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Diurnal cycle of O 3 at three sites in New England. Dates correspond to selected examples in Figure 4.
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Data obtained from spring 2001 to summer 2003 in New England by the Atmospheric Investigation, Regional Modeling, Analysis and Prediction (AIRMAP) program were used to document the diurnal characteristics of O3, CO2, NO, and during selected intervals hydrocarbon and oxygenated species. The diurnal cycles of O3 and oxygenated species showed a monoto...
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Context 1
... To further examine the characteristics of the O 3 diurnal cycle across New England, we examined it on four nights at Chicopee, Ware, and Boston, Massachusetts ( Figure 5). All three sites showed similar trends in both O 3 depletion and replenishment which mimicked the fea- tures of the diurnal cycle at TF (Figure 2). ...
Context 2
... three sites showed similar trends in both O 3 depletion and replenishment which mimicked the fea- tures of the diurnal cycle at TF (Figure 2). Using the hourly averaged data presented in Figure 5, the rates of depletion and replenishment were calculated for these four time periods. The results are summarized and compared with results for TF in Table 2. Overall the agreement was reasonable given there are undoubtedly local terrain and other micrometeorological features that cause differences in the actual rates between each location. ...
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This paper studied the method for converting the aerosol extinction to the mass concentration of particulate matter (PM) and obtained the spatio-temporal distribution and transportation of aerosol, nitrogen dioxide (NO2), sulfur dioxide (SO2), and formaldehyde (HCHO) based on multi-axis differential optical absorption spectroscopy (MAX-DOAS) observ...
Citations
... Ground-level ozone (O 3 ) is mainly produced by precursors such as nitrogen oxides (NOx) and volatile organic compounds (VOCs) under light conditions [1][2][3][4][5]. Due to the synthetic effects of photochemical generation, dry depositions, and boundary layer entrainment, the diurnal variation in ozone concentration usually presents a unimodal distribution; that is, the concentration is higher during the daytime and lower at nighttime, which has been confirmed by a large number of field observations [6][7][8][9][10][11]. However, in recent years, many studies have demonstrated that nocturnal ozone concentrations are surging and that a nocturnal ozone peak occurs, which has been widely observed in the United States, Europe, and China [12][13][14][15][16][17][18][19]. ...
In recent years, nocturnal ozone enhancement (NOE) events have emerged as a prominent research focus in the field of the atmospheric environment. By using statistical analysis methods, we conducted a comparative investigation of nocturnal ozone concentrations and NOE events in Dongying, the central city of the Yellow River Delta, China, in 2022 and 2023, and further explored the effects of NOE events on O3 and PM2.5 on the same night and the subsequent day. The results showed that from 2022 to 2023, in Dongying, the annual average nocturnal ozone concentrations increased from 51 μg/m3 to 59 μg/m3, and the frequency of NOE events was higher in the spring, summer, and autumn, and lower in the winter. The NOE events not only exhibited promoting effects on nocturnal O3 and Ox, and on the daily maximum 8 h average concentration of O3 (MDA8-O3) on the same day (comparatively noticeable in summer and autumn), but also demonstrated a clear impact on nocturnal PM2.5 and PM2.5-bounded NO3− and SO42− (especially in winter). Additionally, the NOE events also led to higher concentrations of O3 and Ox, as well as higher MDA8-O3 levels during the subsequent day, with more observable impacts in the summer. The results could strengthen our understanding about NOE events and provide a scientific basis for the collaborative control of PM2.5 and O3 in urban areas in the Yellow River Delta in China.
... Ground-level ozone (O3) is mainly produced by precursors such as nitrogen oxides (NOx) and volatile organic compounds (VOCs) under light conditions [1][2][3][4][5]. Due to the synthetic effects of photochemical generation, dry depositions and boundary layer entrainment, the diurnal variation of ozone concentration usually presents a unimodal distribution, that is, the concentration is higher during the daytime and lower at nighttime, which has been confirmed by a large number of field observations [6][7][8][9][10][11]. However, in recent years, many studies have found that the nocturnal ozone concentrations are surging rather than falling, and the nocturnal ozone peak occurs, which has been widely observed in the United States, Europe, and China [12][13][14][15][16][17][18][19]. ...
In recent years, nocturnal ozone enhancement(NOE) events have become a hot research topic in the field of atmospheric environment. By using statistically-based data analysis, we made a comparative investigation about nocturnal ozone concentrations and NOE events in 2022 and 2023, and further explored the effects of NOE events on O3 and PM2.5 at the night of the same day and the next day. The results showed that from 2022 to 2023 in Dongying, the annual average nocturnal ozone concentrations increased from 51 μg/m3 to 59 μg/m3, and the frequency of NOE events was higher in spring, summer and autumn, and lower in winter. NOE events not only had an obvious promoting effect on nocturnal O3 and Ox, and the daily maximum 8 h average concentration of O3 (MDA8-O3) of the same day (more apparent in summer and autumn), and nocturnal PM2.5 and PM2.5-bounded NO3- and SO42- (more apparent in winter), but also had a distinct influence on O3, Ox and MDA8-O3 of the next day (more apparent in summer). The results could strengthen the understanding of the phenomena of NOE events, and provide a scientific basis for the collaborative control of PM2.5 and ozone in urban areas in the Yellow River Delta, China.
... During the operation of the Tekran instruments, the sampling inlet was set at ∼ 1.5 m above the instrument platform (shown in Fig. S2). Considering the high altitude at which the instrument was installed, as well as to mitigate the impacts of low atmospheric pressures on the pump's operation, a low air sampling rate of 7 L min −1 for the pump model and 0.75 L min −1 (at standard pressure and temperature) for model 2537B were applied, based on the previous studies (Swartzendruber et al., 2009;Lin et al., 2019). Air was drawn in from the atmosphere into the Tekran instrument, and the Hg was divided into GOM, PBM, and GEM inside the instrument for analysis. ...
... The decrease in GEM concentration at night may be due to the interaction of pollutants from regional emissions and long-range transport (Fu et al., 2008(Fu et al., , 2010. After sunrise, partial GEM re-emission occurs in the sunlight, along with the mixing effect of the residual boundary layer downward, which may lead to an increase in GEM concentration (Mao and Talbot, 2012;Selin et al., 2007;Weiss-Penzias et al., 2009;Talbot et al., 2005). The height of the boundary layer increases after noon during the daytime, which produces dilution of GEM at the surface and may be the reason for the decrease in GEM concentration in the afternoon. ...
The Tibetan Plateau is generally considered to be a significantly clean area owing to its high altitude; however, the transport of atmospheric pollutants from the Indian subcontinent to the Tibetan Plateau has influenced the Tibetan environments. Nyingchi is located at the end of an important water vapor channel. In this study, continuous monitoring of gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particle-bound mercury (PBM) was conducted in Nyingchi from 30 March to 3 September 2019, to study the influence of the Indian summer monsoon (ISM) on the origin, transport, and behavior of Hg. The GEM and PBM during the preceding Indian summer monsoon (PISM) period (1.20±0.35 ng m−3 and 11.4±4.8 pg m−3 for GEM and PBM, respectively) were significantly higher than those during the ISM period (0.95±0.21 ng m−3, and 8.8±6.0 pg m−3); the GOM during the PISM period (13.5±7.3 pg m−3) was almost at the same level as that during the ISM period (12.7±14.3 pg m−3). The average GEM concentration in the Nyingchi region, obtained using passive sampler, was 1.12±0.28 ng m−3 (from 4 April 2019 to 31 March 2020). The GEM concentration showed that the sampling area was very clean compared to other high-altitude sites. The GEM has several patterns of diurnal variation during different periods. Stable high GEM concentrations occur at night and low concentrations occur in the afternoon during PISM, which may be related to the nocturnal boundary layer structure. High values occurring in the late afternoon during the ISM may be related to long-range transport. Low concentrations of GEM observed during the morning
in the ISM may originate from vegetation effects. The results of the
trajectory model demonstrate that the sources of pollutants at Nyingchi are
different with different circulation patterns. During westerly circulation
in the PISM period, pollutants mainly originate from central India, northeastern India, and central Tibet. During the ISM period, the pollutants mainly originate from the southern part of the SET site. The strong precipitation
and vegetation effects on Hg species during the ISM resulted in low Hg
concentrations transmitted to Nyingchi during this period. Further,
principal component analysis showed that long-distance transport, local
emissions, meteorological factors, and snowmelt factors are the main factors affecting the local Hg concentration in Nyingchi. Long-distance transport factor dominates during PISM and ISM3, while local emissions is the major contributor between PISM and ISM3. Our results reveal the Hg species distribution and possible sources of the most important water vapor channel in the Tibetan Plateau and could serve as a basis for further transboundary transport flux calculations.
... The nighttime mixing ratio of hourly O3 drops close to zero in Bode, Paknajol and Pulchowk in the winter season. This is a typical characteristic of many urban areas where reaction with NO at night depletes O3 from the boundary layer (e.g., Talbot et al., 2005). In the pre-monsoon and monsoon months, the titration is not as strong and the hourly O3 falls, but generally remains above 10 ppb. ...
South Asia is a rapidly developing, densely populated and highly polluted region that is facing the impacts of increasing air pollution and climate change, and yet it remains one of the least studied regions of the world scientifically. In recognition of this situation, this thesis focuses on studying (i) the spatial and temporal variation of key greenhouse gases (CO2 and CH4) and air pollutants (CO and O3) and (ii) the vertical distribution of air pollutants (PM, BC) in the foothills of the Himalaya. Five sites were selected in the Kathmandu Valley, the capital region of Nepal, along with two sites outside of the valley in the Makawanpur and Kaski districts, and conducted measurements during the period of 2013-2014 and 2016. These measurements are analyzed in this thesis. The CO measurements at multiple sites in the Kathmandu Valley showed a clear diurnal cycle: morning and evening levels were high, with an afternoon dip. There are slight differences in the diurnal cycles of CO2 and CH4, with the CO2 and CH4 mixing ratios increasing after the afternoon dip, until the morning peak the next day. The mixing layer height (MLH) of the nocturnal stable layer is relatively constant (~ 200 m) during the night, after which it transitions to a convective mixing layer during the day and the MLH increases up to 1200 m in the afternoon. Pollutants are thus largely trapped in the valley from the evening until sunrise the following day, and the concentration of pollutants increases due to emissions during the night. During afternoon, the pollutants are diluted due to the circulation by the valley winds after the break-up of the mixing layer. The major emission sources of GHGs and air pollutants in the valley are transport sector, residential cooking, brick kilns, trash burning, and agro-residue burning. Brick industries are influential in the winter and pre-monsoon season. The contribution of regional forest fires and agro-residue burning are seen during the pre-monsoon season. In addition, relatively higher CO values were also observed at the valley outskirts (Bhimdhunga and Naikhandi), which indicates the contribution of regional emission sources. This was also supported by the presence of higher concentrations of O3 during the pre-monsoon season. The mixing ratios of CO2 (419.3 ±6.0 ppm) and CH4 (2.192 ±0.066 ppm) in the valley were much higher than at background sites, including the Mauna Loa observatory (CO2: 396.8 ± 2.0 ppm, CH4:1.831 ± 0.110 ppm) and Waligaun (CO2: 397.7 ± 3.6 ppm, CH4: 1.879 ± 0.009 ppm), China, as well as at an urban site Shadnagar (CH4: 1.92 ± 0.07 ppm) in India. The daily 8 hour maximum O3 average in the Kathmandu Valley exceeds the WHO recommended value during more than 80% of the days during the pre-monsoon period, which represents a significant risk for human health and ecosystems in the region. Moreover, in the measurements of the vertical distribution of particulate matter, which were made using an ultralight aircraft, and are the first of their kind in the region, an elevated polluted layer at around ca. 3000 m asl. was detected over the Pokhara Valley. The layer could be associated with the large-scale regional transport of pollution. These contributions towards understanding the distributions of key air pollutants and their main sources will provide helpful information for developing management plans and policies to help reduce the risks for the millions of people living in the region.
... This phenomenon abnormalities in comparison with those on 11 May and 13 May, when there was gradual decline in O3 concentration by 00:00 and 06:00 LT. As Talbot et al. (2005) suggested that the second surface O 3 maxima at night were likely due to downward mixing of RL O 3 . Based on this hypothesis, we further observed the vertical distribution of O 3 concentrations during this time (Fig. 5(c)) and found a significant decrease in O 3 concentration at~0.5-1.0 km and an increasing tendency in the lower atmospheric layers. ...
Aim at the effects of the coastal characteristic on ozone pollution in the Yangtze River Delta (YRD), a campaign was launched at the Ningbo, China in the summer of 2020, which mainly covered the monitoring of the vertical profiles of ozone (O3) concentration, three-dimensional wind field, temperature and humidity profiles and parameters of boundary layer dynamic-thermodynamic structure. At the coastal research station, a sea-land breeze (SLB) circulation accompanied by a concurrent coastal low-level jets (CLLJ) structure was observed and identified during 11-12 May 2020. The sea breeze first formed at 10:00 LT on 11 May, turned to land breeze at night, and returned to sea breeze again at 10:00 LT the next morning, prevailing at altitudes of 0-0.5 km and 0-0.3 km respectively. Land breeze at night carries O3 from the inland to the sea forming high ozone levels over the sea, and the shift of the sea breeze during daytime further blew pollution back to the land. Additionally, the conversion of SLB contributed to the occurrence of CLLJ at the altitudes of ~0.3-0.7 km by 02:00 and 06:00 LT, of which the center of wind speed reached ~13 m s-1. The CLLJ-induced turbulent activity decoupled the residual layer (RL) and stable boundary layer, leading to a reduction of RL-O3 levels and an increase of ~50 μg m-3 in surface-O3 concentration. The YRD's unique coastal characteristics make O3 pollution causes in coastal areas more complicated.
... There was a slight increase in the concentration of acetonitrile during the daytime, and a slow decrease after sunset. This curve was similar to that of acetonitrile concentration data from a rural site in New Hampshire, USA (Jordan et al., 2009), and was likely due to dry deposition (Talbot et al., 2005). The specific and detailed source apportionment for VOCs is discussed in Section 3.4. ...
Volatile organic compounds (VOCs) that are emitted from biomass burning, vehicle exhaust, and industrial emissions play a vital role in the formation of ozone (O3) and secondary organic aerosols (SOA). Since VOCs are the precursors of O3 and aerosol pollution which have become the world’s most emergent environmental problems, a field measurement study focused on VOCs was carried out from 27 August to 10 October 2018 in a rural coastal site in Hong Kong. During the campaign, 13 VOC species were detected continuously with proton-transfer-reaction quadrupole mass spectrometry, and their effects on photochemical air pollution were studied. Methanol was the most abundant species among the measured VOCs (average concentration, 3.73 ± 3.26 ppb), and higher concentrations of oxygenated VOCs were found than reported in previous studies of atmospheric chemistry in rural areas. Diurnal variations were observed in the concentrations of various VOC species, indicating that the VOC concentrations were influenced by photochemical reactions. The amount of O3 formation was estimated based on the maximum incremental reactivity scale of the VOCs. The top five contributors to O3 formation in Hong Kong (in order) were isoprene (13.46 μg/m³), methyl ethyl ketone (12.74 μg/m³), xylene (8.52 μg/m³), acetaldehyde (8.22 μg/m³), and acrolein (4.32 μg/m³). Receptor model positive matrix factorization (PMF) was used to identify the dominant emission sources and evaluate their corresponding contributions to VOCs. Five major VOC sources were identified with the PMF method, including (1) industry and vehicle-related sources (8.1%), (2) biogenic emissions (5.5%), (3) biomass burning (63.7%), (4) secondary formation (9.2%), and (5) ship-related emissions (13.5%). The source apportionment results from PMF analysis show that the sampling site at the southeastern tip of Hong Kong was strongly influenced by urban plumes from the Guangdong–Hong Kong–Macao Greater Bay Area/Pearl River Delta region and by oceanic emissions.
... Additional sources include secondary production from the reactions of methylperoxy radicals (CH 3 O 2 ) with CH 3 O 2 and other organic peroxy radicals (Madronich & Calvert, 1990;Tyndall et al., 2001), as well as emissions from oceans (Heikes et al., 2002;Millet et al., 2008), biomass burning (e.g., Akagi et al., 2013;Hornbrook et al., 2011;Wentworth et al., 2018), and anthropogenic sources including solvent use, vehicular exhaust, and industrial processes (Legreid et al., 2007;Olivier et al., 1994;Velasco et al., 2009). Its sinks include reaction with OH (Sander et al., 2006), surface deposition (Karl et al., 2005(Karl et al., , 2004Mao et al., 2006;Talbot et al., 2005), and uptake by the ocean (Yang, Beale et al., 2014;Yang et al., 2013). ...
Methanol is the second‐most abundant organic gas in the remote atmosphere after methane, but its sources are poorly understood. Here, we report a global budget of methanol constrained by observations from the ATom aircraft campaign as implemented in the GEOS‐Chem global atmospheric chemistry model. ATom observations under background marine conditions can be fit in the model with a surface ocean methanol concentration of 61 nM and a methanol yield of 13% from the newly implemented CH3O2 + OH reaction. While terrestrial biogenic emissions dominate the global atmospheric methanol budget, secondary production from CH3O2 + OH and CH3O2 + CH3O2 accounts for 29% of the total methanol source, and makes up the majority of methanol in the background marine atmosphere sampled by ATom. Net emission from the ocean is comparatively minor, particularly because of rapid deposition from the marine boundary layer. Aged anthropogenic and pyrogenic plumes sampled in ATom featured large methanol enhancements to constrain the corresponding sources. Methanol enhancements in pyrogenic plumes did not decay with age, implying in‐plume secondary production. The atmospheric lifetime of methanol is only 5.3 days, reflecting losses of comparable magnitude from photooxidation and deposition. GEOS‐Chem model results indicate that methanol photochemistry contributes 5%, 4%, and 1.5% of the tropospheric burdens of formaldehyde, CO, and ozone, respectively, with particularly pronounced effects in the tropical upper troposphere. The CH3O2 + OH reaction has substantial impacts on radical budgets throughout the troposphere and should be included in global atmospheric chemistry models.
... Stability plots generated using NOAAS's HRRR model indicate that during each night of the sampling campaign, atmospheric inversions occurred, creating a moderately stable to extremely stable (Pasquill atmospheric stability class) boundary layer in the valley with a depth of approximately 50 to 100m (Figure 3.5A). These conditions are consistent with the documented trapping of chemical constituents, such as ozone and carbonyl sulfide, within the boundary layer and their depletion by chemical reactions or by vegetation uptake(Talbot et al., 2005;Wehr et al., 2017). Hg 0 is an additional atmospheric constituent that is similarly trapped within the boundary layer resulting in diel variations in THg and Hg isotopic composition(Fu et al., 2016b;Jiskra et al., 2019;Kellerhals et al., 2003;Poissant et al., 2008;. ...
Mercury (Hg) is a neurotoxic trace metal that is globally distributed and has important implications for human health. The anthropogenic use of Hg has caused the concentration of Hg in the environment to approximately triple since ~1850. It is therefore imperative to understand the historical deposition trends and modern biogeochemical cycling of Hg to better inform future policy actions regarding the release of Hg to the environment. In this dissertation, measurements of Hg stable isotopes were applied to an environmental record of historical Hg deposition and to remote, low Hg concentration atmospheric samples to answer outstanding questions regarding both historical and modern biogeochemical conditions and processes driving Hg cycling in the environment. To discern past sources of Hg and conditions controlling Hg isotope fractionation in the atmosphere, a sediment core from a remote, high elevation lake near Jackson, Wyoming was collected. Lake sediments were dated to understand temporal changes in the deposition of Hg to the sediments, revealing an approximate 3.8-fold increase in Hg flux from 1850 to the present. Additionally, measurements of Hg stable isotope ratios in the sediments indicated a shift in atmospheric chemical reactions over the same period. Analyses of wet precipitation and snow collected in the lake’s vicinity were utilized to explain the modern Hg isotopic composition observed in the lake sediments and motivated further investigation into Hg isotope fractionation in snow. Measurements of 12-hour atmospheric gaseous Hg samples were collected continuously for one week from two high elevation sites (Mount Bachelor, Oregon and Camp Davis, Wyoming) with contrasting geographic characteristics to understand both the vertical gradient of Hg in the atmosphere and better understand the role of vegetation in Hg isotope fractionation in the atmosphere. Analyses of Hg isotope ratios from samples at Mount Bachelor (mountaintop site) revealed diel variation in the isotopic composition of Hg. Nightly measurements indicated a dominant influence from the free troposphere with a distinct isotopic composition. Near the end of the sampling period, the diel variation dissipated due to a nearby forest fire that came to dominate both daytime and nighttime samples. At Camp Davis (valley site), diel variation in the isotopic composition of Hg was also observed, however, the variation at this site contrasted with observations at Mount Bachelor. Nightly inversions trapping Hg in the valley at Camp Davis and the subsequent uptake of Hg from the atmosphere by vegetation explains the fractionation observed in the residual Hg. Motivated by the analysis of Hg isotopes in snow from Lost Lake, WY, five time series of snow samples (with sampling every 12 hours for 48 hours) were collected at two sites in Michigan (Dexter and Pellston). A time series collected in Dexter during a polar vortex revealed progressively more negative odd-mass independent fractionation (MIF), similar to observations in Arctic snow. In contrast, the fractionation of Hg isotopes in all of the other snow samples progressed towards more positive odd-MIF, indicating a difference in oxidants and binding ligands associated with the Hg in snow. Finally, snow samples indicative of snowmelt were used to estimate the Hg isotopic composition of Hg deposited to mid-latitude ecosystems via snow. As a whole, this dissertation begins to answer outstanding questions concerning processes that control the fractionation and distribution of Hg in the environment and creates a platform for future researchers to expand upon.
... In metropolises, most of 95 percent of all CO emissions may come from vehicle exhaust, Other sources of CO releases include manufacturing processes, non-transportation fuel ignition, and natural foundations such as wildfires. Ultimate CO concentrations naturally occur during the colder months of the year when CO automobile discharges are greater and at nighttime inversion conditions (where air pollutants are confined near the ground underneath a layer of warm air) this more frequent (Talbot et al., 2005;Inness, et al. 2013). on other hand CO 2 consider as retaining gas for radiation energy, thus Without CO 2 the Earth will be unbearably cold, but its elevated levels, cause temperatures to rise, leading to what is known as global warming. ...
Boundary layer height played an important role in the vertical diffusion of the pollutants .In this research total carbon monoxide column concentration data TCCO, boundary layer height (in meter) (BLH) is consider to study relationship between it. Data taken from (European-Centre for-Medium Range-Weather-Forecasts) (ECMWF) at lat. 33.34 and long. 44.44 (degree) this Situated near the middle of Baghdad city at 8 hours (00,03,06,09,12,15,18,21) from every day for years (2003-2012). The MATLAB program was used to convert data formula from NetCDF (network Common Data Form) nc. File format to excel file. Correlation between boundary layer height (BLH) in (meter) and TCCO concentration column in (Kg/m 2) is accomplished, and consequences display correlation between this column gas and BLH. Two cases were used to find this relationship, First, the daily behaviour, and results show the BLH is low while the CO is high, and gradually decreases to lowest value about (0.67 Kg/m 2) at the hour 12 , when the BLH is at its maximum value (4975 m), then it decreases at night, where TCCO concentration increases. In Second fallouts, monthly and seasonal outlines are consequential and results show that during winter the BLH decrease, the CO concentration reaches its maximum value in February is (1.42 Kg/m 2), and then decrease during March to October until it reaches the lowest value (0.6009 Kg/m 2) during 2012 and increase during October to December, while the BLH is the maximum in May (4991m) in 2008 and gradually decreasing to December. The vital of this revision fundamentally situated in valuation airborne quality of Baghdad city, results show not direct relation between TCCO and BLH, because photochemical reaction in upper atmosphere.
... Moreover, due to less convective activities, the winter season is characterised by shallow boundary layer, which confines the O 3 close to surface thus increasing the concentration level at near surface region. In addition, during winter as the air begins to cool drastically, O 3 rich air grows denser and sinks to lower altitudes, resulting in an increase in lower tropospheric O 3 (Talbot et al., 2005;Seinfeld and Pandis, 2016). This also explains the low O 3 mixing ratio at upper tropospheric region in winter. ...
This paper deals with the temporal changes in the altitude distribution of tropospheric ozone (O3) based on the measurements by balloon-borne O3-sondes during 2011–2014, conducted at the tropical, coastal site Thiruvananthapuram on the south west coast of India. This is the first study from this region addressing the highly dynamic nature of tropospheric O3 profiles (in terms of their vertical structure and short-term changes) and attempting to categorise them based on 121 in-situ measured O3 profiles. The tropospheric O3 profiles could be categorised into four major groups namely (i) those with steady O3 mixing ratio (ii) with increasing mixing ratio, (iii) with mid-tropospheric enhancement and (iv) with multiple layers/laminar nature. The causative mechanisms of these different categories were examined. The observed differences in the tropospheric O3 distribution are attributed to meteorological conditions in particular the synoptic scale systems, long range transport, intrusion from stratosphere and photochemistry, most importantly, the effect of water vapour content. Water vapour and O3 showed complex dependence with positive and negative association depending on the precursor levels and availability of water vapour. The altitudinal changes in O3 also exhibited close association with those of potential temperature and equivalent potential temperature. An analysis of the seasonal characteristics of vertical distribution of tropospheric O3 also carried out along with the altitude-dependent seasonal behaviour. In general, the total column O3 estimated by the integration of O3-sonde retrieved profiles differed by about ±10% with those retrieved by satellite-based measurements. The TCO contributes to about 16% (34 DU) of the total column O3, with minimum of 9% in October and maximum of 27% on March. In general, the OMI retrievals under-estimates the O3-sonde derived TCO by 5–10 DU in all the seasons.
