three AEROENET sites are taken into consideration. ...
This study validated MODIS (Moderate Resolution Imaging Spectroradiometer) of the National Aeronautics and Space Agency, USA, Aqua and Terra Collection 6.1, and MERRA-2 (Modern-ERA Retrospective Analysis for Research and Application) Version 2 of aerosol optical depth (AOD) at 550 nm against AERONET (Aerosol Robotic Network) ground-based sunphotometer observations over Turkey. AERONET AOD data were collected from three sites during the period between 2013 and 2017. Regression analysis showed that overall, seasonally and daily statistics of MODIS are better than MERRA-2 by the mean of coefficient of determination (R 2), mean absolute error (MAE), and relative root mean square deviation (RMSD rel). MODIS combined Terra/Aqua AOD and MERRA-2 AOD corresponding to morning and noon hours resulted in better results than individual sub datasets. A clear annual cycle in AOD was detected by the three platforms. However, overall, MODIS and MERRA-2 tend to overestimate and underestimate AOD, respectively, in comparison with AERONET. MODIS showed higher efficiency in detecting extreme events than MERRA-2. There was no clear relation found between the accuracy in MODIS/MERRA-2 AOD and surface relative humidity (RH).
... A major limitation of balloon-based conversions is the uncertainty in the conversion factors (on the order of ±30 %-40 %; Deshler et al., 2003;Kovilakam and Deshler, 2015) and the requirement for ongoing OPC launches to accurately observe both zonal and seasonal variability. The primary limitation of lidar conversions is the challenge of appropriately selecting S. Indeed, Kar et al. (2019) showed that S is both altitude and latitude dependent and varies from 20 to 30 sr, while other reports (Wandinger et al., 1995;Kent et al., 1998) have shown S to go as high as 70 during background conditions. While a lidar ratio of 40-50 sr has been regarded as a satisfactory assumption, S is ultimately uninformed about the atmosphere in which the measurement was recorded, making appropriate selection of S difficult. ...
Aerosol backscatter coefficients were calculated using multiwavelength
aerosol extinction products from the SAGE II and III/ISS instruments (SAGE: Stratospheric Aerosol and Gas Experiment). The
conversion methodology is presented, followed by an evaluation of the
conversion algorithm's robustness. The SAGE-based backscatter products were
compared to backscatter coefficients derived from ground-based lidar at
three sites (Table Mountain Facility, Mauna Loa, and Observatoire de
Haute-Provence). Further, the SAGE-derived lidar ratios were compared to
values from previous balloon and theoretical studies. This evaluation
includes the major eruption of Mt. Pinatubo in 1991, followed by the
atmospherically quiescent period beginning in the late 1990s.
Recommendations are made regarding the use of this method for evaluation of
aerosol extinction profiles collected using the occultation method.
... Some ground-based measurements from Australia have shown evidence of the outflow plume 26,28 but spaceborne measurements that can conclusively track the outflow from the source region to the Australian continent have been lacking. There were initial spaceborne lidar measurements made by the Laser In-space Technology Experiment (LITE) 42 that appear to capture a feature similar to those described in this paper during September 1994. As LITE was a technology demonstration onboard the Space Shuttle, those measurements were limited in coverage and, moreover, the 1064 nm data from LITE was never calibrated. ...
Biomass burning in Africa provides a prolific source of aerosols that are transported from the source region to distant areas, as far away as South America and Australia. Models have long predicted the primary outflow and transport routes. Over time, field studies have validated the basic production and dynamics that underlie these transport patterns. In more recent years, the advancement of spaceborne active remote-sensing techniques has allowed for more detailed verification of the models and, importantly, verification of the vertical distribution of the aerosols in the transport regions, particularly with respect to westerly transport over the Atlantic Ocean. The Cloud-Aerosol Transport System (CATS) lidar on the International Space Station has detection sensitivity that provides observations that support long-held theories of aerosol transport from the African subcontinent over the remote Indian Ocean and as far downstream as Australia.
Significance:
• Biomass burning in Africa can have impacts as far away as Australia.
• Flow of aerosols from Africa towards Australia has long been postulated by transport models, but has been poorly characterised due to a lack of measurements.
• The CATS instrument on the International Space Station has detection sensitivity that captures aerosol transport from Africa over the Indian Ocean to Australia.
Open data set:
https://cats.gsfc.nasa.gov/
... Later, the Lidar In-space Technology Experiment (LITE) (Mc-Cormick et al., 1993;Winker et al., 1996), the precursor to CALIPSO, provided the first opportunities to observe vertical distributions of aerosol globally. Kent et al. (1998) first described the long-range transport of biomass burning aerosols and characterized the optical properties using a similar lidar retrieval for LITE. Ground-and ship-based measurements by micropulse lidar (Spinhirne, 1993) provided case studies of biomass burning (Campbell et al., 2003), maritime and polluted maritime (Welton et al., 2002), and dust aerosols (Welton et al., 2000;Powell et al., 2000). ...
The NASA Langley Research Center (LaRC) airborne High Spectral Resolution Lidar (HSRL) on the NASA B200 aircraft has acquired extensive datasets of aerosol extinction (532 nm), aerosol optical depth (AOD) (532 nm), backscatter (532 and 1064 nm), and depolarization (532 and 1064 nm) profiles during 18 field missions that have been conducted over North America since 2006. The lidar measurements of aerosol intensive parameters (lidar ratio, depolarization, backscatter color ratio, and spectral depolarization ratio) are shown to vary with location and aerosol type. A methodology based on observations of known aerosol types is used to qualitatively classify the extensive set of HSRL aerosol measurements into eight separate types. Several examples are presented showing how the aerosol intensive parameters vary with aerosol type and how these aerosols are classified according to this new methodology. The HSRL-based classification reveals vertical variability of aerosol types during the NASA ARCTAS field experiment conducted over Alaska and northwest Canada during 2008. In two examples derived from flights conducted during ARCTAS, the HSRL classification of biomass burning smoke is shown to be consistent with aerosol types derived from coincident airborne in situ measurements of particle size and composition. The HSRL retrievals of AOD and inferences of aerosol types are used to apportion AOD to aerosol type; results of this analysis are shown for several experiments.
... Globally and annually averaged extinction in Ctrl is smaller than the detection limit at all heights above roughly 2-3 km altitude, which clearly makes the model vs. CALIOP validation of the aerosol profile in the upper troposphere very uncertain. Assuming a background aerosol extinction of 0.005 km −1 (instead of zero), which is on the high end of a range of background values from satellite retrievals (Kent et al., 1998) and actually close to the globally averaged extinction at 10 km height in CAM4-Oslo (0.006 km −1 ), Koffi et al. (2012a) found that this was enough to reverse the conclusion about the AeroCom model performance in the upper atmosphere, with the exception for one model which still overestimated the extinction at high altitudes. ...
... Data on concentrations as well as characteristics of particulate matter are almost non-existent in developing countries most of which are in the Southern Hemisphere. For instance, there is almost no routine monitoring of aerosol data in Africa except for South Africa, Egypt and Tunisia (Landsberger and Biegalski, 1995;Kent et al., 1998). In Ghana, collaboration between the US Environmental Protection Agency (US-EPA) and the United Nations Environmental Program (UNEP) started in 2005. ...
Particulate matter and black carbon concentration levels in Ashaiman, a semi-urban area of Ghana was assessed. Using IVL PM 2.5 and PM 10 particle samplers, airborne particulate matter was sampled on Teflon filters for a period of three months. In addition to determination of particulate mass in the two fractions by gravimetrical method, aerosol filters were analyzed to determine Black Carbon (BC) concentration levels using the black smoke method. BC fractions in fine and coarse, together with PM 2.5 to PM 10 ratio were determined. PM 2.5 mass concentrations determined averaged 23.26 :g/m 3 (3.85 -46.43 :g/m 3) and that of PM 10 was 96.56 :g/m 3 (37.10-293.06 :g/m 3). The results were compared with some literature values and World Health Organization guideline values. The values obtained for PM 2.5 to PM 10 ratio and for PM 10-2.5 concentrations, suggest that, the semi-urban background aerosol is not only largely made up of combustion generated carbonaceous particles but also particulate matter emissions from natural activities.
... Globally and annually averaged extinction in Ctrl is smaller than the detection limit at all heights above roughly 2-3 km altitude, which clearly makes the model vs. CALIOP validation of the aerosol profile in the upper troposphere very uncertain. Assuming a background aerosol extinction of 0.005 km −1 (instead of zero), which is on the high end of a range of background values from satellite retrievals (Kent et al., 1998) and actually close to the globally averaged extinction at 10 km height in CAM4-Oslo (0.006 km −1 ), Koffi et al. (2012a) found that this was enough to reverse the conclusion about the AeroCom model performance in the upper atmosphere, with the exception for one model which still overestimated the extinction at high altitudes. ...
According to the 4th assessment report of IPCC, major sources of
uncertainty in anthropogenic climate change projections are inaccurate
model description and weak knowledge of aerosols and their interactions
with radiation and clouds, as well as the cloud feedback to radiative
forcing. One important aspect of the associated uncertainty is the
natural atmosphere. Anthropogenic climate change is an increment caused
by anthropogenic emissions relative to the properties of the climate
system untouched by man. This is crucial for the direct and indirect
effects of aerosols, since the amount, size and physical properties of
natural background particles strongly influence the same properties of
the anthropogenic aerosol components. In many climate models where
CDNC is calculated explicitly, CDNC is constrained by prescribing a
lower bound below which calculated values are not allowed. This is done
in order to keep the aerosol in-direct effect within estimated values.
The rationale for using such a lower bound is to keep the aerosol
radiative forcing constrained by the forcing of green-house gases and
20th century climate.We hypothesize this lower bound can be removed or
made less strict by including aerosols of biogenic origin. We will
present results and sensitivity studies from simulations with the NorESM
where we have added contributions from organic carbon of natural origin
both from vegetation and oceanic sources. By including aerosols of
biogenic origin we obtain close to the median indirect radiative forcing
reported by IPCC AR4, as well as reproducing the temperature increase in
the 20th century. NorESM is based on the Earth system model CCSM4.0
from NCAR, but is using CAM4-Oslo instead of CAM4 as atmosphere model
and an updated version of MICOM from the Bergen Climate Model (BCM)
instead of the ocean model POP2. The aerosol module includes sea-salt,
dust, sulphate, black carbon (BC) and particulate organic matter (OM).
Primary aerosol size-distributions are modified by condensation,
coagulation, and wet-phase processes. Aerosol optics and size
distributions for calculation of cloud droplet activation use look-up
tables constructed from first principles. The model is used for CMIP5
simulations.
... The transmittance measurements are inverted using the "onion-peeling" approach (Antuña et al., 2002) to retrieve the aerosol extinction coefficient (km −1 ). SAGE II has a horizontal resolution of the order of 200 km and a vertical resolution of 1 km (Kent et al., 1998). The 0.525 µm extinction data used here are based on the v6.2 retrieval algorithm. ...
An eight member ensemble of ECHAM5-HAMMOZ simulations for the year 2003
is analyzed to study the transport of aerosols in the Upper Troposphere
and Lower Stratosphere (UTLS) during the Asian Summer Monsoon (ASM).
Simulations show persistent maxima in black carbon, organic carbon,
sulfate, and mineral dust aerosols within the anticyclone in the UTLS
throughout the ASM (period from July to September) when convective
activity over the Indian subcontinent is highest. Model simulations
indicate boundary layer aerosol pollution as the source of this UTLS
aerosol layer and identify ASM convection as the dominant transport
process. Evidence of ASM transport of aerosols into the stratosphere is
observed in HALogen Occultation Experiment (HALOE) and Stratospheric
Aerosol and Gas Experiment (SAGE) II aerosol extinction. The impact of
aerosols in the UTLS region is analyzed by evaluating the differences
between simulations with (CTRL) and without aerosol (HAM-off) loading.
The transport of anthropogenic aerosols in the UTLS increases cloud ice,
water vapour and temperature, indicating that aerosols play an important
role in enhancement of cloud ice in the Upper-Troposphere (UT). Aerosol
induced circulation changes include a weakening of the main branch of
the Hadley circulation and increased vertical transport around the
southern flank of the Himalayas and reduction in monsoon precipitation
over the India region.
... The transmittance measurements are inverted using the "onion-peeling" approach ( Antuña et al., 2002) to retrieve the aerosol extinction coefficient (km −1 ). SAGE II has a horizontal resolution of the order of 200 km and a vertical resolution of 1 km ( Kent et al., 1998). The 0.525 µm extinction data used here are based on the v6.2 retrieval algorithm. ...
An eight-member ensemble of ECHAM5-
HAMMOZ simulations for a boreal summer season is analysed
to study the transport of aerosols in the upper troposphere
and lower stratosphere (UTLS) during the Asian summer
monsoon (ASM). The simulations show persistent maxima
in black carbon, organic carbon, sulfate, and mineral
dust aerosols within the anticyclone in the UTLS throughout
the ASM (period from July to September), when convective
activity over the Indian subcontinent is highest, indicating
that boundary layer aerosol pollution is the source
of this UTLS aerosol layer. The simulations identify deep
convection and the associated heat-driven circulation over
the southern flanks of the Himalayas as the dominant transport
pathway of aerosols and water vapour into the tropical
tropopause layer (TTL). Comparison of model simulations
with and without aerosols indicates that anthropogenic
aerosols are central to the formation of this transport pathway.
Aerosols act to increase cloud ice, water vapour, and
temperature in the model UTLS. Evidence of ASM transport
of aerosols into the stratosphere is also found, in agreement
with aerosol extinction measurements from the Halogen Occultation
Experiment (HALOE) and Stratospheric Aerosol
and Gas Experiment (SAGE) II. As suggested by the observations,
aerosols are transported into the Southern Hemisphere
around the tropical tropopause by large-scale mixing
processes. Aerosol-induced circulation changes also include
a weakening of the main branch of the Hadley circulation and
a reduction of monsoon precipitation over India.
... Globally and annually averaged extinction in Ctrl is smaller than the detection limit at all heights above roughly 2-3 km altitude, which clearly makes the model vs. CALIOP validation of the aerosol profile in the upper troposphere very uncertain. Assuming a background aerosol extinction of 0.005 km −1 (instead of zero), which is on the high end of a range of background values from satellite retrievals (Kent et al., 1998) and actually close to the globally averaged extinction at 10 km height in CAM4-Oslo (0.006 km −1 ), Koffi et al. (2012a) found that this was enough to reverse the conclusion about the AeroCom model performance in the upper atmosphere, with the exception for one model which still overestimated the extinction at high altitudes. ...
The objective of this study is to document and evaluate recent changes and updates to the module for aerosols and aerosol–cloud–radiation interactions in the atmospheric module CAM4-Oslo of the core version of the Norwegian Earth System Model (NorESM), NorESM1-M. Particular attention is paid to the role of natural organics, sea salt, and mineral dust in determining the gross aerosol properties as well as the anthropogenic contribution to these properties and the associated direct and indirect radiative forcing.
The aerosol module is extended from earlier versions that have been published, and includes life-cycling of sea salt, mineral dust, particulate sulphate, black carbon, and primary and secondary organics. The impacts of most of the numerous changes since previous versions are thoroughly explored by sensitivity experiments. The most important changes are: modified prognostic sea salt emissions; updated treatment of precipitation scavenging and gravitational settling; inclusion of biogenic primary organics and methane sulphonic acid (MSA) from oceans; almost doubled production of land-based biogenic secondary organic aerosols (SOA); and increased ratio of organic matter to organic carbon (OM/OC) for biomass burning aerosols from 1.4 to 2.6.
Compared with in situ measurements and remotely sensed data, the new treatments of sea salt and dust aerosols give smaller biases in near-surface mass concentrations and aerosol optical depth than in the earlier model version. The model biases for mass concentrations are approximately unchanged for sulphate and BC. The enhanced levels of modeled OM yield improved overall statistics, even though OM is still underestimated in Europe and overestimated in North America.
The global anthropogenic aerosol direct radiative forcing (DRF) at the top of the atmosphere has changed from a small positive value to −0.08 W m<sup>−2</sup> in CAM4-Oslo. The sensitivity tests suggest that this change can be attributed to the new treatment of biomass burning aerosols and gravitational settling. Although it has not been a goal in this study, the new DRF estimate is closer both to the median model estimate from the AeroCom intercomparison and the best estimate in IPCC AR4. Estimated DRF at the ground surface has increased by ca. 60%, to −1.89 W m<sup>−2</sup>. We show that this can be explained by new emission data and omitted mixing of constituents between updrafts and downdrafts in convective clouds.
The increased abundance of natural OM and the introduction of a cloud droplet spectral dispersion formulation are the most important contributions to a considerably decreased estimate of the indirect radiative forcing (IndRF). The IndRF is also found to be sensitive to assumptions about the coating of insoluble aerosols by sulphate and OM. The IndRF of −1.2 W m<sup>−2</sup>, which is closer to the IPCC AR4 estimates than the previous estimate of −1.9 W m<sup>−2</sup>, has thus been obtained without imposing unrealistic artificial lower bounds on cloud droplet number concentrations.
... f S reported at 690 nm; method requires significant wavelength and angular adjustments, based on assumed size distributions and Mie calculations Rosen and Kjome, 1997]. g S at 532 nm is constrained to values yielding physically plausible lidar retrievals under the assumption of constant S throughout upper troposphere [Kent et al., 1998]. h S at 308 nm determined from independent extinction measurements using nitrogen-Raman; one profile [Ansmann et al., 1992]. ...
The extinction-to-backscatter ratio S is a crucial parameter for quantitative interpretation of lidar data, yet empirical knowledge of S for tropospheric aerosols is extremely limited. Here we review that knowledge and extend it using a recently developed in situ technique that employs a 180… backscatter nephelometer. This technique allows robust quantification of measurement uncertainties and permits correlations with other aerosol and meteorological properties to be explored. During 4 weeks of nearly continuous measurements in central Illinois, S was found to vary over a wide range, confirming previous indications that geographical location by itself is not necessarily a good predictor. The data suggest a modest dependence of S on relative humidity, but this explains only a small portion of the variation. Most variation was associated with changes between two dominant air mass types: rapid transport from the northwest and regional stagnation. The latter category displayed much higher aerosol concentrations and a systematically higher and more tightly constrained range of S. Averages and standard deviations were 64 ± 4 sr for the stagnant category and 40 ± 9 sr for the rapid transport category. Considering the 95% confidence precision uncertainty of the measurements, the difference between these averages is at least 13 sr and could be as large as 35 sr. The wavelength dependence of light scattering, as measured by a conventional nephelometer, is shown to have some discriminatory power with respect to S.
... This is not to suggest that variability in the vertical is unimportant; indeed, it is essential to recognize and understand it. There are many facets of such variability that can be immediately recognized: layers of aerosol in the stratosphere from volcanic injection, visible and subvisible cirrus, the stability (on average) of the planetary boundary layer and the existence therein of higher aerosol concentrations than in the free troposphere, the observations of laminae of higher and lower aerosol content within and above the PBL (e.g., Kent et al. 1998;Hobbs 2002), the likely correlation of such layers with the thermodynamic state of the air (especially the relative humidity), and the complex effects of mixing processes in the vertical dimension that change both the aerosol extensive properties and the thermodynamic state. The present study does not address these issues except to comment, in the discussion section, on the consequences of horizontal variability on aircraft observations of vertical profiles. ...
ABSTRACT Tropospheric aerosols are calculated to cause global-scale changes in the earth’s heat balance, but these forcings are space/time integrals over highly variable quantities. Accurate quantification of these forcings will require an unprecedented synergy among satellite, airborne, and surface-based observations, as well as models. This study considers one aspect of achieving this synergy—the need to treat aerosol variability in a consistent and realistic way. This need creates a requirement,to rationalize the differences in spatiotemporal,resolution and coverage among the various observational and modeling approaches. It is shown, based on aerosol optical data from diverse regions, that mesoscale variability (specifically, for horizontal scales of 40‐400 km and temporal scales of 2‐48 h) is a common,and perhaps universal feature of lower-tropospheric aerosol light extinction. Such variation is below the traditional synoptic or ‘‘airmass’’ scale (where the aerosol is often assumed,to be essentially homogeneous,except for plumes from point sources) and below the scales that are readily resolved by chemical,transport models. The present study focuses on documenting,this variability. Possible physical causes and practical implications for coordinated observational strategies are also discussed.
... Later, the Lidar In- 20 space Technology Experiment (LITE) (Winker et al., 1996), the precursor to CALIPSO, provided the first opportunities to observe vertical distributions of aerosol globally. Kent et al. (1998) first described the long-range transport of biomass burning aerosols and characterized the optical properties using a similar lidar retrieval for LITE. Ground-and ship-based measurements by micropulse lidar (Spinhirne, 1993) provided case studies 25 of biomass burning (Campbell et al., 2003), maritime and polluted maritime (Welton et al., 2002), and dust aerosols (Welton et al., 2000; Powell et al., 2000). ...
The NASA Langley Research Center (LaRC) airborne High Spectral Resolution Lidar (HSRL) on the NASA B200 aircraft has acquired extensive datasets of aerosol extinction (532 nm), aerosol optical thickness (AOT) (532 nm), backscatter (532 and 1064 nm), and depolarization (532 and 1064 nm) profiles during 18 field missions that have been conducted over North America since 2006. The lidar measurements of aerosol intensive parameters (lidar ratio, depolarization, backscatter color ratio, and spectral depolarization ratio) are shown to vary with location and aerosol type. A methodology based on observations of known aerosol types is used to qualitatively classify the extensive set of HSRL aerosol measurements into eight separate types. Several examples are presented showing how the aerosol intensive parameters vary with aerosol type and how these aerosols are classified according to this new methodology. The HSRL-based classification reveals vertical variability of aerosol types during the NASA ARCTAS field experiment conducted over Alaska and northwest Canada during 2008. In two examples derived from flights conducted during ARCTAS, the HSRL classification of biomass burning smoke is shown to be consistent with aerosol types derived from coincident airborne in situ measurements of particle size and composition. The HSRL retrievals of AOT and inferences of aerosol types are used to apportion AOT to aerosol type; results of this analysis are shown for several experiments.
... ( Rosen et al., 1997a, b) g S at 532 nm is constrained to values yielding physically plausible lidar retrievals under the assumption of constant S throughout upper troposphere. ( Kent et al., 1998) h S at 532 nm determined from independent extinction measurements using nitrogen-Raman. ( Ansmann et al., 1992;Muller et al., 1998)But it is not enough to have individual measurements of S a that are not coupled to other aerosol and atmospheric properties. ...
The goal of this project is to facilitate the integration of Light Detection and Ranging (lidar) measurements in building a global aerosol climatology. Lidar is sensitive to aerosol even at modest concentrations, provides vertical resolution, and is capable of surveying large regions of the troposphere - indeed, upcoming satellite missions will provide truly global surveys. However, quantitative interpretation of lidar data requires knowledge of the extinction-to-backscatter ratio, S a. As such, the focus of our project has been to develop a database and an approach to constraining S a. As shown below, knowledge of Sa for tropospheric aerosols was highly unconstrained at the outset of this project and is much better constrained today (due to the efforts of several groups, not just our own). Two case studies using spaceborne lidar data are used below to demonstrate the vast reduction of uncertainty in lidar retrievals that is associated with moving from unconstrained Sa to constrained S a. Prior to this project, our group developed a method for calibrated, in situ measurement of Sa (Doherty et al., 1999, Appl. Optics, 38, 1823). During the course of this project, we deployed this technology in several field campaigns to obtain characteristic values for a wide range of aerosol types and to study the physical/chemical/thermodynamic parameters that control variations in S a. This grant was the primary source of support for one field campaign (LINC) and for the analysis of Sa from all field campaigns. While considerable progress has been made over the three years of this project, a great deal remains to be done. As documented below, the recent increase in data on S a has been extraordinary. These data come from our approach as well as approaches based on Raman lidar, slant-path lidar, and sunphotometry. The challenge ahead is to integrate these data and to develop and test methods of constraining S a throughout the global troposphere. The planned launch in 2004 of the ESSP3-CENA satellite (a lidar satellite to fly in formation with the many aerosol sensors on NASA's Aqua satellite) provides a compelling target date for achieving this larger goal. As described herein, our research has helped to lay the foundation for this larger project, which will form the basis of a subsequent proposal.
... . A variety of lidar–ratio observations are available for northern midlatitudes [e.g., Takamura et al., 1994; Anderson et al., 2000; Ansmann et al., 2001], a few have been published for the southern hemisphere [e.g., Young et al., 1993; Kent et al., 1998] but no measurements were done in the tropics before INDOEX. ...
Observations of the extinction-to-backscatter ratio (lidar ratio) of South and Southeast Asian aerosol par-ticles are presented for the wavelength of 532 nm. Raman lidar measurements were performed in the Maldives (4.1 • N, 73.3 • E) in the framework of the Indian Ocean Experiment (INDOEX) in 1999/2000. These observations in the tropics are an important contribution to a growing global lidar– ratio climatology which is needed for an improved determi-nation of the particle optical depth with ground–based and spaceborne lidars. The lidar ratio was found to be a use-ful quantity to trace back different pollution sources and to identify less and considerably light–absorbing particles. During the winter/spring seasons heavily polluted air from India and Southeast Asia was advected to the lidar site. Un-der these conditions lidar ratios up to 110 sr were observed in the lofted pollution plumes above 1000 m height. According to backward trajectories the highest lidar ratios were found for airmasses which crossed the eastern and northeastern parts of India. Large lidar ratios >70 sr indicate small, con-siderably absorbing aerosol particles. Below 1000 m height, the lidar ratio typically ranged from 30–60 sr. The marine boundary layer contained a mixture of marine and anthro-pogenic particles. Under clean, marine conditions in Octo-ber 1999, lidar ratios <30 sr were found.
... In our study, atmospheric layers with no detected aerosol are assumed to have zero aerosol extinction. Previous studies based on the LITE (Lidar-In-SpaceTechnology Experiment) and on SAGE (Stratospheric Aerosol and Gas Experiment) global satellite data sets [Kent et al., 1993 [Kent et al., , 1998 show aerosol background extinctions mainly ranging between 0.001 and 0.005 km À1 on average in the upper troposphere (from 6 km to the tropopause) of the Southern Hemisphere. In order to test the impact of an eventual presence of background aerosol below the detection limit, we conducted a sensitivity test by assuming 0.001 km À1 and 0.005 km À1 extinction over atmospheric layers where no aerosol layer was detected by the CALIOP retrieval algorithm. ...
1] The CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) layer product is used for a multimodel evaluation of the vertical distribution of aerosols. Annual and seasonal aerosol extinction profiles are analyzed over 13 sub-continental regions representative of industrial, dust, and biomass burning pollution, from CALIOP 2007–2009 observations and from AeroCom (Aerosol Comparisons between Observations and Models) 2000 simulations. An extinction mean height diagnostic (Z a) is defined to quantitatively assess the models' performance. It is calculated over the 0–6 km and 0–10 km altitude ranges by weighting the altitude of each 100 m altitude layer by its aerosol extinction coefficient. The mean extinction profiles derived from CALIOP layer products provide consistent regional and seasonal specificities and a low inter-annual variability. While the outputs from most models are significantly correlated with the observed Z a climatologies, some do better than others, and 2 of the 12 models perform particularly well in all seasons. Over industrial and maritime regions, most models show higher Z a than observed by CALIOP, whereas over the African and Chinese dust source regions, Z a is underestimated during Northern Hemisphere Spring and Summer. The positive model bias in Z a is mainly due to an overestimate of the extinction above 6 km. Potential CALIOP and model limitations, and methodological factors that might contribute to the differences are discussed. Citation: Koffi, B., et al. (2012), Application of the CALIOP layer product to evaluate the vertical distribution of aerosols estimated by global models: AeroCom phase I results, J. Geophys. Res., 117, D10201, doi:10.1029/2011JD016858.
INTRODUCTIONSIZE DISTRIBUTION, COMPOSITION, AND CONCENTRATIONAEROSOL SOURCESHETEROGENEOUS CHEMISTRYCLIMATE FORCINGTROPOSPHERIC AND STRATOSPHERIC AEROSOLS: REMOTE SENSING
In this paper, two inversion algorithms considering multiple scattering are proposed to retrieve aerosol/cloud extinction coefficient profiles from LITE data, which are called as the Iterative Forward Integration Algorithm (IFIA) and the Iterative Forward-Backward Integration Algorithm (IFBIA). In IFIA, at first assuming no multiple scattering, retrieve the extinction coefficient profile by the forward integration algorithm. Then, using the profile and assuming an aerosol/cloud phase scattering function, calculate the multiple scattering component by a parameterized multiple scatter lidar equation (or Monte Carlo calculation) and then yield new extinction coefficient profile solution. By using IFIA and IFBIA, some typical LITE data are selected to derive aerosol/cloud extinction coefficient profiles. As shown in the inversion results, if the multiple scattering is neglected, there may be a very large uncertainty in the retrieved aerosol/cloud extinction coefficient profile, especially for the shorter-wavelength 355nm channel and the case of cloud layer. The present algorithms considering multiple scattering can produce more reasonable aerosol/cloud extinction coefficient retrievals.
Three methods of analyzing Stratospheric Aerosol and Gas Experiment (SAGE) II tropospheric aerosol extinction data are described and intercompared in terms of global maps and vertical contour plots of the extinction coefficient, or its equivalent. The first method, which has been in use for several years, is found to be biased toward smaller aerosols (effective radius 0.25 mum). The third method which, unlike the first two methods, is capable of producing an altitude resolved aerosol climatology down to about 1 km above the earth's surface, requires an assumption about the amount of cloud contamination in the data set. Given the correctness of this assumption, the method is able to derive the total extinction due to both large and small aerosols. Aerosol climatologies produced by all three methods are shown and intercompared, with particular emphasis on the lofting of dust from Asian and other Northern Hemisphere deserts and its subsequent advection over the western Pacific Ocean.
A model is proposed for identifying the aerosol mode of the second Stratospheric Aerosol and Gas Experiment (SAGE II) 1.02-mum extinction coefficient measurements at altitudes below 6.5 km, which also contain cloud samples. This development allows one to extend the SAGE II satellite data analysis from the lower limit at 6.5 km of the SAGE II two-wavelength method into the lower troposphere. Thus the proposed model provides opportunities for fully utilizing the SAGE II tropospheric measurements important to the understanding of the global behavior of tropospheric aerosols, clouds, and ozone and related transports. The effectiveness of this model is examined by using the SAGE II two-wavelength technique at 6.5 km. Sample applications of the proposed model reveal encouraging results. To assess the quality of the aerosol 1.02-mum data, it is recommended that a comprehensive data comparison analysis be conducted by using tropospheric measurements from different instruments.
Lidar (extinction-to-backscatter) ratios are computed at 0.55, 1 and 10 µm, based upon a recently published summary of the physicochemical properties of climatically relevant aerosol species. The results agree very well with previously measured values in the literature, indicating that low Sa values for desert dust (15-30) and maritime (30-45) aerosols are clearly distinguishable from biomass burning (55-65) and urban/industrial pollution (55-80). The results show that most aerosol types can be discriminated by their absorption and scattering characteristics through use of spectral lidar ratios, except between biomass burning and pollution aerosols. Predictions of on- and off-axis scattering in the presence of these aerosol types illustrate the range of signal that may be expected in a bistatic lidar system in such cases, and indicate that bistatic lidar may be successfully used to detect a source lidar signal and discriminate the aerosol species present. These findings strongly suggest that a combination of passive and active remote sensing systems operating simultaneously (e.g., ground-based sky radiance and bistatic lidar), would be capable of directly measuring the absorption and scattering characteristics required to describe the optical behaviour of the aerosol with vertical resolution. This is expected to be of great utility to climate researchers or other communities interested in comprehensively measuring atmospheric optical properties.
A detailed analysis has been made of Stratospheric Aerosol and Gas Experiment I and II aerosol extinction data for the upper troposphere (6-km altitude to the seasonally averaged tropopause) taken between 1979 and 1998. An improved method of separation of the volcanic and surface-derived components of the aerosol optical depth has been used. The mean extinction, at a wavelength of 1.02 mum, of the nonvolcanic component of the upper tropospheric aerosol is found to increase from approximately 1×10-4km-1 at 70°S to about 7 times that value at 70°N. Maximum downward transfer of volcanic material into the upper troposphere is observed to take place in local spring in each hemisphere, occurring at a latitude of 70°S or greater in the southern hemisphere and at about 50°N in the northern hemisphere. The almost 20-year data sequence (1979-1981, 1984-1991, 1994-1998) has been examined for evidence of any long-term trends in the aerosol optical depth of the upper troposphere. It is unlikely that any change in the upper tropospheric 1-mum aerosol optical depth greater than 1% per year has taken place when averaged over either hemisphere.
Three Pacific atmospheric chemistry experiments (PACE I, II, and III) were conducted each in January 1994, October 1994, and July 1995. The objective was to investigate the latitudinal distribution, transportation, and possible sources of tropospheric ozone over the western Pacific. Measurements of ozone using UV absorption method were taken by an aircraft flying at a maximum altitude of 6000 m between the Southern Hemisphere (SH) and the Northern Hemisphere (NH) (38°S–38°N) and within the longitudinal range of 120°E–150°E. The PACE aircraft missions first provide the middle troposphere (5000–6000 m) ozone distributions in three different seasons over this region in relatively short periods of time (within 9 days). On the basis of one-minute average data, ozone mixing ratios in the middle troposphere were significantly higher in the NH midlatitudes (39–92 ppbv) and the SH midlatitudes (30–71 ppbv) than in the tropics (10–35 ppbv) during three different seasons. In particular, an air mass with low ozone mixing ratios (2–28 ppbv) extended from 7°S to 29°N in July during PACE III, in contrast to the air mass with higher ozone mixing ratios (39–57 ppbv) observed in the NH midlatitudes (>21°N) in January during PACE I. Several episodes of increased ozone were observed during the PACE missions. In the SH subtropics (16°S–22°S), photochemical ozone production in a biomass-burning smoke, probably emitted from the northwest part of Australia, caused relatively high ozone mixing ratios (maximum 89 ppbv) at 5200 m during PACE II. In contrast, ozone transport over a long distance from the upper troposphere south of Africa brought about a maximum ozone mixing ratio of 64 ppbv in the same geographical region above 4500 m during PACE III. Large-scale circulation coupled with a typhoon was found to impact ozone transport from the NH midlatitudes to the tropics. An air mass with ozone mixing ratios averaging about 50 ppbv at 5200 m, which originated over the Asian continent in the NH midlatitudes, was transported to the tropical western Pacific region (9°N–14°N) by a circulation coupled with a typhoon during PACE II. Summer observations of tropopause folding over the western Pacific are rare, yet high potential vorticity (PV) value (>1 PV unit) above 600 hPa level at 35°N shows that the tropopause folding beneath the subtropical jet occurred in the NH midlatitudes over the western Pacific in summer during PACE III. In this measurement, the increased ozone (maximum 92 ppbv) combined with decreased water vapor mixing ratio (
Monthly sonde data for Lauder in Central Otago, New Zealand show
profiles of aerosol backscatter from the surface to over 30 km altitude.
The tropospheric data vary by season, with greater aerosol backscatter
throughout the free troposphere in springtime. Aerosol mixing ratios in
layers in the upper troposphere at these times are often much higher
than anywhere else above the boundary layer, suggesting that they arise
from horizontal transport. Ozone measurements from the sonde show
correspondence in vertical structure to the backscatter data and also
seasonal enhancement. The latter correlates with aerosol, but competing
causes of ozone enhancement make the correspondence indistinct. High
concentrations of carbon monoxide are observed by Fourier transform
spectroscopy in spring, and altitude profiles derived from line shape
suggest that the peak in CO occurs in the same altitude range as the
aerosol enhancement.
We derive profiles of the aerosol extinction-to-backscatter ratio, Sa, at 355 nm using aerosol extinction and backscatter profiles measured during 1998 and 1999 by the operational Raman lidar at the Department of Energy Atmospheric Radiation Measurement program Southern Great Plains site in north central Oklahoma. Data from this Raman/Rayleigh-Mie lidar, which measures Raman scattering from nitrogen as well as the combined molecular (Rayleigh) and aerosol (Mie) scattering at the laser wavelength, are used to derive aerosol extinction and backscattering independently as a function of altitude. Because this lidar operates at 355 nm, where molecular backscattering is comparable with aerosol backscattering, Sa retrievals are generally limited to conditions where the aerosol extinction at 355 nm is > 0.03 km-1. The mean value of Sa at 355 nm derived for this period was 60 sr with a standard deviation of 12 sr. Sa was generally about 5-10 sr higher during high aerosol optical thickness (AOT) (> 0.3) conditions than during low AOT (< 0.1). A similar increase in Sa was found when the relative humidity increased from 30 to 80%. Large (> 15%) variations in the vertical profile of Sa occurred about 30% of the time, which implies significant variability in the vertical distribution of aerosol size distribution, shape, and/or composition often occurs. The Raman lidar measurements of Sa were compared with estimates of particle size and refractive index derived from an algorithm that uses ground-based Sun photometer measurements of Sun and sky radiance. For 17 cases of coincident Raman lidar and Sun and sky radiance measurements, Sa was linearly correlated with the aerosol fine mode effective radius and the volume ratio of fine/coarse particles.
Airborne particles have been collected using a dichotomous virtual impactor at three different locations in the eastern part of Botswana: Serowe, Selibe-Phikwe, and Francistown. The particles were separated into two fractions (fine and coarse). Sampling at the three locations was done consecutively during the months of July and August, which are usually dry and stable. The sampling time for each sample was 12 hours during the day. For elemental composition, energy-dispersive x-ray fluorescence technique was used. Correlations and principal component analysis with varimax rotation were used to identify major sources of aerosol particles. In all the three places, soil was found to be the main source of aerosol particles. A copper-nickel mine and smelter at Selibe-Phikwe was found to be not only a source of copper and nickel particles in Selibe-Phikwe but also a source of these particles in far places like Serowe. In Selibe-Phikwe and Francistown, car exhaust was found to be the major source of fine particles of lead and bromine.
Air samples at 8-13 km were collected regularly using a commercial airliner to obtain long-term measurements of carbon monoxide (CO) mixing ratio in the upper troposphere over the western Pacific between Australia and Japan during April 1993-December 1997. The measurements in 1997 clearly reveal an anomalous CO increase during September to November in the Southern Hemisphere, with a maximum of 320-380 ppb around 20°S in October. Tropical biomass burning, not urban/industrial emissions, was the main source for the enhanced CO in 1997. A similar southern-spring increase due to biomass burning was observed in previous years. The peaks showed a large interannual variation associated with the El Niño/Southern Oscillation (ENSO) events. The largest CO spring peak appeared during the strong El Niño even in 1997, while the weak La Niña year of 1996 was marked by a largely suppressed CO spring peak. The outgoing longwave radiation (OLR) anomaly is largest during the El Niño events indicating that the events cause a longer drought in the tropics and significantly influence the enlargement of biomass burning in tropical Southeast Asia. Thus the most likely cause for the ENSO-cycle CO variability is a year-to-year change of biomass-burning emissions mainly from Southeast Asia. The appearance of the CO spring peak in the southern subtropics is discussed on the basis of the possible long-range transport of biomass-burning CO from Southeast Asia to the upper troposphere over the western South Pacific.
Altitude profiles of Carbon Monoxide (CO) and aerosols have been
compared from the Network for Stratospheric Change (NDSC) mid-latitude
southern hemisphere site at Lauder, New Zealand. The CO mixing ratio
profile was derived from infrared spectra recorded with a very high
resolution Fourier Transform interferometer using three lines of the
(1-0) band between 2057 and 2160 cm-1. The
aerosol surface area was derived from balloon-borne backscatter
radiation at 940 nm. Both datasets show significant enhancements
occurring over the observation site in the austral spring. When
displayed together their combined effect illustrates the close
correlation between CO and aerosols. Peak concentrations are
consistently recorded between September and October over a five year
time frame (1994-1999), with the enhancements typically occurring
at heights of between 3 to 8 km. The temporal and spatial correlation
between the aerosol plumes and enhanced CO concentrations are
interpreted in terms of the effect of long range transport of biomass
burning plumes in combination with the El Nino-Southern Oscillation
(ENSO) cycles influence on southern hemisphere climate dynamics.
In situ and laser remote measurements of gases and aerosols were made
with airborne instrumentation to establish a baseline chemical signature
of the atmosphere above the South Pacific Ocean during the NASA Global
Tropospheric Experiment (GTE)/Pacific Exploratory Mission-Tropics A
(PEM-Tropics A) conducted in August-October 1996. This paper discusses
general characteristics of the air masses encountered during this
experiment using an airborne lidar system for measurements of the
large-scale variations in ozone (O3) and aerosol
distributions across the troposphere, calculated potential vorticity
(PV) from the European Centre for Medium-Range Weather Forecasting
(ECMWF), and in situ measurements for comprehensive air mass
composition. Between 8°S and 52°S, biomass burning plumes
containing elevated levels of O3, over 100 ppbv, were
frequently encountered by the aircraft at altitudes ranging from 2 to 9
km. Air with elevated O3 was also observed remotely up to the
tropopause, and these air masses were observed to have no enhanced
aerosol loading. Frequently, these air masses had some enhanced PV
associated with them, but not enough to explain the observed
O3 levels. A relationship between PV and O3 was
developed from cases of clearly defined O3 from stratospheric
origin, and this relationship was used to estimate the stratospheric
contribution to the air masses containing elevated O3 in the
troposphere. The frequency of observation of the different air mass
types and their average chemical composition is discussed in this paper.
The Lidar in Space Technology Experiment (LITE) is a three-wavelength backscatter lidar developed by NASA Langley Research Center to demonstrate and explore the capabilities of space lidar. LITE was flown on space shuttle Discovery in September 1994. Among the primary experimental objectives of LITE was the measurement of stratospheric aerosols. High-quality stratospheric aerosol measurements at 532 nm and 355 nm were obtained during nighttime, high-gain operation. These LITE data provide a detailed global view of the vertical structure and optical properties of the stratospheric aerosols. The data are also used to study the transport processes influencing the aerosol spatial distribution. LITE data compare well with measurements made by the Stratospheric Aerosol and Gas Experiment (SAGE) II. Individual profile comparisons and comparisons of more global features reinforce and extend the validation of the LITE stratospheric data. LITE demonstrates that a spaceborne lidar, with its high vertical resolution and global coverage, is a powerful tool for tracing atmospheric transport.
[1] Several studies have observed midtropospheric atmospheric composition anomalies and suggested a link to tropical biomass burning. Such anomalies complicate the use of trace gas profiles in remote regions to infer their surface sources/sinks based on the vertical gradients. The Southern African Regional Science Initiative (SAFARI 2000) campaign in Africa and coordinated downwind measurements in Australia provided an opportunity to confirm this link and elucidate the specific surface and atmospheric processes. Five aircraft missions were conducted by Commonwealth Scientific and Industrial Research Organisation (CSIRO) Atmospheric Research during the campaign. They were scheduled after African outflows of polluted air were observed in satellite images over the Indian Ocean flowing east toward Australia. Air samples collected from near the surface to 7 km were analyzed for a suite of trace gases (12CO2, CH4, CO, H2, N2O, and C2 and C3 hydrocarbons) and one isotopomer (13CO2) to provide vertical composition profiles. Ozone was monitored continuously during flight while a ground-based lidar was employed in the Melbourne region to detect aerosol layers. A preliminary statistical analysis on the Australian data confirms covarying midtroposphere enhancements in the biomass burning products. Making rudimentary corrections for photochemical evolution during transit, the trace gas enhancement ratios in affected air samples are comparable to emission ratios in fresh biomass burning plumes. The 13CO2/12CO2 ratios are also consistent with a source from terrestrial plants. Back-trajectory analysis for strongly enhanced samples suggests long-range transport from tropical regions in Africa or South America, the proof of which requires a follow-on analysis with a global chemistry transport model.
A field campaign focused primarily on free tropospheric aerosol measurements over Mildura, Australia, at 34°S (Mildura Aerosol Tropospheric Experiment (MATE 98)) was conducted in the austral spring of 1998 to test for the current presence of a seasonal aerosol layering activity observed in the 1970s and to obtain additional characteristics that would lead to a better understanding of the phenomenon. Ground-based lidar as well as balloon-borne optical particle counters and backscattersondes with ozone sensors were employed. The results indicate that large horizontal scale layers are present and show that their structure is highly correlated with excess ozone. Particle concentrations in the layers were sufficient to measurably affect aerosol optical depth. The probable source is distant biomass burning regions, but a detailed understanding of associated smoke transport and evolution as observed over Mildura is incomplete.
More than three decades have passed since the launch of the first satellite instrument used for atmospheric aerosol detection. Since then, various powerful satellite remote sensing technologies have been developed for monitoring atmospheric aerosols. The application of these new technologies to differ-ent satellite data have led to the generation of mul-tiple aerosol products, such as aerosol spatial distri-bution, temporal variation, fraction of fine and coarse modes, vertical distribution, light absorption, and some spectral characteristics. These can be used to infer sources of major aerosol emissions, the transportation of aerosols, interactions between aerosols and energy and water cycles, and the involvement of aerosols with the dynamic system. The synergetic use of data from different satellite sensors provides more comprehen-sive information to better quantify the direct and indi-rect effects of aerosols on the Earth's climate. This paper reviews how satellite remote sensing has been used in aerosol monitoring from its earliest beginnings and highlights future satellite missions.
We present the seasonal cycle of optical and physical particle properties over the Indian Ocean based on case studies of six-wavelength aerosol lidar observations performed in the framework of the Indian Ocean Experiment. From February 1999 to March 2000 the lidar system made routine measurements at the International Airport of the Maldives on Hulule island (4.1°N, 73.3°E). The measurement from February 18, 1999, during the northeast monsoon showed high optical depths of 0.27 at 532 nm. Lidar ratios of 40-45 sr at 532 nm indicated the long-range transport of well-aged polluted air from Southeast Asia. Effective radii of 1.6-0.01i, and single-scattering albedos of 0.86-0.93 at 532 nm were retrieved for the pollution layer above 1000 m height. Similar physical particle properties followed from the measurement on March 22, 2000. Optical depths were 0.4; lidar ratios of 60 sr at 1000 m height indicated anthropogenic pollution from India. Values of 45 sr at 3000 m height indicated a considerable influence by clean-continental particles from Africa and Arabia. Accordingly, the single-scattering albedo ranged from 0.85-0.9. During the southwest monsoon in July, air was advected from the western Indian Ocean, eastern Africa, and Arabia. Particles were transported into maximum heights of 5 km from these regions on July 12, 1999. Small effective radii of 0.18-0.3 mum, considerably lower mean complex refractive indices of 0.88 indicated the prevailing clean-marine and clean-continental conditions. The intermonsoon season was characterized by strong washout processes during the crossover of the Intertropical Convergence Zone. On October 16, 1999, low optical depths of 0.13 and lidar ratios from 20-35 sr indicated clean-marine conditions. Accordingly, large effective radii of 0.2-0.35 mum, low complex refractive indices ~1.45-0.005i, and single-scattering albedos >0.95 were found.
For the first time the seasonal cycle of optical and physical particle properties over the Indian Ocean was observed with a six-wavelength aerosol lidar. The measurements were performed in the framework of the Indian Ocean Experiment at the Maldives International Airport in Hulule island (4.1°N, 73.3°E) from February 1999 until March 2000. The advection of air masses from India and Southeast Asia during the northeast monsoon season showed multiple particle layers with strong backscatter and extinction coefficients to heights of 4 km. Approximately 30-45% of the monthly-mean optical depth was contributed by elevated particle layers above 1000 m height. Mean optical depths of ~0.32 at 530 nm and mean extinction-to-backscatter (lidar) ratios from 45 to 75 sr at 532 nm indicated heavily polluted aerosols. Observations during March 2000 showed that 56% of the mean optical depth of 0.29 was contributed by elevated particle layers. Lidar ratios between 45 and 60 sr were in the same range as in the previous year. The southwest monsoon season was characterized by the advection of particles in maximum heights of 5000 m from Africa and Arabia. Backscatter coefficients were considerably less compared to the findings during the winter monsoon. Low mean optical depths of 0.15, with a contribution of 48% from layers above 1000 m, and lidar ratios from 35-55 sr indicated a mixture of clean-marine and clean-continental aerosol conditions. During the intermonsoon season, clean-marine conditions prevailed with particles to maximum heights of 2.5 km, mean optical depths of 0.13, and lidar ratios from 20 to 30 sr.
A major pollution outbreak from the Indian subcontinent was observed
with a six-wavelength aerosol lidar at Hulule island (4.1°N,
73.3° E) on 25 March 1999 during the Indian Ocean Experiment. From
the observed backscatter and extinction data, vertical profiles of
physical particle properties were determined. Effective radii between
0.14 and 0.22 µm, complex refractive indices of about 1.65 in real
part and between 0.03i and 0.08i in imaginary part, and
single-scattering albedos between 0.79 and 0.86 at 532 nm suggest
anthropogenic activities and biomass burning as source of the particles.
First estimates from radiative-transfer calculations show a cooling of
-6.4 W/m² at the top of the atmosphere and -70
W/m² at ground level.
The lidar ratios at 500 and 1020 nm were derived from POM 01L sun-sky scanning photometer measurements taken at Belsk Geophysical Observatory (long. 20A degrees 47', lat. 51A degrees 50') in the period from 2002 to 2006. The most frequently occurring lidar ratio values for the study period are 50 sr and 30 sr at 500 nm and 1020 nm, respectively. Calculations of lidar ratios for summer and winter seasons have been made as well. Back trajectory analysis was also performed to final aerosol source of origin.
An elevated layer was observed during airborne lidar and in-situ aerosol measurements, leeward of the European Alps and Apennine mountains. The layer was encountered during the Mesoscale Alpine Program (MAP) in autumn 1999, and extended >200 km at an altitude ~4100 m asl over the northern Adriatic sea. Detailed meteorological analysis suggested that mountain venting followed by advection was responsible for formation of the layer. Evidence for particle nucleation was found in six profiles, and was associated with regions of cloud outflow. Despite determination of the average aerosol mass concentration within the layer (~1.8 (SO4) mug m-3), an estimate of export to the free troposphere was complicated by the complex structure of the planetary boundary layer.
Absorption of sunlight by sub-micron particles is an important factor in calculations of the radiation balance of the earth and thus in climate modelling. Carbon-containing particles are generally considered as the most important in this respect. Major sources of these particles are generally considered to be bio-mass burning and vehicle exhaust. In order to characterise size fractionated particulate matter in a rural village in Botswana with respect to light absorption and elemental content experiments were performed, in which simultaneous sampling was made with a dichotomous impactor and a laboratory-made sampler, made compatible with black carbon analysis by reflectometry. The dichotomous impactor was equipped with Teflon filters and the other sampler with glass fibre filters. Energy dispersive X-ray fluorescence was used for elemental analysis of both kinds of filters. It appeared that Teflon filters were the most suitable for the combination of mass-, elemental- and black carbon measurements. The black carbon content in coarse (2.5–10 μm) and fine (<2.5 μm) particles was determined separately and related to elemental content and emission source. The results show that the fine particle fraction in the aerosol has a much higher contribution of black particles than the coarse particle fraction. This observation is valid for the village in Botswana as well as for a typical industrialised city in Sweden, used as a reference location.
Biomass burning contributes significantly to the global budgets of a number of atmospheric trace gases and particles. The gaseous emissions are involved in photochemical formation of tropospheric ozone. The particulate emissions contribute to the direct and indirect effect of the aerosol on the radiation budget of the earthatmosphere system. In this thesis, atmospheric processes in young biomass burning plumes in the first tens of minutes are investigated. For this purpose, the three-dimensional (3D) atmospheric plume model ATHAM is used to simulate the evolution of a biomass burning plume. The simulation represents the situation during the Quinault prescribed fire conducted during the Smoke, Cloud, and Radiation-C (SCAR-C) experiment. The model reproduces well the general appearance of the plume and the observed aerosol mass concentrations. Remaining differences between the model results and the measurements are attributed to limited meteorological and fire emission information. Remote sensing measurements indicate a lower limit for the single-scattering albedo, ω, of the emitted biomass burning aerosol at 550 nm of 0.94. The calculation of ω based on in situ measurements results in a significant lower value of 0.85. Possible reasons for this discrepancy are discussed. Three-dimensional solar radiative transfer simulations show that horizontal photon transfer significantly influences the actinic flux in the center of the biomass burning plume. The magnitude of this 3D radiation effect depends on the absorbing properties of the aerosol and can influence photochemistry in biomass burning plumes and other phenomena of similar dimensions, e.g., convective clouds. For the interpretation of measurements of the upward irradiance above finite plumes, the use of one-dimensional (1D) radiative transfer models is inappropriate because of the decreasing solid angle of the plume with increasing altitude. This effect cannot be taken into account in 1D radiative transfer simulations. Atmospheric photochemistry in young biomass burning plumes leads to the formation of ozone and nitrogen reservoir species. The simulated ozone mixing ratio of about 70 ppb agrees well with the observations from the Quinault fire. Significant production of nitrogen reservoir species is simulated with HNO3 and peroxyacetyl nitrate (PAN) accounting for about ∼60% and ∼30%, respectively. The availability of radicals is the limiting factor for photochemistry in the plume. Production of radicals is dominated by photolysis of formaldehyde (∼80% of the total radical production). The concentrations of the alkenes are significantly reduced by oxidation in the plume. Neglecting the emission of formaldehyde from the fire leads to unrealistic low ozone concentrations. Decreasing the emissions of nitrogen oxides as well as neglecting aerosol absorption lead to an increase in the ozone concentrations within the range of observations. Overall, it appears that young biomass burning plumes are a highly interesting research field for several disciplines in the atmospheric science. Plumes from vegetation fires include a number of atmospheric processes and offer the potential to combine field observations and modeling studies.
Measurements of aerosol sulfuric acid in the free troposphere were performed in the vicinity of Tenerife, Canary Islands (28degreesN, 16degreesW), in July 1997. These measurements were carried out on board a Dutch Cessna Citation 11 research aircraft within the framework of the second Aerosol Characterization Experiment (ACE 2). We used the Volatile Aerosol Component Analyzer for the detection of sulfuric acid (H2SO4). Vertical profiles between 2 and 13 km altitude were obtained. Typically, H2SO4 mixing ratios ranged between 10 and 120 pptv. Between 4 and 6 km altitude a distinct H2SO4 aerosol layer was observed repeatedly with H2SO4 mixing ratios of up to 550 pptv. The measurements are in agreement with total aerosol mass concentrations inferred from simultaneous measurements of aerosol size distributions using two condensation particle counters, a differential mobility analyzer, and an optical aerosol counter. At altitudes above 4 km the predominant aerosol component was sulfuric acid, frequently correlated with ozone, suggesting photochemical air pollution as a common source. Sulfur dioxide measured by chemical ionization mass spectrometry technique revealed typical mixing ratios between 10 and 60 pptv at altitudes above 6 kin and up to 200 pptv in the lower troposphere.
The recent literature on satellite remote sensing of air quality is reviewed. 2009 is the 50th anniversary of the first satellite atmospheric observations. For the first 40 of those years, atmospheric composition measurements, meteorology, and atmospheric structure and dynamics dominated the missions launched. Since 1995, 42 instruments relevant to air quality measurements have been put into orbit. Trace gases such as ozone, nitric oxide, nitrogen dioxide, water, oxygen/tetraoxygen, bromine oxide, sulfur dioxide, formaldehyde, glyoxal, chlorine dioxide, chlorine monoxide, and nitrate radical have been measured in the stratosphere and troposphere in column measurements. Aerosol optical depth (AOD) is a focus of this review and a significant body of literature exists that shows that ground-level fine particulate matter (PM2.5) can be estimated from columnar AOD. Precision of the measurement of AOD is +/-20% and the prediction of PM2.5 from AOD is order +/-30% in the most careful studies. The air quality needs that can use such predictions are examined. Satellite measurements are important to event detection, transport and model prediction, and emission estimation. It is suggested that ground-based measurements, models, and satellite measurements should be viewed as a system, each component of which is necessary to better understand air quality.
The latest in a series of solar occultation satellite instruments, Stratospheric Aerosol and Gas Experiment (SAGE) III, was placed into orbit in December 2001, and data were obtained until March 2006. Measurements were made of the extinction attributable to aerosols and cloud at a number of wavelengths between 290 and 1550 nm. The analysis of data obtained by its predecessor, SAGE II, has shown that an intercomparison of such data at two or more wavelengths may be used to separate the effects of cloud and aerosol. This analysis has been done on a routine basis for many years using SAGE II data at 525 and 1020 nm and applied extensively to global studies of tropospheric cloud and aerosol. Here we describe the aerosol-cloud separation algorithm developed for use with the SAGE III data, which uses the extinction at 525, 1020, and 1550 nm. This algorithm is now being used to produce vertical profiles of cloud presence as a standard SAGE III data product. These profiles have a vertical resolution of 0.5 km and cover the altitude range from 6.0 to 30.0 km, and data are presently available from March 2002 onward. An outline is given of the development of this algorithm, the nature of the SAGE III data, and the algorithm performance. To maintain continuity with SAGE II cloud data, the relative performances of the SAGE II and SAGE III algorithms are also examined. An example of the application of the algorithm to SAGE III tropospheric data is shown and discussed.
The seasonal tropospheric ozone maximum in the tropical South Atlantic, first recognized from satellite observations [Fishman et al., 1986, 1991], October 1992. Along with a new TOMS-based method for deriving tropospheric column ozone, we used the TRACE A/SAFARI 1992 data set to put together a regional picture of the 0 3 distribution during this period. Sondes and aircraft profiling showed a troposphere with layers of high O3 (->90 ppbv) all the way to the tropopause. These features extend in a band from 0 ø to 25øS, over the SE Indian Ocean, Africa, the Atlantic, and eastern South America. A combination of trajectory and photochemical modeling (the Goddard (GSFC) isentropic trajectory and tropospheric point model, respectively) shows a strong connection between regions of high ozone and concentrated biomass burning, the latter identified using satellite-derived fire counts [Justice et al., this issue]. Back trajectories from a high-O3 tropical Atlantic region (column ozone at Ascension averaged 50 Dobson units (DU)) and forward trajectories from fire-rich and convectively active areas show that the Atlantic and southern Africa are supplied with O3 and O3-forming trace gases by midlevel easterlies and/or recirculating air from Africa, with lesser contributions from South American burning and urban pollution. Limited sampling in the mixed layer over Namibia shows possible biogenic sources of NO. High-level westerlies from Brazil (following deep convective transport of ozone precursors to the upper troposphere) dominate the upper tropospheric 03 budget over Natal, Ascension, and Okaukuejo (Namibia), although most enhanced O3 (75% or more) equatorward of 10øS was from Africa. Deep convection may be responsible for the timing of the seasonal tropospheric 0 3 maximum: Natal and Ascension show a 1-to 2-month lag relative to the period of maximum burning [cf. Baldy et al., this issue; Olson et al., this issue]. Photochemical model calculations constrained with TRACE A and SAFARI airborne observations of O3 and 03 precursors (NOx, CO, hydrocarbons) show robust ozone formation (up to 15 ppbv O3/d or several DU/d) in a widespread, persistent, and well-mixed layer to 4 km. Slower but still positive net 03 formation took place throughout the tropical upper troposphere [cf. Pickering et al., this issue (a); Jacob et al., this issue]. Thus whether it is faster rates of 0 3 formation in source regions with higher turnover rates or slower 03 production in long-lived stable layers ubiquitous in the TRACE A region, 10-30 DU tropospheric 03 above a -25-DU background can be accounted for. In summary, the 03 maximum studied in October 1992 was caused by a coincidence of abundant 03 precursors from biomass fires, a long residence time of stable air parcels over the eastern Atlantic and southern Africa, and deep convective transport of biomass burning products, with additional NO from lightning and occasionally biogenic sources.
In situ and laser remote measurements of gases and aerosols were made with airborne instrumentation to investigate the sources and sinks of tropospheric gases and aerosols over the tropical South Atlantic during the NASA Global Tropospheric Experiment (GTE)/Transport and Atmospheric Chemistry Near the Equator-Atlantic (TRACE A) field experiment conducted in September-October 1992. Gases from extensive fires in Brazil were transported by convective storms into the upper troposphere where tropospheric ozone (O3) was photochemically produced and advected eastward over the South Atlantic. In central Africa, the fires were widespread, and in the absence of deep convection, the fire plumes were advected at low altitudes (below ~6 km) over the Atlantic. There was a positive correlation between O3 and aerosols found in the plumes that were not involved in convection. High O3 [>75 parts per billion by volume (ppbv)] was observed in the low-altitude plumes, and also in the upper troposphere where O3 often exceeded 100 ppbv with low aerosol loading. The average tropospheric O3 distributions were determined for the following: Brazil and western South Atlantic, eastern and central South Atlantic, central and east coast of Africa, and the entire South Atlantic Basin. The tropopause heights and O3 columns across the troposphere were calculated for individual flights and for the average O3 distributions in the above regions. A maximum tropospheric O3 column of 56 Dobson units (DU) was found over the biomass burning region in Zambia and in the subsidence region over the central South Atlantic. The high O3 region over the South Atlantic from 4° to 18°S corresponded with the latitudinal extent of the fires in Africa. In situ and laser remote measurements were used to determine the frequency of observation and chemical composition of nine major air mass types. Biomass burning emissions contributed to most of the air masses observed over the South Atlantic Basin, and biomass burning was found to contribute up to half (28 DU) of the O3 column across this region.
The NASA Global Tropospheric Experiment (GTE) Transport and Atmospheric
Chemistry Near the Equator-Atlantic (TRACE A) expedition was conducted
September 21 through October 26, 1992, to investigate factors
responsible for creating the seasonal South Atlantic tropospheric ozone
maximum. During these flights, fine aerosol (0.1-3.0 μm) number
densities were observed to be enhanced roughly tenfold over remote
regions of the tropical South Atlantic and greater over adjacent
continental areas, relative to northern hemisphere observations and to
measurements recorded in the same area during the wet season. Chemical
and meteorological analyses as well as visual observations indicate that
the primary source of these enhancements was biomass burning occurring
within grassland regions of north central Brazil and southeastern
Africa. These fires exhibited fine aerosol (N) emission ratios relative
to CO (dN/dCO) of 22.5 ± 9.7 and 23.6 ± 15.1
cm-3 parts per billion by volume (ppbv)-1 over
Brazil and Africa, respectively. Convection coupled with
counterclockwise flow around the South Atlantic subtropical anticyclone
subsequently distributed these aerosols throughout the remote South
Atlantic troposphere. We calculate that dilute smoke from biomass
burning produced an average tenfold enhancement in optical depth over
the continental regions as well as a 50% increase in this parameter over
the middle South Atlantic Ocean; these changes correspond to an
estimated net cooling of up to 25 W m-2 and 2.4 W
m-2 during clear-sky conditions over savannas and ocean
respectively. Over the ocean our analyses suggest that modification of
CCN concentrations within the persistent eastern Atlantic marine
stratocumulus clouds by entrainment of subsiding haze layers could
significantly increase cloud albedo resulting in an additional surface
radiative cooling potentially greater in magnitude than that caused by
direct extinction of solar radiation by the aerosol particles
themselves.
Global distributions of UV-absorbing aerosols are obtained using measured differences between the 340 and the 380 nm radiances from the Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) for the years 1979-1993. Time series are shown for major sources of biomass burning and desert dust giving the frequency of occurrence and areal coverage over land and oceans. Minor sources of UV-absorbing aerosols in the atmosphere are also discussed (volcanic ash and oil fires). Relative values of year-to-year variability of UV-absorbing aerosol amounts are shown for major aerosol source regions: (1) central South America (Brazil) near 10°S latitude; (2) Africa near 0°-20°S and 0° to 10°N latitude; (3) Saharan Desert and sub-Saharan region (Sahel), Arabian Peninsula, and the northern border region of India; (4) agricultural burning in Indonesia, Eastern China, and Indochina, and near the mouth of the Amazon River; and (5) coal burning and dust in northeastern China. The first three of these regions dominate the injection of UV-absorbing aerosols into the atmosphere each year and cover areas far outside of their source regions from advection of UV-absorbing particulates by atmospheric wind systems. During the peak months, smoke and dust from these sources are transported at altitudes above 1 km with an optical depth of at least 0.1 and can cover about 10% of the Earth's surface. Boundary layer absorbing aerosols are not readily seen by TOMS because the small amount of underlying Rayleigh scattering leads to a small signal. Significant portions of the observed dust originate from agricultural regions frequently within arid areas, such as in the Sahel region of Africa, especially from the dry lake-bed near Lake Chad (13.5°N, 14°E), and intermittently dry drainage areas and streams. In addition to drought cycle effects, this suggests there may be an anthropogenic component to the amount of dust injected into the atmosphere each year. Detection of absorbing aerosols and calculation of optical depths are affected by the presence of large-scale and subpixel clouds in the TOMS field of view.
The National Oceanic and Atmospheric Administration (NOAA) advanced very high resolution radiometer (AVHRR) is an instrument on a polar orbiting satellite that provides information on global aerosol distributions. The remote sensing algorithm is based on measurements of backscattered solar radiation which yield a measure of the "radiatively equivalent" aerosol optical thickness Z^sat (EAOT) over the oceans. Seasonally composited EAOT data for the period July 1989 to June 1991 reveal many spatially coherent plume-like patterns that can usually be interpreted in terms of known (or reasonably hypothesized) sources in association with climatological wind fields. The largest and most persistent areas of high EAOT values are associated with wind-blown dust and biomass burning sources; especially prominent are sources in Africa, the middle East, and the Asian subcontinent. Prominent plumes over the midlatitude North Atlantic are attributed to pollution emissions from North America and Europe. Large plumes attributed to pollution aerosols and dust from sources in Asia are clearly visible over the western and central North Pacific. On a global scale the annually averaged northern hemisphere EAOT values are about 1.7 times greater than those in the southern hemisphere. Considering each hemisphere separately, EAOT values in summer are about twice those in winter. Within the midlatitude band 300-60 ø (i.e., where anthropogenic emissions are greatest) the summer/winter ratio is about 3. The temporal variability of monthly mean EAOT in specific ocean regions often shows characteristic seasonal patterns that are usually consistent with aerosol measurements made in the marine boundary layer. Nonetheless, there are features in the EAOT distributions that can not be readily interpreted at this time. The AVHRR EAOT distributions demonstrate that satellite products can serve as a useful tool for the planning and implementation of focused aerosol research programs and that they will be especially important in studies of climate- related processes.
Tropospheric air trajectories that occurred during the Southern African Fire-Atmosphere Research Initiative (SAFARI) in August-October 1992 are described in terms of a circulation classification scheme and the vertical stability of the atmosphere. Three major and frequently occurring stable discontinuities are found to control vertical transport of aerosols in the subtropical atmosphere at the end of the dry season. Of these, the main subsidence-induced feature is a spatially ubiquitous and temporally persistent absolutely stable layer at an altitude of about 5 km (3.5 km above the interior plateau elevation). This effective obstacle to vertical mixing is observed to persist without break for up to 40 days. Below this feature an absolutely stable layer at 3 km (1.5 km above the surface) prevails on and off at the top of the surface mixing layer for up to 7 days at a time, being broken by the passage of regularly occurring westerly wave disturbances. Above the middle-level discontinuity a further absolutely stable layer is frequently discerned at an altitude of about 8 km. It is shown that five basic modes can be used to describe horizontal aerosol transportation fields over southern Africa. Dominating these is the anticyclone mode which results in frequent recirculation at spatial scales varying from hundreds to thousands of kilometers. In exiting the anticyclonic circulation, transport on the northern periphery of the system is to the west over the Atlantic Ocean via a semistationary easterly wave over the western part of the subcontinent. On the southern periphery, wave perturbations in the westerly enhance transports which exit the subcontinent to the east into the Indian Ocean. Independently derived data suggest that during SAFARI only 4% of the total transport of air from three locations south of 18°S was into the Atlantic Ocean. Over 90% of the transport was into the Indian Ocean across 35°E. This result reflects circulation fields typical of the extremely dry conditions prevailing in 1992. The integrated effect of the control exerted by atmospheric stability on vertical mixing, on the one hand, and the nature of the horizontal circulation fields, on the other, is to produce a distinctive suite of transport patterns that go a long way to explain the observed high concentrations of tropospheric aerosols and trace gases observed over the subcontinent in winter and spring, as well as over the tropical South Atlantic and southwestern Indian Oceans.
Temporal variations of the aerosol optical depth of the Antarctic
troposphere and stratosphere are considered on the basis of long-term
Sun photometer and actinometer measurements which have been made at
Mirny and Georg Forster stations since 1956 and 1988, respectively. This
data is supplemented by measurements of the stratospheric aerosol
optical depth by the satellite-borne stratospheric aerosol measurement
II instrument. These observations indicate that under undisturbed
conditions, the stratospheric aerosol optical depth represents
approximately 25% of the total atmospheric aerosol optical depth. The
aerosol optical depth in the Antarctic is most notably affected by
volcanic eruptions, such as El Chichon in 1982 and Mount Pinatubo and
Cerro Hudson in 1991, and by the occurrence of polar stratospheric
clouds during Antarctic winter and spring. Apart from these episodic
events, no long-term trend in the aerosol optical depth can be discerned
from the nearly 40-year record.
This paper explores some of the mechanisms governing the accumulation of passive tracers over the tropical southern Atlantic Ocean during the northern hemisphere fall season. There has been a pioneering observation regarding ozone maxima over the South Atlantic during austral spring. The understanding of the formation of this maxima has been the prime motivation for this study. Using a global model as a frame of reference, we have carried out three kinds of experiments during the period of the Transport and Atmospheric Chemistry Near the Equator-Atlantic (TRACE A) project of 1992. The first of these is a simple advection of total ozone (a passive tracer) in time using the Florida State University global spectral model. Integration over the period of roughly 1 week showed that the model quite closely replicates the behavior of the observed total ozone from the total ozone mapping spectrometer (TOMS). This includes many of the changes in the features of total ozone over the tropical and subtropical region of the southern Atlantic Ocean. These studies suggest a correlation of 0.8 between the observed ozone over this region and ozone modeled from ``dynamics alone,'' i.e., without recourse to any photochemistry. The second series of experiments invoke sustained sources of a tracer over the biomass burn region of Africa and Brazil. Furthermore, sustained sources were also introduced in the active frontal ``descending air'' region of the southern hemisphere and over the Asian monsoon's east-west circulation. These experiments strongly suggest that air motions help to accumulate tracer elements over the tropical southern Atlantic Ocean. A third series of experiments address what may be required to improve the deficiencies of the vertical stratification of ozone predicted by the model over the flight region of the tropical southern Atlantic during TRACE A. Here we use the global model to optimally derive plausible accumulation of burn elements over the fire count regions of Brazil and Africa to provide passive tracer advections to closely match what was observed from reconnaissance aircraft-based measurements of ozone over the tropical southern Atlantic Ocean.
A series of large mesoscale convective systems that occurred during the Brazilian phase of GTE/TRACE A (Transport and Atmospheric Chemistry near the Equator-Atlantic) provided an opportunity to observe deep convective transport of trace gases from biomass burning. This paper reports a detailed analysis of flight 6, on September 27, 1992, which sampled cloud- and biomass-burning-perturbed regions north of Brasilia. High-frequency sampling of cloud outflow at 9-12 km from the NASA DC-8 showed enhancement of CO mixing ratios typically a factor of 3 above background [200-300 parts per billion by volume (ppbv) versus 90 ppbv] and significant increases in NOx and hydrocarbons. Clear signals of lightning-generated NO were detected; we estimate that at least 40% of NOx at the 9.5-km level and 32% at 11.3 km originated from lightning. Four types of model studies have been performed to analyze the dynamical and photochemical characteristics of the series of convective events. (1) Regional simulations for the period have been performed with the NCAR/Penn State mesoscale model (MM5), including tracer transport of carbon monoxide, initialized with observations. Middle-upper tropospheric enhancements of a factor of 3 above background are reproduced. (2) A cloud-resolving model [the Goddard cumulus ensemble (GCE) model] has been run for one representative convective cell during the September 26-27 episode. (3) Photochemical calculations (the Goddard tropospheric chemical model), initialized with trace gas observations (e.g., CO, NOx, hydrocarbons, O3) observed in cloud outflow, show appreciable O3 formation postconvection, initially up to 7-8 ppbv O3/d. (4) Forward trajectories from cloud outflow levels (postconvective conditions) put the ozone-producing air masses in eastern Brazil and the tropical Atlantic within 2-4 days and over the Atlantic, Africa, and the Indian Ocean in 6-8 days. Indeed, 3-4 days after the convective episode (September 30, 1992), upper tropospheric levels in the Natal ozone sounding show an average increase of ~30 ppbv [3 Dobson units (DU) integrated] compared to the September 28 sounding. Our simulated net O3 production rates in cloud outflow are a factor of 3 or more greater than those in air undisturbed by the storms. Integrated over the 8- to 16-km cloud outflow layer, the postconvection net O3 production (~5-6 DU over 8 days) accounts for ~25% of the excess O3 (15-25 DU) over the South Atlantic. Comparison of TRACE A Brazilian ozonesondes and the frequency of deep convection with climatology [Kirchhoff et al., this issue] suggests that the late September 1992 conditions represented an unusually active period for both convection and upper tropospheric ozone formation.
Emissions of sulfur gases from both natural and anthropogenic sources strongly influence the chemistry of the atmosphere. To assess the relative importance of these sources we have combined the measurements of sulfur gases and fluxes during the past decade to create a global emission inventory. The inventory, which is divided into 12 latitude belts, takes into account the seasonal dependence of sulfur emissions from biogenic sources. The total emissions of sulfur gases from natural sources are approximately 0.79 Tmol S/a. These emissions are 16% of the total sulfur emissions in the Northern Hemisphere and 58% in the Southern Hemisphere. The inventory clearly shows the impact of anthropogenic sulfur emissions in the region between 35-degrees and 50-degrees-N.
We present a methodology for objective and automated determination of the uncertainty in aerosol measurements made by lidar. The methodology is based on standard error-propagation procedures, a large data base on atmospheric behavior, and considerable experience in processing lidar data. It yields algebraic expressions for probable error as a function of the atmospheric, background lighting, and lidar parameters. This error includes contributions from (1) lidar signal; (2) molecular density; (3) atmospheric transmission; and (4) lidar calibration. The validity of the algebraic error expressions is tested by performing simulated measurements and analyses, in which random errors of appropriate size are injected at appropriate steps. As an example, the methodology is applied to a new airborne lidar system used for measurements of the stratospheric aerosol. It is shown that for stratospheric measurements below about 25 km, molecular density uncertainties are the dominant source of error for wavelengths shorter than about 1.1 microm during nonvolcanic conditions. Because the influence of molecular scattering (relative to particulate scattering) decreases with increasing wavelength, stratospheric measurements with a Nd:YAG lidar can thus be more accurate than those made with a ruby lidar, provided that a suitable detector is used.
Savannas consist of a continuous layer of grass interspersed with scattered trees or shrubs, and cover approx. 10 million square kilometers of tropical Africa. African savanna fires, almost all resulting from human activities, may produce as much as a third of the total global emissions from biomass burning. Little is known, however, about the frequency and location of these fires, and the area burned each year. Emissions from African savanna burning are known to be transported over the mid-Atlantic, south Pacific and Indian oceans; but to study fully the transport of regional savanna burning and the seasonality of the atmospheric circulation must be considered simultaneously. Here we describe the temporal and spatial distribution of savanna fires over the entire African continent, as determined from night-time satellite imagery. We find that, contrary to expectations, most fires are left to burn uncontrolled, so that there is no strong diurnal cycle in the fire frequency. The knowledge gained from this study regarding the distribution and variability of fires will aid monitoring of the climatically important trace gases emitted from burning biomass.
A global three-dimensional model of the atmospheric mineral dust cycle is developed for the study of its impact on the radiative balance of the atmosphere. The model includes four size classes of minearl dust, whose source distributions are based on the distributions of vegetation, soil texture and soil moisture. Uplift and deposition are parameterized using analyzed winds and rainfall statistics that resolve high-frequency events. Dust transport in the atmosphere is simulated with the tracer transport model of the Goddard Institute for Space Studies. The simulated seasonal variations of dust concentrations show general reasonable agreement with the observed distributions, as do the size distributions at several observing sites. The discrepancies between the simulated and the observed dust concentrations point to regions of significant land surface modification. Monthly distribution of aerosol optical depths are calculated from the distribution of dust particle sizes. The maximum optical depth due to dust is 0.4-0.5 in the seasonal mean. The main uncertainties, about a factor of 3-5, in calculating optical thicknesses arise from the crude resolution of soil particle sizes, from insufficient constraint by the total dust loading in the atmosphere, and from our ignorance about adhesion, agglomeration, uplift, and size distributions of fine dust particles (less than 1 micrometer).
The annual cycle of the net mass transport across the extratropical tropopause is examined. Contributions from both the global-scale meridional circulation and the mass variation of the lowermost stratosphere are included. For the northern hemisphere the mass of the lowermost stratosphere has a distinct annual cycle, whereas for the southern hemisphere, the corresponding variation is weak. The net mass transport across the tropopause in the northern hemisphere has a maximum in late spring and a distinct minimum in autumn. This variation and its magnitude compare well with older estimates based on representative Sr-90 mixing ratios. For the southern hemisphere the seasonal cycle of the net mass transport is weaker and follows roughly the annual variation of the net mass flux across a nearby isentropic surface.
Measurements of ozone (O3) and aerosol distributions were made with an airborne lidar system in the lowland and boreal forest regions of eastern Canada during July - August 1990 as part of the NASA Global Tropospheric Experiment/Arctic Boundary Layer Expedition (ABLE) 3B. Aerosol and O3 profiles were measured simultaneously above and below the Electra aircraft from near the surface to above the tropopause on long-range flights over these important ecosystems. A broad range of atmospheric conditions were encountered during repeated flights over intensive study sites in the Hudson Bay lowlands near Moosonee, Ontario, and over the boreal forest near Schefferville, Quebec. The tropospheric composition in this high-latitude region was found to be strongly influenced by stratospheric intrusions. Regions of low aerosol scattering and enhanced O3 mixing ratios were correlated with descending air from the lower stratosphere. Over 33% of the troposphere (0-12 km) along our flight track at latitudes from about 45 deg to 55 deg N had significantly enhanced O3 due to stratospheric intrusions, and in the middle to upper troposphere the extent of the enhanced O3 gnerally exceeded 40%. Ozone mixing ratios of 80 parts per billion by volume (ppbv) near 6 km were common in strong intrusions. In the boundary layer over the lowlands, O3 was in the 20-30 ppbv range with a vertical O3 gradient of 6.7 ppbv/km to about 45 ppbv at 3 km. Above 6 km the background tropospheric O3 profile was nearly constant with an average value of 53 ppbv. Due to forest fires in Canada and Alaska, plumes from biomass-burning sources were observed on many flights. Biomass-burning plumes influenced about 25% of the free troposphere below 4 km, and in some of the plumes, O3 was enhanced by 10-20 ppbv over ambient levels of 30-45 ppbv. Several air masses transported from the tropical Pacific were observed over Canada in the middle to upper troposphere with O3 levels 10-20 ppbv below background values of 50-55 ppbv.
The Lidar-In-space Technology Experiment (LITE) has been used to study stratospheric aerosols over the Atlantic Ocean and elevated tropospheric aerosol layers in the Southern Hemisphere. Stratospheric aerosols showed distinct geographical variations and were still (September 1994) dominated by the lingering presence of aerosol from the eruption of Mount Pinatubo in June 1991. The upper tropospheric aerosol layers are thought to originate primarily as smoke from biomass burning.
The Measurement of Air Pollution from Satellite (MAPS) experiment measured the distribution of middle tropospheric carbon monoxide (CO) from the space shuttle during October 1984. The data represent average mixing ratios in the middle troposphere between 57°N and 57°S. The data are presented in maps that show the CO mixing ratios averaged over 5° latitude by 5° longitude areas for 6 days of the mission. Comparisons with concurrent, direct measurements taken aboard aircraft show that the inferred concentrations are systematically low by 20-40% depending upon which direct measurement calibration standard is used. The data show that there are very large CO sources resulting from biomass burning over South America and southern Africa. Measured mixing ratios were high over NE Asia and were highly variable over Europe. -from Authors
Total particle concentrations have been measured from ground level to about 6000 m over more than 100,000 km of flight path in the Australian region during seven separate excursions spread over three years. Median concentrations in maritime air masses below the lowest temperature inversion averaged about 220 cm−3 except in the vicinity of extensive coral reefs where the median concentration was 1590 cm−3 Average median concentrations were 160 cm−3 above temperature inversions in maritime air masses and 220 cm −3 above temperature inversions in continental air masses. Median concentrations in the well-mixed lower layer of continental air masses averaged only 680 cm−3, far lower than that found by most other observers in tropical or temperate lands.Particle production by one of the smaller metropolitan areas at 4.1019 s−1 was shown to exceed the estimates from all natural sources, indicating that particles resulting from human activities may often dominate the continental aerosol over much of Australia.
Monthly balloon borne aerosol backscatter measurements at ~173° and wavelengths of 940 and 490 nm have been made at Lauder, New Zealand (45°S), and Laramie, Wyoming (41°N), since 1992. The presentation and analysis here focus on the trophosperic results which suggest that a similar quasi steady state background aerosol appears over both sites with superimposed seasonal disturbances that are probably related to arid region dust storms and biomass burning. Volcanic influences on the free troposphere are found to be minimal. In contrast to the background aerosol, the aerosol disturbances show a strong hemispheric difference with fewer perturbations over Lauder as well as significantly differing vertical profile structure, as might be expected at a remote site. Our interpretation of the higher resolution backscatter measurements substantially supports the conclusions drawn from the lower resolution stratospheric aerosol and gas experiment (SAGE) and stratospheric aerosol measurement (SAM) satellite observations.
We use the natural radionuclides 7Be and 210Pb as aerosol tracers in a three-dimensional chemical tracer model (based on the Goddard Institute for Space Studies general circulation model (GCM) 2) in order to study aerosol transport and removal in the troposphere. Beryllium 7, produced in the upper troposphere and stratosphere by cosmic rays, and 210Pb, a decay product of soil-derived 222Rn, are tracers of upper and lower tropospheric aerosols, respectively. Their source regions make them particularly suitable for the study of vertical transport processes. Both tracers are removed from the troposphere primarily by precipitation and are useful for testing scavenging parameterizations. In particular, model convection must properly transport and scavenge both ascending 210Pb and descending 7Be. The ratio 7Be/210Pb cancels most model errors associated with precipitation and serves as an indicator of vertical transport. We show that over land the annual average 7Be/210Pb ratio for surface concentrations and deposition fluxes vary little globally. In contrast, the seasonal variability of the 7Be/210Pb concentration ratio over continents is quite large; the ratio peaks in summer when convective activity is maximum. The model overestimates 7Be in the tropics, a problem which we relate to flaws in the GCM parameterization of wet convection (excessive convective mass fluxes and no allowance for entrainment). The residence time of tropospheric 7Be calculated by the model is 23 days, in contrast with a value of about 9 days calculated for 210Pb, reflecting the high-altitude versus low-altitude source regions of these two tracers.
Measurements made of the concentration of aerosol particles in the diameter range 0.15-3 mum in the free troposphere over the southwest Pacific show evidence of a seasonal cycle. The maximum concentration occurs in spring and the minimum in autumn. The amplitude of the cycle is greatest at 30°S where, in early spring, the mean aerosol concentration is 18 cm-3, more than 10 times the mean autumn value. In the vicinity of the equator the seasonality disappears and concentrations throughout the year apparently remain close to the autumn levels at other latitudes. A number of different mechanisms could account for the observed seasonality but it is probable that the peak in early spring is due to biomass burning through the tropical southern hemisphere dry season. Continued high values through late spring and early summer are thought to be the result of the seasonal production of aerosol precursors, such as dimethyl sulfide (DMS), at middle and high latitudes.
Data from the Stratospheric Aerosol and Gas Experiment II (SAGE II) solar occultation satellite instrument have been used to study the properties of tropical cloud over the altitude range 10.5-18.5 km. By virtue of its limb viewing measurement geometry, SAGE II has good vertical resolution and sensitivity to subvisual cloud not detectable by most other satellite instruments. The geographical distribution and temporal variation of the cloud occurrence have been examined over all longitudes on timescales from less than 1 day to that of the El Nino-Southern Oscillation (ENSO) cycle. Significant variations in cloud occurrence are found on each of these scales and have been compared with the underlying surface temperature changes.
Measurements using balloonborne optical particle counters at Laramie,
Wyoming during the summer of 1991 are used to study the particle-size
distribution and volatility of the aerosol which formed in the
stratosphere following the mid-June eruptions of Mt. Pinatubo. Enhanced
aerosol layers were observed below 20 km as early as 16 July, about 1
month after the eruption. During late July, a transient though
substantial particle layer was observed in the 23 km region. High
concentrations of large particles in this high-altitude layer resulted
in aerosol-mass mixing ratios as large as 0.5 ppm, considerably larger
than observed following the eruption of El Chichon. Aerosol volatility
tests indicated that well over 90 percent of the particles were composed
of an H2SO4/H2O solution in all layers observed, indicating rapid
conversion of SO2 to H2SO4 and subsequent droplet growth. High
concentrations of droplets suggest homogeneous or ion nucleation as the
most likely aerosol-production mechanism.
A comparison of global stratospheric aerosol levels measured in 1979 by the Stratospheric Aerosol and Gas Experiment (SAGE) and in 1989-1991 by SAGE II is presented. These periods exhibit the lowest stratospheric aerosol levels in the era of modern measurements and are often referred to as background periods. We find that, depending on latitude, the 1-/,m aerosol optical depth in 1989-1991 was 10 to 30% higher than that observed in 1979. We demonstrate that the latter period (prior to the June 1991 eruption of Mount Pinatubo) was characterized by an ongoing global recovery from the eruptions of E1 Chich6n in 1982 and Nevado del Ruiz in 1985, with a further complication introduced by the February 1990 Kelut eruption. Therefore the differences between 1979 and 1989-1991 cannot be completely attributed to nonvolcanic sources.
An overview of the Transport and Atmospheric Chemistry near the EquatorsAtlantic (TRACE A) field mission is presented. TRACE A was conducted to provide a comprehensive investigation of the chemical composition, transport, and chemistry of the atmosphere over the tropical South Atlantic Ocean and the adjacent South American and African continents. Measurements for TRACE A consisted of a remote sensing component to derive tropospheric ozone and biomass burning patterns, an airborne atmospheric chemistry component to determine the composition of the air in the most pristine areas of our research domain as well as to characterize the photochemistry and transport of trace gas emissions from both fire and biogenic sources, a series of ozonesonde observations, and an enhanced radiosonde network and airborne meteorological measurements that provided information about the transport of trace gases and the physical processes that were responsible for their observed distributions. The data were inter- preted through the use of both photochemical and meteorological numerical models. The picture that emerges from TRACE A is that widespread biomass burning in both South America and southem Africa is the dominant source of the precursor gases necessary for the formation of the huge amounts of ozone over the South Atlantic Ocean. In addition, however, the meteorology in this region of the world is favorable for the accumulation of these pollutants over the tropical Atlantic basin so that photochemical processes produce large quantifies of ozone in situ. The generation of ozone occurs over scales of thousands of kilometers and is unusually enhanced in the upper troposphere where relatively high concentrations of nitrogen oxides (NOx) prevail. This latter finding suggests that convective processes (or other lifting mechanisms) may play an impor- tant role in the generation of tropospheric ozone or that there may be an additional significant upper tropospheric source of NOx, such as from lightning.
Measurements of the aerosol/molecular extinction ratio at 1-μm wavelength, obtained from the SAGE I, SAGE II, and SAM II solar occultation satellite experiments between 1978 and 1986, have been used to study the global-scale behavior of the upper troposphere aerosol. The distribution of extinction ratio values shows a pronounced mode between about 0.5 and 5 in all data subsets, regardless of latitude and season. Within a given latitude band and season the mode value is nearly constant over the altitude range from about 5 km above Earth's surface to 3 km below the tropopause. The mode shows a distinct seasonal variation, with maxima in local spring and summer, and is significantly enhanced following volcanic injection of material into the stratosphere. South of latitude 20°N, mode values in the absence of volcanic contamination are normally between 0.5 and 1.0. North of 20°N, values up to about 5 are observed, probably associated with aerosol derived from surface dust or anthropogenic sources. A secondary mode, with extinction ratios of 30 or greater and little or no variation of extinction with wavelength, is apparent just below the tropopause. This mode is believed to be associated with thin cloud along the ray path from the Sun to the satellite.
We have developed detailed emission inventories for the amount of both black and organic carbon particles from biomass burning sources (wood fuel, charcoal burning, dung, charcoal production, agricultural, savanna, and forest fires). We have also estimated an inventory for organic carbon particles from fossil fuel burning and urban activities from an existing inventory for fossil fuel sources of black carbon. We also provide an estimate for the natural source of organic matter. These emissions have been used together with our global aerosol model to study the global distribution of carbonaceous aerosols. The accuracy of the inventories and the model formulation has been tested by comparing the model simulations of carbonaceous aerosols in the atmosphere and in precipitation with observations reported in the literature. For most locations and seasons, the predicted concentrations are in reasonable agreement with the observations, although the model underpredicts black carbon concentrations in polar regions. The predicted concentrations in remote areas are extremely sensitive to both the rate of removal by wet deposition and the height of injection of the aerosols. Finally, a global map of the aerosol single scattering albedo was developed from the simulated carbonaceous particle distribution and a previously developed model for aerosol sulfates. The computed aerosol single scattering albedos compare well with observations, suggesting that most of the important aerosol species have been included in the model. For most locations and seasons, the single scattering albedo is larger than 0.85, indicating that these aerosols, in general, lead to a net cooling.
The tendency for more frequent El Niño events and fewer La Niña events since the late 1970's has been linked to decadal changes in climate throughout the Pacific basin. Aspects of the most recent warming in the tropical Pacific from 1990 to 1995, which are connected to but not synonymous with El Niño, are unprecedented in the climate record of the past 113 years. There is a distinction between El Niño (EN), the Southern Oscillation (SO) in the atmosphere, and ENSO, where the two are strongly linked, that emerges clearly on decadal time scales. In the traditional El Niño region, sea surface temperature anomalies (SSTAs) have waxed and waned, while SSTAs in the central equatorial Pacific, which are better linked to the SO, remained positive from 1990 to June 1995. We carry out several statistical tests to assess the likelihood that the recent behavior of the SO is part of a natural decadal-timescale variation. One test fits an autoregressive-moving average (ARMA) model to a measure of the SO given by the first hundred years of the pressures at Darwin, Australia, beginning in 1882. Both the recent trend for more ENSO events since 1976 and the prolonged 1990-1995 ENSO event are unexpected given the previous record, with a probability of occurrence about once in 2,000 years. This opens up the possibility that the ENSO changes may be partly caused by the observed increases in greenhouse gases.
Meteorological satellite data from 1982 to 1990 were used to identify
areas of significant association between tropical Pacific sea surface
temperature (SST) and remotely sensed normalized difference vegetation
index (NDVI) anomalies, here taken as a surrogate for rainfall
anomalies. During this period, large areas of arid and semi-arid Africa,
Australia and South America experienced NDVI anomalies directly
correlated to tropical Pacific SST anomalies. The results are limited by
the relatively short time period of analysis. However, they confirm the
disruptive effects of large-scale tropical Pacific SST variations on
arid and semiarid continental rainfall patterns in Africa, Australia,
and South America, as reported previously.
NOAA lidars at wavelengths of 0.574, 0.694, and 10.591 μm have observed ejecta from the eruption of Mt. Pinatubo both in the troposphere and stratosphere over Boulder, Colorado, since July 27, 1991. Multilayered clouds have been highly variable on time scales of minutes to days. Measurements at multiple wavelengths provide valuable information on size distribution and the wavelength dependence of backscatter and optical depth.
Measurements of aerosol extinction at wavelengths of 0.525 and 1.02 μm, made by the Stratospheric Aerosol and Gas Experiment (SAGE) II solar occultation satellite experiment, have been used to study the global-scale characteristics of the upper tropospheric aerosol. Extinction measurements in which only aerosols occurred along the optical path, have been separated from those that included high-altitude cloud by examining the wave-length variation of the extinction. Data for the time period October 1984 to May 1991 show that the two main influences on the upper tropospheric aerosol were seasonal lifting of material from below and downward transfer of volcanic aerosol from the stratosphere. -from Authors
A 14.5 year gridded data set of tropospheric absorbing aerosol index was derived from the Nimbus-7 Total Ozone Mapping Spectrometer (TOMS) reflectivity difference between 340 and 380 nm channels. Based upon radiative transfer calculations, the reflectivity anomaly between these two UV wavelength channels is very sensitive to smoke and soot aerosols from biomass burning and forest fires, volcanic ash clouds as well as desert mineral dust. The authors demonstrate the ability of the TOMS instrument to detect and track smoke and soot aerosols generated by biomass burning in South America. TOMS data can clearly distinguish between absorbing particles (smoke and dust) and non-absorbing aerosols (clouds and haze). For South American fires, comparisons of TOMS data are consistent with the limited amount of ground-based observations (Porto Nacional, Brazil) and show generally good agreement with other satellite imagery. TOMS data shows large-scale transport of smoke particulates generated by the burning fires in the South America, which subsequentially advects smoke aerosols as far as the Atlantic Ocean east of Uruguay. 15 refs., 4 fig.
Results from a wide range of techniques for measuring stratospheric
aerosol parameters are discussed. The techniques used included impactor
sampling, filter sampling, photoelectric particle counting, lidar system
sensing, satellite-extinction sensing, and optical depth probing. An
outline of the main methods of determining the vertical profiles of
stratospheric aerosols is given. Size distribution for stratospheric
aerosols and their association with volcanic activity are discussed. The
use of models for the estimation of radiative effects of stratospheric
aerosols is described.
A Conceptual model of the Saharan Air Layer (SAL) and easterly wave
disturbance is presented in light of diagnostic analyses of dust
outbreaks.Numerical simulations of the SAL were carried out to 5 days
for two case studies using the Penn State/NCAR limited-area tropical
model. The region of simulations encompasses North Africa and the
eastern tropical Atlantic Ocean. One set of simulations used a
horizontal resolution of 220 km. Analysis of the simulations emphasize
the structure of the SAL and easterly wave disturbance and evaluation is
made with reference to available observations and a conceptual model.
Because both cases are similar, emphasis of the sensitivity tests is
placed on the August 1974 case only. These tests include the effect of
enhancing the SAL in the initial conditions, the role of surface
sensible heating, the role of latent heating in the atmosphere, and the
effect of heating due to radiative warming of the aerosol. A fine-mesh
simulation of 110 km was also made to resolve the mesoscale features of
the SAL.Topics treated in the discussion include 1) the interaction of
the SAL with attendant easterly wave disturbances, 2) the frontal
structure of the SAL along the leading and southern boundary of the SAL,
3) forcing of vertical motions and the transverse/vertical circulations
in the SAL front, 4) the nature of the anticyclonic curvature of the SAL
plume along the coast of Africa and 5) the role of aerosol radiative
heating in preserving the characteristics of the SAL as it moves toward
the west. A significant conclusion is that the SAL contributes to
forcing of vertical motions and cumulus convection and is therefore
important (if not necessary) in the initial development of some easterly
wave disturbances. Without surface heating over the Sahara or a proper
initialization of the desert mixing layer, atmospheric forcing tends to
be very much weaker than for the cam where a deep SAL is present.
Measurements are presented of the amount of scattering produced by the atmosphere from an intense light-beam projected vertically from a ruby laser. These observations have been made in Kingston, Jamaica. They show that the scattering observed from heights up to eighty kilometres may be interpreted principally in terms of Rayleigh Scattering from atmospheric molecules. In addition, superimposed on this, an enhancement of the scattered signal between fifteen and thirty kilometres is observed, which is believed to be due to the concentration of aerosols in this height range.An account is given of the theory of the experiment and of the equipment used. The limitations of the method are discussed and a comparison is made of daytime and night-time observations, showing the different problems which arise in these two cases.
During NASA's Global Backscatter Experiment (GLOBE) mission flights in November 1989 and May 1990, a DC-8 research aircraft probed the Pacific Basin free troposphere for about 90 flight hours in each month between +72 and −62 degrees latitude, +130 and −120 degrees longitude, and up to 39,000 feet pressure altitudes. Aerosols were sampled continuously in situ by optical particle counters to measure concentration and particle size, and during 48 10-min intervals during each mission by wire impactors for concentration, size, composition, phase and shape analyses. The optical particle counters cover a particle diameter range between 0.3 and 20 μm; wire impactors extend the range down to 0.03 μm.Results of particle number, size, shape, together with the assumption of a refractive index corresponding to (NH4)2SO4 to account for the prevalence of aerosol sulfur, were utilized in a Mie algorithm to calculate aerosol extinction and backscatter for a range of wavelengths (0.385 < λ < 10.64 μm). Computations for 22 randomly selected size distributions yield coefficients of extinction E0.525=(2.03±1.20) × 10−4 km−1 and backscatter β0.525=(6.45±3.49) × 10−6 km−1 sr−1 in the visible, and E10.64=(8.13±6.47) × 10−6 km−1 and β10.64=(9.98±10.69) × 10−8 km−1 sr−1 in the infra-red, respectively. Large particles (D > 0.3 μm) contribute two-thirds to the total extinction in the visible (λ=0.525 μm), and almost 100% in the infra-red (λ= 10.64 μm). These results have been used to define an IR optical aerosol climatology of the Pacific Basin free troposphere, from which it follows that the infra-red backscatter coefficient at λ=9.25 μm wavelength fluctuates between 5.0 × 10−10 and 2.0 × 10−7 km−1 sr−1 with a modal value 2.0 × 10−8 km−1 sr−1.
Biomass burning is one of the major pollution sources of the Earth's
atmosphere, releasing large amounts of gaseous (CO, NOX ,
hydrocarbons, etc.) and particulate (aerosol) emissions [Crutzen and
Andreae, 1990; Andreae, 1991; Leuine, 1991]. Furthermore, fires in
forests and savannas have an important ecological influence, especially
in the tropics [Goldammer, 1990; Kozlowski and Ahlgren, 1974]. Yet, no
global survey of fire incidence as a function of time and geographic
region is available at this time. The recent release of the NASA/Space
Shuttle Earth Observations Project (SSEOP) data base provides an
opportunity to investigate the global distribution of large fires.
The distribution of middle tropospheric carbon monoxide measure by the Measurement of Air Pollution from Satellites (MAPS) instrument carried aboard the space shuttle is reported. The data represent average mixing ratios in the middle troposphere and are presented in the form of maps that show the carbon monoxide mixing ratios averaged for 6 days of the mission. Comparisons with concurrent, direct measurements taken aboard aircraft show that the inferred concentrations are systematically low by from 20 to 40 percent depending upon which direct measurement calibration standard is used. The data show that there are very large CO sources resulting from biomass burning over South America and southern Africa. Measured mixing ratios were high over northeast Asia and were highly variable over Europe.
An analysis of more than 22,000 ozone profiles from Stratospheric Aerosol and Gas Experiment I (SAGE I) (1979-1981) and SAGE II (1984-1987) between 50 deg N and 50 deg S is used in conjunction with 9 years (1979-1987) of daily global depictions of total ozone from the TOMS instrument aboard Nimbus 7 to investigate the spatial distribution and seasonal cycle of the integrated amount of ozone in the troposphere. In the tropics, highest concentrations are found in the eastern Atlantic Ocean downwind (west) of Africa and maximize during the time when biomass burning is most prevalent, between July and October. A different seasonal cycle in the tropics is also observed over Indonesia, where a relative maximum is present in the March-April time frame, likewise consistent with when biomass burning is most prevalent. At mid-latitudes, highest concentrations are found downwind of Asia and maximize in the summer. Relatively higher amounts of tropospheric ozone are similarly observed downwind of North America and Europe. At mid-latitudes, the ratio between the amount of tropospheric ozone in the Northern Hemisphere and the amount in the Southern Hemisphere is 1.4, in good agreement with in situ measurements.
The SAGE-I and SAM-II satellite sensors were designed to measure, with global coverage, the 1 micron extinction produced by the stratospheric aerosol. In the absence of high altitude clouds, similar measurements may be made for the free tropospheric aerosol. Median extinction values at middle and high latitudes in the Northern Hemisphere, for altitudes between 5 and 10 km, are found to be one-half to one order of magnitude greater than values at corresponding latitudes in the Southern Hemisphere. In addition, a seasonal increase by a factor of 1.5-2 was observed in both hemispheres, in 1979-80, in local spring and summer. Following major volcanic eruptions, a long-lived enhancement of the aerosol extinction is observed for altitudes above 5 km.
The various methods of calculating the atmospheric aerosol backscattering function, beta(CO2), both from measured aerosol characteristics and from optical measurements made at other wavelengths, are discussed in detail, with limits placed on their accuracy. The most significant factor in determining beta(CO2) is found to be the aerosol size distribution and concentration; this should be known accurately for particle radii up to at least 1 micron for stratospheric particles and 5 microns for tropospheric particles. Results are then presented from the modeling of the aerosol backscattering function at a wavelength of 10.6 microns in the lowest 20 km of the atmosphere. It is found that beta(CO2) varies from 10 to the -6th per m per sr in the planetary boundary layer to less than 10 to the -11th per m per sr in the stratosphere. It is next shown that, with the exception of (NH4)2SO4-containing aerosols, whose size distributions have relatively large numbers of small particles, the variation of backscattering with CO2 wavelength is less than a factor of approximately 3. For such (NH4)2SO4 aerosol distributions, however, the variation of backscatter function with CO2 wavelengths between 9.1 and 11.1 microns may reach one order of magnitude.
The Stratospheric Aerosol and Gas Experiment (SAGE) II satellite experiment measures the extinction due to aerosols and thin cloud, at wavelengths of 0.525 and 1.02 micrometers, down to an altitude of 6 km. The wavelength dependence of the extinction due to aerosols differs from that of the extinction due to cloud and is used as the basis of a model for separating these two components. The model is presented and its validation using airborne lidar data, obtained coincident with SAGE II observations, is described. This comparison shows that smaller SAGE II cloud extinction values correspond to the presence of subvisible cirrus cloud in the lidar record. Examples of aerosol and cloud data products obtained using this model to interpret SAGE II upper tropospheric and lower stratospheric data are also shown.
SAGE II multiwavelength aerosol extinction measurements are used to estimate mass- and extinction-to-backscatter conversion parameters. The basis of the analysis is the principal component analysis of the SAGE II extinction kernels to estimate both total aerosol mass and aerosol backscatter at a variety of wavelengths. Comparisons of coincident SAGE II extinction profiles with 0.694-micron aerosol backscatter profiles demonstrate the validity of the method.
The Lidar In-space Technology Experiment (LITE) is a
three-wavelength backscatter lidar developed by NASA Langley Research
Center to fly on the Space Shuttle. LITE flew on Discovery in September
1994 as part of the STS-64 mission. The goals of the LITE mission were
to validate key lidar technologies for spaceborne applications, to
explore the applications of space lidar, and to gain operational
experience which will benefit the development of future systems on
free-flying satellite platforms. The performance of the LITE instrument
was excellent, resulting in the collection of over 40 GBytes of data.
These data present us with our first highly detailed global view of the
vertical structure of cloud and aerosol from the Earth's surface through
the middle stratosphere. This paper will discuss the LITE instrument,
the LITE mission, and briefly present some results from the Experiment.
These preliminary results highlight the benefits to be obtained from
long duration satellite lidars
Fischer, b: Phys. and Chem. Prop
- Meteorology
Meteorology, edited by G. Fischer, b: Phys. and Chem. Prop. of the Air, pp. 391-457, Springer-Verlag, New York, 1988.
Analysis and numerical simula-tions of the Saharan air layer and its effect on easterly wave distur-bances High altitude atmo-spheric scattering of light from a laser beam
- V M Karyampudi
- T N Carlsonkent
- B R Clemesha
- R W Wright
Karyampudi, V. M., and T. N. Carlson, Analysis and numerical simula-tions of the Saharan air layer and its effect on easterly wave distur-bances, J. Atmos. $ci., 45, 3102-3136, 1988..Kent, G. S., B. R. Clemesha, and R. W. Wright, High altitude atmo-spheric scattering of light from a laser beam, J. Atmos. Terr. Phys., 20, 169-181,1967.
Atmospheric Sciences Division, NASA Langley Research Center, Mail Stop 475
- Kristi Skeens
- East
- Sun
- C R Com
- D M Trepte
- Winker
Kristi. Skeens@ East. Sun. COM) C. R. Trepte and D. M. Winker, Atmospheric Sciences Division, NASA Langley Research Center, Mail Stop 475, Hampton, VA 23681. (e-mail: c.r.trepte@larc.nasa.gov; d.m.winker@larc.nasa.gov) (Received August 20, 1997; revised January 27, 1998; accepted January 28, 1998.)