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Atmos. Chem. Phys., 7, 1585–1598, 2007
www.atmos-chem-phys.net/7/1585/2007/
© Author(s) 2007. This work is licensed
under a Creative Commons License.
Atmospheric
Chemistry
and Physics
The T1-T2 study: evolution of aerosol properties downwind of
Mexico City
J. C. Doran1, J. C. Barnard1, W. P. Arnott2, R. Cary3, R. Coulter4, J. D. Fast1, E. I. Kassianov1, L. Kleinman5,
N. S. Laulainen1, T. Martin4, G. Paredes-Miranda2, M. S. Pekour1, W. J. Shaw1, D. F. Smith3, S. R. Springston5, and
X.-Y. Yu1
1Pacific Northwest National Laboratory, Richland, WA, USA
2Desert Research Institute, Reno, NV, USA
3Sunset Laboratory, Inc., Tigard, OR, USA
4Argonne National Laboratory, Argonne, IL, USA
5Brookhaven National Laboratory, Upton, NY, USA
Received: 6 November 2006 – Published in Atmos. Chem. Phys. Discuss.: 12 December 2006
Revised: 22 February 2007 – Accepted: 15 March 2007 – Published: 23 March 2007
Abstract. As part of a major atmospheric chemistry and
aerosol field program carried out in March 2006, a study was
conducted in the area to the north and northeast of Mexico
City to investigate the evolution of aerosols and their associ-
ated optical properties in the first few hours after their emis-
sion. The focus of the T1-T2 aerosol study was to investi-
gate changes in the specific absorption αABS (absorption per
unit mass, with unit of m2g−1) of black carbon as it aged
and became coated with compounds such as sulfate and or-
ganic carbon, evolving from an external to an internal mix-
ture. Such evolution has been reported in previous studies.
The T1 site was located just to the north of the Mexico City
metropolitan area; the T2 site was situated approximately
35km farther to the northeast. Nephelometers, particle soot
absorption photometers, photoacoustic absorption spectrom-
eters, and organic and elemental carbon analyzers were used
to measure the optical properties of the aerosols and the car-
bon concentrations at each of the sites. Radar wind profilers
and radiosonde systems helped to characterize the meteorol-
ogy and to identify periods when transport from Mexico City
over T1 and T2 occurred. Organic and elemental carbon con-
centrations at T1 showed diurnal cycles reflecting the noc-
turnal and early morning buildup from nearby sources, while
concentrations at T2 appeared to be more affected by trans-
port from Mexico City. Specific absorption during transport
periods was lower than during other times, consistent with
the likelihood of fresher emissions being found when the
winds blew from Mexico City over T1 and T2. The spe-
Correspondence to: J. C. Doran
(christopher.doran@pnl.gov)
cific absorption at T2 was larger than at T1, which is also
consistent with the expectation of more aged particles with
encapsulated black carbon being found at the more distant lo-
cation. In situ measurements of single scattering albedo with
an aircraft and a ground station showed general agreement
with column-averaged values derived from rotating shadow-
band radiometer data, although some differences were found
that may be related to boundary-layer evolution.
1 Introduction
The MILAGRO (Megacity Initiative: Local and Global
Research Observations; http://www.eol.ucar.edu/projects/
milagro/) field campaign was designed to follow the urban
plume originating in Mexico City in order to study the evo-
lution of the properties of trace gases and aerosols as they
drifted downwind from a megacity. The study was con-
ducted over multiple scales, ranging from ground-based in-
vestigations centered in the Mexico City metropolitan area
to aircraft sampling over distances of hundreds of kilome-
ters. A major component of the MILAGRO campaign was
the MAX-MEX experiment (Megacity Aerosol Experiment
in Mexico City; http://www.asp.bnl.gov/MAX-Mex.html),
which focused on measurements within, over, and a few tens
of kilometers downwind from Mexico City. MAX-MEX was
supported by the U.S. Department of Energy through its At-
mospheric Sciences Program. In this paper we describe some
aspects of one element of the MAX-MEX experiment that we
refer to as the T1-T2 study.
Published by Copernicus GmbH on behalf of the European Geosciences Union.
1586 J. C. Doran et al.: The T1-T2 study
The T1-T2 study was motivated by the desire to investi-
gate the evolution of aerosols and their associated optical
properties in the first few hours after their formation. We
hypothesized that the shift from an externally mixed state
characteristic of fresh emissions to an internally mixed one
would result in significant modifications to the optical prop-
erties of aerosols as they were advected downstream from
Mexico City. Evidence for changes in aerosol composition
due to aging in the Mexico City area has been found by other
investigators (e.g., Baumgardner et al., 2000; Johnson et al.,
2005). We were particularly interested in changes in the mass
absorption efficiency or specific absorption αABS (absorption
per unit mass, with unit of m2g−1)of black carbon (BC) be-
cause of its potential importance for climate change. The
specific absorption is one of two key parameters that deter-
mine the magnitude of the aerosol forcing by BC, the other
being the atmospheric burden (Chung and Seinfeld, 2002;
Sato et al., 2003). In this paper we will use the terms black
carbon and elemental carbon (EC) interchangeably, but we
note here that Andreae and Gelencs´
er (2006) warn that the
term “black carbon” has been used somewhat carelessly in
the literature. For the moment we will not deal with this dif-
ficulty but will return to it when we discuss our absorption
measurements in Sect. 5.
In the past, field estimates of the specific absorption of
black carbon have varied by an order of magnitude, ranging
from 2m2g−1to 25m2g−1at ∼500nm. (e.g., Waggoner
et al., 1981; Horvath, 1993; Liousse et al., 1993; Petzold et
al., 1997; Penner et al., 1998; Moosm¨
uller, 1998; Marley et
al., 2001; Baumgartner, 2002; Arnott et al., 2003; Schnaiter
et al., 2003; Schuster et al., 2005; Mikhailov et al., 2006).
While BC is normally assumed to be present as an external
mixture close to the source (e.g., Mallet et al., 2003; Jacob-
son and Seinfeld, 2004), processes such as heterocoagulation
and condensation can alter the mixing state as the particles
age (e.g., Jacobson, 2001). Such processes likely have con-
tributed to the order of magnitude differences in the estimates
of αABS noted above. Recently, Bond and Bergstrom (2006)
suggested that freshly emitted, uncoated soot particles have a
value of αABS=7.5±1.2 m2g−1at 550 nm. This “uncoated”
value will increase as the particles age and become coated,
e.g., with substances such as sulfate or organic carbon, but
the rates at which aging occurs and direct observations of
the change in αABS as this occurs downstream from a source
have been difficult to determine in the field. In contrast, the
theoretical study of Fuller et al. (1999) suggests that αABS is
unlikely to exceed 10 m2g−1unless mostof the BC is encap-
sulated in aerosol distributions whose geometric mean radius
is greater than 0.06µm. In light of these and other studies,
we are led to ask: to what extent and at what rate does aging
affect αABS for BC-containing aerosols that were originally
uncoated?
The campaign in the Mexico City area provided an op-
portunity to address this question by measuring values of
αABS at two or more locations, one relatively close to ma-
jor sources of BC and one farther downstream. Unless an
aerosol is somehow tagged to a specific source, the precise
location and time of the formation, t0, of an aerosol over
Mexico City is essentially impossible to determine after it
has drifted downwind by a few kilometers. Once outside the
primary source region, however, it is possible, at least in prin-
ciple, to track air masses containing aerosols. Our approach
was to measure aerosol properties at two sites downwind of
the Mexico City urban area and to use meteorological mea-
surements and modeling to determine periods when aerosols
originating in Mexico City passed over the two sites. During
those periods a segment of the urban plume can be assumed
to arrive at the first downwind location at a time t1and at the
second location at a time t2, thereby giving rise to the T1-T2
identifying label for this study. The differences in aerosol
properties as a function of the time difference t2–t1can then
be studied.
In the remainder of this paper we describe the design of the
T1-T2 study, compare some of the meteorological conditions
found over the T1 and T2 sites, identify periods when the
Mexico City plume was expected to flow over both sites, and
present some preliminary comparisons of specific absorption
and other aerosol properties at the two locations. More exten-
sive and detailed comparisons, as well as analyses of results
obtained at T0, will be the subjects of future papers.
2 Study design
The T1 site was located at Tecamac University at 19.703N
latitude and 98.982W longitude, at an altitude of 2273 m.
This site is near built-up regions and a major highway
and is just to the north of the main metropolitan area
of Mexico City. Because the site is close to the city
we expected it would frequently be affected by the ur-
ban plume drifting over the region. An analysis com-
bining computer modeling, radar wind profiler and ra-
diosonde data collected during the 1997 IMADA-AVER ex-
periment (Doran et al., 1998), and rawinsonde data from
2002 and 2003 (http://mirage-mex.acd.ucar.edu/Documents/
MIRAGE-Mex SOD 040324.pdf) indicated that, in the up-
per portions of the boundary layer and above, flows from
Mexico City to the north, northeast, or northwest could be ex-
pected roughly 50% of the time during the month of March.
Closer to the surface the winds tend to be lighter and less
well organized. It was also estimated that winds throughout
the depth of the boundary layer would carry the Mexico City
urban plume to the north-northeast over the city of Pachuca
between 20 and 30% of the time. Additional information
about the meteorology encountered during the MILAGRO
campaign will be published elsewhere.
Although the expected frequency of favorable wind direc-
tions was not as high as we might have wished, it was de-
cided nonetheless that it would still be worthwhile to locate
the T2 site in the general area of Pachuca. Accordingly, the
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J. C. Doran et al.: The T1-T2 study 1587
Fig. 1. Map of study area showing locations of T1 and T2 sites.
Topography contour intervals are 200 m.
T2 site was situated 35 km to the north-northeast of T1 at
Rancho la Bisnaga, 20.010 N latitude and 98.909 W longi-
tude, at an altitude of 2542 m. The locations of these two
sites are shown in Fig. 1. The region between T1 and T2
is much less densely developed than the Mexico City urban
area. Some local sources of BC and other aerosols can be
found between T1 and T2, primarily along the highway be-
tween Mexico City and Pachuca, but their impact was ex-
pected to be small compared to that of the urban plume orig-
inating in Mexico City. Also shown in Fig. 1 is the location of
the Insituto Mexicano del Petr´
oleo, labeled T0 in the figure,
which was an observation site considered to be representative
of the Mexico City metropolitan area.
The T1 site was one of the two major ground sites for
the MILAGRO campaign, with investigators from approxi-
mately a dozen laboratories and universities making a wide
range of meteorological, chemical, aerosol, and atmospheric
radiation measurements. In contrast, the T2 site featured a
much smaller number of investigators and instruments, con-
sistent with the expected infrequency of urban plume pas-
sages over that location. In this paper we will consider data
from pairs of instruments, with one of each pair located at
both T1 and T2: radar wind profilers, radiosondes, organic
and elemental carbon (OCEC) analyzers, photoacoustic ab-
sorption spectrometers (PASs), 3-wavelength nephelometers,
particle soot absorption photometers (PSAPs), and multi-
filter rotating shadowband radiometers (MFRSRs). Informa-
tion about these instruments is provided below. In addition to
these ground-based instruments, several aircraft made flights
over T1 and T2 during the campaign. We will briefly dis-
Fig. 2. Time series of temperature, humidity, and total solar radia-
tion measured with surface instruments located at T2.
cuss some results from measurements with a nephelometer
and PSAP on board DOE’s Gulfstream-1 (G-1) aircraft taken
during periods when the Mexico City plume was expected to
drift over those sites.
Although some instruments were operational earlier, full-
scale operations at T1 and T2 commenced approximately on
day of year (DOY) 70 (11 March). Measurements at both
sites continued from that time through DOY 87 (28 March).
Figure 2 shows time series of temperature, humidity, and to-
tal solar radiation measured with surface instruments located
at T2. The first two weeks of the measurement period were
generally warmer, drier, and had less cloudiness than the last
week. Rain showers occurred on a daily basis during the last
week.
3 Instruments
The radar wind profilers (RWPs) used at T1 and T2 were
915MHz models manufactured by Vaisala. They were oper-
ated in a 5-beam mode with nominal 192-m range gates. We
applied the NCAR Improved Moment Algorithm (Morse et
al., 2002) to the moments data from the profilers to obtain
30-min average consensus winds. The radiosonde systems
were Vaisala DigiCORA™systems and used model RS-92
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1588 J. C. Doran et al.: The T1-T2 study
sondes, also manufactured by Vaisala. The sondes released
at T2 measured temperature and humidity while some of the
sondes released at T1 also measured winds with a GPS sys-
tem. On days when the G-1 was expected to fly in the vicinity
of T1, up to five radiosondes were released there, nominally
at 09:00, 11:00, 13:00, 15:00, and 17:00 local standard time
(LST). If the anticipated G-1 flights included sampling in the
T2 area, up to three additional sondes were launched at T2 at
11:00, 13:00, and 15:00 LST.
The OCEC analyzers were manufactured by Sunset Labo-
ratory Inc., and are similar to the thermal-optical instrument
described in Birch and Cary (1996). Samples collected on
a quartz filter are volatilized under controlled conditions of
temperature and processing gas mixtures. The process is
used to differentiate between organic and elemental carbon,
with an optical correction technique applied to correct for
pyrolytically generated carbon. Sampling times were 45 min
in length, with an additional 15 min allocated to analysisand
cooling after each sampling period. The OCEC unit was cal-
ibrated on-site using external standard filters before ambi-
ent sampling. The external standard was analyzed in off-line
mode. The data acquisition parameters were adjusted after
the calibration to reflect the known OC/EC ratio. The esti-
mated detection limit is 0.02 µgC m−3and the estimated un-
certainty of the OCEC measurement is 0.2 µgC m−3. Quartz
filters were changed every few days before a significant re-
duction in the intensity of the laser signal used for the optical
corrections was observed. A blank sample was scheduled
for midnight (LST) daily. We restricted our analyses to pe-
riods when the OC and EC concentrations were greater than
or equal to 0.1µgC m−3.
The PAS measures sound pressure produced by light ab-
sorption in an acoustic resonator and has been described by
Arnott et al. (1999). The instruments at T1 and T2 operated
at a wavelength of 870nm. Operation at 870 nm is advan-
tageous because BC does absorb at this wavelength but the
absorption of other atmospheric constituents such as dust and
OC is quite weak (Jacobson, 1999; Sokolik and Toon, 1999;
Sato et al., 2003; Kirchstetter et al., 2004). Average absorp-
tion values were computed every two minutes, and we com-
bined absorption values averaged over 46-min periods with
EC values from the OCEC analyzers to obtain estimates of
the specific absorption of EC at T1 and T2.
The nephelometers (TSI 3563) and PSAPs (Radiance Re-
search) at T1 and T2 were configured as parts of aerosol op-
tical measurement systems developed by NOAA’s Climate
Modeling & Diagnostics Laboratory (CMDL). Ambient air
is diverted alternately through 1µm and 10µm impactors at
6-min intervals, and the humidity is controlled so that it is
less than or equal to 40% RH before the air is introduced
into the nephleometer or PSAP. The nephelometers oper-
ated at three wavelengths: 450 nm, 550nm, and 700nm; the
PSAPs also operated at three wavelengths: 470 nm, 530nm,
and 660nm. Scattering and absorption measurements were
averaged and recorded every minute. The nephelometer
and PSAP on the G-1 operated at the same wavelengths as
the surface units. Corrections to the PSAP absorption and
the nephelometer scattering were carried out using the ap-
proach described by Bond et al. (1999) and Anderson and
Ogren (1998). Additional information on the nephelome-
ters and PSAPs can be found at the CMDL website http:
//www.cmdl.noaa.gov/aero/instrumentation/instrum.html.
The MFRSRs (Harrison et al., 1994) measured the direct
normal, diffuse horizontal, and total horizontal components
of the broadband irradiances using a silicon detector, and
these same components at six wavelengths (415, 500, 615,
673, 870, and 940 nm) with an interference filter/silicon de-
tector combination having a nominal passband of 10nm at
each wavelength. Data were averaged and recorded at 20-
s intervals. From these measurements, we can determine
aerosol optical thickness, τ. By applying the technique of
Kassianov et al. (2005) it is also possible to determine the
aerosol single scattering albedo, $0, and asymmetry param-
eter, g. In contrast to the aerosol quantities measured by our
other surface instruments, these quantities should be consid-
ered as averages over the atmospheric column.
4 Boundary-layer structure and winds
Doran et al. (1998) found that on clear days the bound-
ary layer (BL) in Mexico City during March typically grew
slowly after sunrise (approximately 06:40 LST) to a depth
on the order of 1000m by 11:00 or 12:00 LST, after which
it grew rapidly to heights of 3000m or more in the next few
hours. This behavior was observed at T1 during the MILA-
GRO campaign as well, although the heights tended to be
somewhat smaller during this campaign, and the BL behav-
ior at the T1 and T2 sites appeared to be similar. Figure 3
shows a comparison of BL depths estimated from potential
temperature profiles measured with radiosondes at ∼11:00,
13:00, and 15:00 LST on days for which nearly simulta-
neous releases were made at the two sites. The regression
line is given by h2=0.90×h1+25m, with R2=0.89, where h1
and h2 are the BL depths at T1 and T2, respectively. We
have also compared boundary-layer depths at T1 and T2 by
examining the signal-to-noise ratio of the reflected signals
from the RWPs at the two sites (not shown). The agreement
between the two sites is again reasonable (R2=0.90), with
h2=0.79×h1+260m. The similarity of the boundary layer
depths at the two sites simplifies some aspects of the anal-
ysis. It makes plume trajectory analyses easier (see below)
and it helps ensure that aircraft operations in the mixed layer
at one site will also be in the mixed layer at the second site,
providing the flight altitudes are not too different. Further in-
formation on boundary layer characteristics during the cam-
paign will be the subject of other papers.
We were particularly interested in days when the winds
were likely to blow the urban plume over both T1 and T2.
A detailed analysis identifying times when this occurred will
Atmos. Chem. Phys., 7, 1585–1598, 2007 www.atmos-chem-phys.net/7/1585/2007/
J. C. Doran et al.: The T1-T2 study 1589
be carried out in the future using a mesoscale model that in-
corporates a data assimilation scheme, parameterizations of
vertical mixing, and estimates of source locations in the Mex-
ico City metropolitan area. For this initial analysis, however,
we have simply used the wind data obtained from the RWP
at T1 to calculate forward and back trajectories of air masses
that passed over T1. Figure 4 shows examples of trajectories
calculated at 1000m above ground level (AGL) for daylight
hours over a 20-day period. The most favorable conditions
for T1-T2 transport are seen to have occurred on DOY 69,
77, 78, 79, and 83, (10, 18, 19, 20, and 24 March, respec-
tively) with briefer periods of preferred wind directions oc-
curring on several other days. On some days (e.g., DOY 81
[22 March]) the calculated trajectories showed the Mexico
City plume traveled close to T2, and lateral spreading of the
plume may well have resulted in some impact of Mexico City
emissions on both T1 and T2. On other days (e.g., DOY 74
[14 March]), the trajectories show near-misses at T2 but the
back trajectories at T1 suggest that the air mass traveling
from T1 to T2 did not originate over the urban region. In
some cases the wind directions exhibited substantial vertical
shear (not shown), which makes interpretation of the plume
transport problematic.
On many days, and especially on those with wind direc-
tions favorable for T1-T2 transport, the boundary layer was
quite dry. For example, by 11:00 LST on each of DOY 70,
77, 78, and 79 (10, 18, 19, and 20 March) radiosonde sound-
ings at T1 showed relative humidities in the boundary layer
of 46% or less (usually much less), with similar values found
from the T2 sondes. Above the boundary layer there was
often a very dry layer that could extend over depths of hun-
dreds of meters or more. Figure 5 shows an example of this
feature. Hygroscopic growth of aerosols and the consequent
effects on aerosol light scattering can be assumed to be es-
sentially negligible under such dry conditions (Nemesure et
al., 1995; Baumgardner and Clarke, 1998; Im et al., 2001;
Redemann et al., 2001; Carrico et al., 2003; Markowicz et
al., 2003). Thus, the optical properties of aerosols measured
near the surface are likely to be similar to those aloft in a
well-mixed boundary layer, which is advantageous for some
radiative closure analyses.
5 Results and discussion
5.1 OC and EC concentrations
Figure 6 shows plots of the organic (OC) and elemental (EC)
carbon concentrations at T1 and T2 as a function of time. At
T1 there was often a strong diurnal variation in OC and EC
concentrations during the first two weeks, with concentra-
tions increasing during the nighttime hours and the largest
values occurring in the morning hours around sunrise or
shortly after. This behavior is consistent with the trapping
of pollutants in the Mexico City area overnight and during
Fig. 3. Comparison of boundary-layer depths at T1 and T2 esti-
mated from potential temperature profiles measured with radioson-
des at ∼11:00, 13:00, and 15:00 LST on days for which nearly
simultaneous releases were made at the two sites: 11:00 LST
(DOY 68, 74, 77-78), 13:00 LST (DOY 68, 69, 76–79), and
15:00 LST (DOY 68 and 79).
the morning hours in a shallow surface layer before the rapid
growth of the mixed layer commences in the late morning.
Although the boundary layer structure and evolution at T2
were similar to those at T1, the OC and EC behavior at T2
was much less regular; there were often multiple peaks dur-
ing a 24-h period, and mid- to late-afternoon peaks were
common. No significant accumulations of pollutants, com-
parable to what was found at T1, developed overnight at T2.
This was expected, given the absence of local sources in the
area around and immediately upwind of T2.
Transport from Mexico City to T2 appears to account for a
substantial fraction of the variations in OC and EC concentra-
tions at T2. OC and EC concentrations were generally lowest
at both sites during the last week of the campaign when the
weather conditions were most unsettled and there were peri-
ods of rain. On days with the most favorable wind directions
for T1-T2 transport (DOY 69, 77, 78, and 79 (10, 18, 19, and
20 March)), both the OC and EC values at T2 tended to reach
higher values than on other occasions, although DOY 70, 71,
and 81 (11, 12 and 22 March) also have elevated readings. At
T1, which was considerably closer to the local and Mexico
City sources than T2, the carbon concentrations showed less
sensitivity to wind directions.
DOY 83 (24 March) has been identified as a transport day
but shows some unusual features. The OC and EC concen-
trations at T1 were low throughout the night, but the OC val-
ues were considerably higher during the day while the EC
values were not. At T2, OC and EC were low at night but
both increased substantially shortly after sunrise. Rain fell
at T1 and T2 just after sunset the previous night and during
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1590 J. C. Doran et al.: The T1-T2 study
Fig. 4. Trajectories of air parcels passing over T1 at 1000m a.g.l. for the hours 06:00–18:00 LST based on 30-min RWP data at T1. Blue
indicates flow into T1 and red indicates flow away from T1. The small box around T2 is 10km on a side; the larger box around T0 is an
approximate boundary of Mexico City.
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J. C. Doran et al.: The T1-T2 study 1591
Fig. 5. Example of relative humidity profiles at T1 and T2 showing dry layer above the boundary layer for sounding taken at 13:00 LST
on DOY 78 (19 March). The stair-step pattern at T1 is due to the limited resolution of the humidity measurements for the system deployed
there.
the afternoon of DOY 83. The rain may have contributed to
the lower carbon concentrations and the specific absorption
values are somewhat higher than the median values found
for other transport periods. While this is consistent with the
results found by Redemann et al. (2001) and Mikhailov et
al. (2006), the sampling period is rather small and we hesi-
tate to attach too much significance to this result at this time.
There is considerable scatter about the means, but for the
period shown in Fig. 6 the OC concentrations at T2 were
approximately 23% lower than at T1 while the EC concen-
trations were smaller by almost a factor of four. The average
EC/OC ratio at T1 was 0.210, while that at T2 was approxi-
mately three times smaller with a value of 0.063. Assuming
the Mexico City metropolitan area is the primary source of
EC in the area, the decrease in this quantity between T1 and
T2 was anticipated, given the dilution expected in traveling
between the two sites. Conversely, the much smaller change
in OC concentrations presumably reflects the rapid and ongo-
ing generation of OC over the region (Volkamer et al., 2006).
5.2 Specific absorption of EC
There is still some debate about exactly what is measured by
an OCEC analyzer. Andreae and Gelencs´
er (2006) propose
an operational definition for apparent elemental carbon, as
“the fraction of carbon that is oxidized above a certain tem-
perature threshold in the presence of an oxygen-containing
atmosphere”, and denote this quantity as ECa. For simplic-
ity in this paper we have retained the more commonly used
designation EC, but we are mindful of this potentially im-
portant distinction. Under some circumstances EC may only
be loosely related to soot particles, but the connection should
be considerably better in regions with high concentrations of
petroleum combustion sources, and Mexico City is one such
region. Andreae and Gelencs´
er also note that BC is often
used to denote the result of a light-absorbing carbon mea-
surement by an optical absorption technique. In any event,
our interest for this study was not the measurement of the
specific absorption of soot per se but rather the absorption
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1592 J. C. Doran et al.: The T1-T2 study
Fig. 6. Time series of the organic (OC) and elemental (EC) carbon concentrations at T1 and T2. Nighttime periods are indicated by the
shaded areas. Note differences in scales for T1 and T2.
per unit mass of EC and how that may be modified as EC be-
comes coated in the hours or days after emission. We again
note that almost all of the absorption measured by the PASs
at T1 and T2 is likely due to the EC at the respective sites
because dust and OC do not absorb significantly at the wave-
length of the PAS measurements. Figure 7 shows time series
of EC concentrations, absorption measured by the PASs, and
specific absorption computed from the ratio of the absorp-
tion determined from the PASs and the EC concentrations
measured by the OCEC analyzers, for a 12-day period dur-
ing which the OCEC analyzers and PASs at both sites had
good data recovery.
Figure 8 shows a scatter plot of the absorption as a func-
tion of EC concentrations at the two sites. There is a greater
degree of scatter at T2 than at T1, which is expected given the
substantially lower EC concentrations and absorption found
at the former site. At T1 the EC concentrations and absorp-
tion values were as much as an order of magnitude higher
than at T2 but the specific absorption was generally higher at
T2 than at T1.
The figure shows that reasonably robust fits to the data at
each site can be obtained with linear fits that are constrained
to go through the origin. The R2values in each case are
∼0.96. The slopes of the lines in the figure correspond to
the mean values of specific absorption at the two sites, with
the value at T2 about 9% higher than that at T1 for the full
sampling period. One can also obtain a least squares fit to
the data at each site while leaving the intercept as a free
Atmos. Chem. Phys., 7, 1585–1598, 2007 www.atmos-chem-phys.net/7/1585/2007/
J. C. Doran et al.: The T1-T2 study 1593
Fig. 7. Time series of EC concentrations, absorption measured by
the PASs, and specific absorption of EC at T1 and T2 for a 12-day
period during which the OCEC analyzers and PASs at both sites had
good data recovery. Transport periods are indicated by the shaded
areas.
parameter. The results are virtually unchanged at T1 but
result in a smaller slope at T2 and an intercept on the or-
der of 0.6Mm−1. A non-zero intercept is unphysical (if the
EC concentration is zero the absorption should be zero) and
may indicate errors in the measurements, another absorbing
species, or that the specific absorption is not necessarily in-
dependent of concentration. At this time we have no way of
distinguishing among these possibilities. Partly for this rea-
son, and partly to mitigate the effects of possible outliers in
the data, we prefer to use the median values of specific ab-
sorption at T1 and T2 rather than mean values. Moreover,
given the skewed nature of the distributions of specific ab-
sorption shown in Fig. 9, we believe the median is a more
useful indicator of the behavior than the mean.
Histograms of specific absorption at the two sites are
shown in Fig. 9 and indicate that the distributions were
skewed toward higher values at T2 but not at T1. This is
what would be expected if the differences between the sites
are attributable to the greater aging of the EC sampled at the
site more distant from sources in Mexico City.
Fig. 8. Scatter plots of absorption as a function of EC concentration
at T1 (top) and T2 (bottom). The straight lines are least squares fits
to the data that are constrained to pass through the origin.
It is interesting to compare the values of specific absorp-
tion at the two sites during times when the Mexico City
plume was traveling from the city over T1 and then T2 with
the values found when such transport was not occurring. Us-
ing the T1 RWP data, we identified “transport” periods when
air parcels passing through T1 were likely to have originated
over the city and subsequently passed within 5km of T2 ap-
proximately 4h or less after exiting the city. In some cases
near-surface trajectories did not have favorable directions but
trajectories further aloft (e.g., 1 km) did. If these cases oc-
curred when a convective boundary layer was present that
could mix air parcels throughout the boundary layer to the
surface, then those periods were also included in the trans-
port category. All other periods were lumped into the “non-
transport” category. Without using a detailed mesoscale
model constrained by data assimilation of RWP data there is
some uncertainty and subjectivity in the choice of the partic-
ular time periods to include in each category, but our results
are not especially sensitive to the details of the selection cri-
teria. Table 1 gives median and 10th and 90th percentile val-
ues of the specific absorption at T1 and T2 for the transport
and non-transport periods. We have also included values of
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1594 J. C. Doran et al.: The T1-T2 study
Fig. 9. Histograms of specific absorption of EC at 870 nm at T1
(top) and T2 (bottom). The histogram for T1 represents 264h of
data while that for T2 represents 236 h.
specific absorption extrapolated to a wavelength of 550nm,
assuming a λ−1dependence for the absorption between 870
and 550nm as suggested by Bergstrom et al. (2002).
Two features stand out in the table. The first is that larger
values of specific absorption are found at both T1 and T2
during non-transport times than during transport times. This
is consistent with the expectation that longer-aged EC with
greater coating is more likely to be found at both sites if the
wind is not blowing directly from the city. The second fea-
ture is that the specific absorption at T2 is larger than at T1
for both transport and non-transport periods. This suggests
that the specific absorption of EC can be modified after only
a few hours of aging, the time required for transport from T1
to T2. On non-transport days the results indicate that fresher
emissions from Mexico City or nearby local sources are still
more likely to be found at T1 than at T2. Although the dif-
ferences in specific absorption noted here are not large, a
Mann-Whitney test indicates that the differences are statis-
tically significant at the 1% level.
The median values extrapolated to 550nm displayed
in the table are considerably larger than the value of
7.5±1.2m2g−1reported by Bond and Bergstrom (2006)
for uncoated soot particles. However, Bond et al. (2006)
suggest that absorption by aged aerosol is about 1.5 times
greater than that of fresh aerosol at 550nm. Johnson et
al. (2005) note that while in Mexico City “fresh particulate
emissions from mixed-traffic are almost entirely carbona-
ceous...ambient soot particles which have been processed
for less than a few hours are heavily internally mixed...”
Our larger values of specific absorption are thus consistent
with this characterization of the emissions.
It is interesting that the 10th percentile value of spe-
cific absorption at T1 during transport conditions is
8.7m2g−1(extrapolated to 550nm), which is near the upper
end of the range of values suggested by Bond and Bergstrom
for freshly emitted soot. In contrast, the 10th percentile value
of specific absorption at T2 is 10.8, well beyond the sug-
gested range. For non-transport periods, the 10th percentile
value at T1 is still 8.7, which is consistent with our expec-
tation that even during non-transport periods T1 would often
be influenced by nearby sources. This is less likely to be
the case at T2, and the 10th percentile value there is indeed
substantially larger than for transport conditions.
The extrapolations of the specific absorption values from
870nm to 550nm shown in the table may be problematic.
Bergstrom et al. (2002) found a σA∼λ−1dependence, where
σAis the cross section for aerosol scattering, but Moosm¨
uller
et al. (1998) derived a λ−2.7relationship from their analysis.
Using PSAP data at three wavelengths we have examined
the wavelength dependence of absorption between 470 and
660nm. We found the median exponent in the expression
σA∼λbis close to −1, but there is considerable scatter about
this value. Moreover, an extrapolation derived from 660 and
530nm PSAP data cannot be applied with much confidence
to the much larger extrapolation of the 870nm photoacoustic
results to 550nm. Thus we do not believe that a point-by-
point comparison of 530nm PSAP and 870nm PAS absorp-
tion values is warranted. The extrapolation of the summary
statistics shown in the table may still be useful, but we sug-
gest that the 550 nm values be treated with considerable cau-
tion.
The G-1 aircraft did not measure EC so it is not possible to
use G-1 data to directly extract values of specific absorption
as it passed through the Mexico City urban plumes.
5.3 Single scattering albedo
We have made an initial examination of the single scatter-
ing albedo determined from the airborne nephelometer and
PSAP data to see if significant changes could be discerned
between the T1 and T2 locations on days when the urban
plume was carried over or near both sites. The single scat-
tering albedo is defined by $0=6S
6S+6A, where 6Sand 6A
are the extinction coefficients for aerosol scattering and ab-
sorption, respectively. Values of $0were calculated when
the G-1 was sampling within 5km of T1 or T2 on DOY 77,
78, and 79 (18, 19, and 20 March). From the PSAP and
Atmos. Chem. Phys., 7, 1585–1598, 2007 www.atmos-chem-phys.net/7/1585/2007/
J. C. Doran et al.: The T1-T2 study 1595
Table 1. Median, 10th and 90th percentile values of specific absorption of EC in m2g−1for transport and non-transport periods.
T1 T2
10th percentile median 90th percentile 10th percentile median 90th percentile
transport peri-
ods 870 nm 5.5 7.7 9.2 6.8 8.3 11.9
non-transport
periods
870nm
5.5 8.1 9.7 7.5 9.0 12.2
transport peri-
ods 550 nm 8.7 12.2 14.5 10.8 13.1 18.8
non-transport
periods
550nm
8.7 12.8 15.4 11.8 14.3 19.3
Table 2. Mean values of $0derived from the T1 and T2 MFRSRs (at 500nm) and from the T2 surface aerosol system (nephelometer and
PSAP) data (at 530 nm) for DOY 71, 78, and 86 (12, 19, and 27 March). Note that surface values were available only at T2.
DOY Time (LST) $0from T1
MFRSR $0from T2 sur-
face system Time (LST) $0from
T2 MFRSR $0from T2 sur-
face system
71 08:00–09:36 0.84 0.88 08:00–09:36 0.91 0.88
78 07:30–09:30 0.85 0.92 07:30–11:28 0.83 0.91
86 08:00–10:00 0.89 0.92 07:30–08:00 0.90 0.90
nephelometer data, $0was on the order of 0.9 for most of
the flights during these transport periods, so that σA∼σS/9.
From Table 1, the median specific absorption at the surface
at T2 was ∼10% larger than that at T1. We can assume that
the variations in the surface absorption values were approxi-
mately the same as those at the G-1 sampling elevations be-
cause the boundary layer was generally well developed at the
times of the G-1 overflights. Thus, reductions in $0between
T1 and T2 arising from changes in σA(and assuming σSwas
unchanged) would only be on the order of 0.01. Given the
observed fluctuations in the values of $0, consistent differ-
ences between T1 and T2 were expected to be difficult to
identify in the G-1’s data stream (although such accuracy has
been suggested as needed for climate studies (Heintzenberg
et al., 1997)). This expectation was confirmed in our initial
examination of the G-1 data time series. Further analyses are
planned using CO measurements to help identify the times
and locations when the G-1 was sampling within the Mexico
City urban plume and to compare in-plume and out-of-plume
values of $0, but at this point we do not anticipate major
changes in $0will be found between T1 and T2.
We have evaluated single scattering albedos at the surface
at T2 for the transport and non-transport periods described
above as well. The distributions of $0for these two peri-
ods are similar, with mean values differing by approximately
0.01 or less. This implies that the direct radiative forcing
at T2 should not be significantly affected by the differences
in the specific absorption of EC between transport and non-
transport periods, although it still could be affected by other
factors such as variations in the total optical depth between
the two sites. It would have been desirable to compare sur-
face results from the T1 and T2 sites to see if those dis-
tributions of $0were similar, but the data recovery for the
nephleometer and PSAP at T1 was poor and we are unable
to provide such a comparison at this time.
We have also used the MFRSR data at T1 and T2 to de-
rive values for $0at 500nm using the approach described by
Kassianov et al. (2005). In contrast to the aerosol quantities
measured by our other surface instruments, $0derived from
the MFRSR data can be considered as an average over the
atmospheric column. The approach is only applicable when
the sky is essentially cloud-free. In practice this often limits
our analysis periods to a few hours in the morning because
cumulus clouds frequently developed in the late morning or
afternoon. Initial results are similar to those obtained from
the G-1 measurements described above, i.e., differences in
$0between T1 and T2 are small (∼0.01–0.02) without an
obvious tendency for values at one or the other site to be
larger.
Table 2 shows some values of $0for a wavelength of
500nm obtained from the MFRSRs and from the T2 sur-
face aerosol system at 530nm on three days with cloud-free
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1596 J. C. Doran et al.: The T1-T2 study
periods. The times of the MFRSR measurements are given in
the table. The agreement between the column averaged val-
ues of $0and the T2 surface value is good for DOY 86 (27
March), fair for DOY 71, and poorer on DOY 78 (19 March).
The discrepancy between MFRSR and surface values of
$0on DOY 78 may be due to boundary-layer structure and
growth. The G-1 made measurements on that day between
∼10:50 and 12:00 LST and found values ranging from 0.87
to 0.90 over T1 and around 0.87 over T2. At T2 this re-
sult is an intermediate one between the surface value and
the column-averaged value. The sampling height of the air-
craft was ∼3100m m.s.l., or approximately 830 m a.g.l. at
T1 and 560m a.g.l. at T2. This would have placed the G-1
within the growing boundary layer at heights of ∼0.5–0.7 of
the mixed layer depths estimated from RWP signal-to-noise
values. It is thus reasonable to assume that the G-1 was sam-
pling in a region where the aerosol mix was influenced by
both near-surface aerosols, with higher values of $0as in-
dicated by the nephelometer and PSAP data, and aerosols
entrained from aloft, with lower values of $0as suggested
by the MFRSR results. Later in the afternoon (∼17:00 LST)
when the boundary layer was deeper, the G-1 was again over
the T2 site and measured a value of 0.90 for $0, which agrees
well with the corresponding surface value at that time of
0.92.
6 Summary
We have carried out an experiment to investigate the evolu-
tion of aerosols and their optical properties downwind from
Mexico City. Our focus has been on the specific absorption
of black carbon and how that is affected as the aerosols age
and become coated, changing from an externally to an inter-
nally mixed state. To accomplish this we deployed instru-
ments at two sites, T1 and T2, to characterize the meteorol-
ogy, solar radiation, aerosol light scattering and absorption,
and OC and EC concentrations. An instrumented aircraft
flew a number of missions over the two sites as well.
The purpose of this paper has been to provide an overview
of the T1-T2 experiment and to present some preliminary re-
sults and highlights from that campaign. OC and EC values
at T1 showed diurnal variations suggesting a buildup of more
polluted air from nearby urban sources during the night and a
subsequent dilution as the boundary layer grew the following
morning. This behavior was not found at the more isolated
T2 site but transport from Mexico City appeared to be a ma-
jor factor in determining OC and EC concentrations. Using
data from a radar wind profiler, we have divided the mea-
surement periods into transport and non-transport periods,
corresponding to conditions when air masses were or were
not expected to travel from Mexico City and then over T1
and T2. The specific absorption of EC during transport peri-
ods was lower than that found during non-transport periods,
which is consistent with the expectation that fresher emis-
sions are more likely to be found at T1 and T2 during trans-
port episodes than at other times. The specific absorption at
T2, the site more distant from Mexico City, was also found
to be larger than at T1, in keeping with the greater age of the
aerosols expected at T2. Initial analyses of the variation of
$0at T2 for transport and non-transport periods reveal only
small differences, although additional analyses are planned.
Comparisons of values from G-1 aircraft data, surface opti-
cal measurements, and column-averaged values derived from
MFRSR data appear broadly consistent, although boundary-
layer structure and growth appear to contribute to some dif-
ferences between the in situ and the column-averaged values.
A number of additional topics are anticipated for future
studies, including detailed examination of the in-plume and
out-of-plume aerosol characteristics derived from the G-1
measurements, more detailed analyses of plume trajectories
and their relation to the specific absorption measured at T1
and T2, assessment of the aerosol optical characteristics at
T0, and further comparisons of in situ and column-averaged
aerosol properties.
Acknowledgements. We thank J. Ogren, B. Andrews, and P. Sheri-
dan of NOAA’s CMDL for their assistance with the construction
of the aerosol optical measurement systems and the subsequent
data processing. We appreciate the assistance of P. Holowecky
and J. Satola of Battelle Columbus for their contributions to the
field measurements. This research was supported by the Office of
Science (BER), U.S. Department of Energy, under the auspices
of the Atmospheric Science Program, under Contract DE-AC05-
76RL01830 at the Pacific Northwest National Laboratory. Pacific
Northwest National Laboratory is operated for the U.S. DOE by
Battelle Memorial Institute.
Edited by: S. Madronich
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