Evolution of Particulates and Direct Radiative Forcing Downwind of

th
7 Annual WRF User’s Workshop, 19-22 June 2006, Boulder, CO
Evolution of Particulates and Direct Radiative Forcing Downwind of Mexico City
during the 2006 MILAGRO Field Campaign
Jerome D. Fast, J. Christopher Doran, James C. Barnard, and Michael L. Alexander
Pacific Northwest National Laboratory, Richand, WA
Lawrence I. Kleinman and Stephen R. Springston
Brookhaven National Laboratory, Upton, NY
1. Introduction
2. 2006 MILAGRO Field Campaign
Much has been learned about the boundary layer
structure and circulations associated with the complex
terrain surrounding Mexico City from previous field
experiments and modeling studies. Heating of the
higher terrain surrounding Mexico City produces
daytime upslope flows that draw air from the plateau
north of the city into the basin [Jauregui, 1988; de Foy et
al., 2005]. Late in the afternoon, the temperature
gradient between the warm basin CBL and the cool
ambient air south of the basin produces strong winds
through the gap in the terrain near Chalco [Doran and
Zhong, 2000]. A propagating density current [Bossert,
1997; Whiteman et al., 2000] is also produced that
brings in cooler air from the Gulf of Mexico into the north
end of the basin. These converging flows may enhance
vertical mixing and venting of pollutants out of the basin
[Fast et al., 1998].
While extensive chemistry and particulate
measurements have been collected at the surface to
characterize air quality over the city, relatively few
measurements have been made aloft and downwind of
the city to characterize the vertical variations of
anthropogenic trace gases and aerosols emitted from
Mexico City and their impact on the local and regional
environment. This knowledge gap has been addressed
by the recent field campaigns conducted in Mexico
during March 2006, as part of the Megacities Initiative:
Local and Global Research Observations (MILAGRO).
This study focuses on simulating the evolution of
particulates and their impact of radiative forcing up to
100 km downwind of Mexico city using WRF-chem
[Grell et al., 2006] with PNNL’s modules [Fast et al.,
2006] for periods in which southerly to westerly synoptic
winds transported anthropogenic pollutants over surface
measurement sites. The ambient flow during these
periods was strong enough to counteract the daytime
thermally-driven circulations that would normally draw
air from the plateau into the city and away from the
surface measurement sites. Various in-situ and remote
sensing measurements are used to evaluate the
predicted boundary layer structure, vertical mixing and
dilution of trace gases and particulates, and the
magnitude and spatial variation in aerosol direct
radiative forcing.
Corresponding author address: Jerome D. Fast, Pacific
Northwest National Laboratory, P.O. Box 999, K9-30,
Richland, WA, 99352, e-mail: [email protected].
During March 2006, MAX-Mex (Megacity Aerosol
Experiment in Mexico City, supported by DOE),
MIRAGE-Mex (Megacity Impacts on Regional and
Global Environments – Mexico City, supported by NSF),
MCMA (Mexico City Metropolitan Area, supported by
NSF/DOE), and INTEX-B (Intercontinental Chemical
Transport Experiment, supported by NASA), field
experiments were conducted in conjunction with field
measurements made by Mexican university scientists to
better understand the evolution of oxidants and aerosols
associated with megacities and their impact on climate
and air quality downwind. Extensive meteorological,
chemical, and particulate measurements were obtained.
Most of the surface instrumentation was deployed
at the three sites depicted in Fig. 1: one within the city,
denoted as T0, and two downwind of the city, denoted
as T1 and T2. Six research aircraft collected data aloft
over central Mexico and up to several hundred
kilometers downwind of the city. The G-1 aircraft,
supported by DOE, usually flew over the city and
downwind of the city over the T1 and T2 sites. In this
way, surface measurements could be put in the context
of measurements aloft at multiple elevations. Example
flight paths on 20 March are shown in Fig. 1.
Five of the research aircraft were based in
Veracruz, located along the Gulf of Mexico and one
aircraft was based in Houston, TX. An operations
center was set up in Veracruz so that the project
scientists could plan and coordinate the aircraft
sampling as well as measurements at ground and
1530
220
1530
220
10
10
UTC
UTC
18.00
1224
176
22.50
1224
176
8
8
Tula
Tula
17.75
T2
22.25
T2
17.50
132
918
22.00
132
918
6
6
17.25
T1
17.00
T0
612
88
16.75
4
21.25
Mexico City
16.50
306
44
21.50
T0
612
88
4
Mexico City
21.75
T1
21.00
306
44
16.25
2
Popocatepetl
Popocatepetl
16.00
0
20.75
2
20.50
0
0
306
44
612
88
132
918
1224
176
1530
220
0
306
44
612
88
132
918
0
220 km
0.0
1224
176
1530
220
0
0.2
0.4
0.6
0.8
1.0
220 km
0.0
0.2
0.4
0.6
0.8
1.0
Fig. 1. G-1 aircraft flight paths during the morning
(left) and afternoon (right) of 20 March 2006 along with
locations of the major surface sampling sites (circles),
locations of the Popocateptl volcano and Tula Power
Facility, and topography of the central Mexican plateau
(shading). Darker shading denotes higher elevations.
mobile sites that were not continuous during the field
campaign. Daily weather briefings were given by the
forecasting / modeling team for the project scientists
that consisted of predictions of the Mexico City pollutant
plume location and predictions of the meteorological
conditions that affect aircraft and surface sampling.
Forecasts made by several local, regional, and global
operational models as well as real-time measurements
were the primary sources of information used by the
forecasting / modeling team. The models included MM5
[de Foy and Molina, 2006], WRF [Skamarock, 2006],
WRF-chem, STEM, FLEXPART, MOZART, GEOSCHEM, and RAQMS. In general, the aircraft missions
met their scientific objectives because the 1-2 day
forecasts from the local and regional models were
reasonably accurate. The performance of these models
will be the subject of future papers.
3. Configuration of WRF-chem
In this study, three nested domains were employed
by PNNL’s version of WRF-chem [Fast et al., 2006] with
horizontal grid spacings of 22.5, 7.5 and 2.5 km. The
outer domain encompassed most of Mexico and the
inner domain included the Mexico City basin and much
of the central plateau north and east of the city where
the G-1 aircraft collected measurements. Initial and
boundary conditions were obtained from NCEP’s Global
Forecasting
System
(GFS).
Meteorological
parameterization options employed in this study include
the Goddard shortwave radiation scheme, RRTM
longwave radiation scheme, the simple thermal diffusion
land surface scheme, the YSU boundary layer scheme,
and the Lin et al. microphysics scheme. For the
chemistry options, we use the modules developed by
PNNL scientists for WRF-chem as described by Fast et
al. [2006] including the CBM-Z gas-phase mechanism,
217.5
1530
10
120 ppb
174.0
1224
8
110
100
130.5
918
6
90
80
4
87.0
612
70
60
2
50
43.5
306
40
0
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0
0.0
0
43.5
306
87.0
612
Fig. 2. Simulated ozone
(arrows) at approximately
March along with the
(dots).between 2030 and
130.5
918
174.0
1224
217.5
1530
(color contours) and winds
950 m AGL at 21 UTC 20
G-1 ozone measurements
2245 UTC. Gray contours
denote topography and white dots denote the T0, T1,
and T2 sites
the MOSAIC sectional aerosol model, the FAST-J
photolysis scheme, and direct radiation feedbacks.
We discuss some of the results from three 48-h
simulation periods that start at 12 UTC on 8, 14, and 19
March. The first simulation day is primarily a spin-up
period and our present analyses employ results on the
second simulation day. Initial and boundary conditions
for trace gases and particulates are set to low values
with some species based on vertical profiles from the
research aircraft. Anthropogenic emission rates of trace
gases and particulates for the metropolitan area of
Mexico City were obtained from the 2000 emissions
inventory [GDF] that employs 2 x 2 kilometer grid cells.
While there are no significant point sources of SO2 in
Mexico City, a power plant and refining complex located
north of Mexico City at Tula (Fig. 1) emits a large
amount of SO2. The Popocateptl volcano, southeast of
Mexico City, is active and emits SO2. These SO2 point
sources are likely important in producing sulfate in the
region and these particles can mix with the
anthropogenic particulate emissions from Mexico City.
Constant emission rates of SO2 reported in the literature
were used for both Tula and Popocateptl sources.
4. Results
Relatively strong southwesterly synoptic winds
were observed over central Mexico between 19 and 20
March as a result of an upper-level trough. The
thermally-driven daytime up-valley flow was suppressed
and transport of pollutants was primarily towards the
northeast. As shown in Fig. 2, the plume of ozone
predicted by WRF-chem during the afternoon of 20
March was towards the northeast and passed over the
T1 and T2 sampling sties. The predicted ozone is at 21
UTC at approximately 950 m AGL, while the aircraft
measurements were between 2030 and 2245 UTC at
various altitudes.
Nevertheless, the comparison
indicates that the spatial extend of the predicted ozone
concentrations was consistent with the aircraft
measurements.
A more rigorous evaluation of the predicted trace
gases and particulates using the G-1 data from both the
morning and afternoon flights is shown in Fig. 3.
Predictions from WRF-chem were interpolated in space
and in time to the locations of the G-1 samples. It is
important to note that these measurements are
preliminary and corrections need to be applied to some
quantities.
In general, the predicted locations and magnitude
of trace gases such as ozone, SO2, NOx, and CO were
predicted well, although the peak ozone values during
the morning were higher than observed. Higher NOx
and CO concentrations are observed and predicted
during the morning as a result of shallow boundary layer
that traps the emissions at the surface.
As the
boundary layer grows during the afternoon, observed
and predicted concentrations of NOx and CO decreased
as ozone increased. The model also captured the SO2
emissions from both the Tula and Popocateptl sources.
Particulate volume from the G-1 was obtained from
T1
city
T1
PCASP measurements for aerosol size diameters
T2
T1
55
55
10
10
16.00
16.25
16.50
16.75
17.00
17.25
17.50
17.75
18.00
18.25
20.50
100
0
16.25
16.50
16.75
17.00
17.25
17.50
17.75
18.00
18.25
NOx (ppb)
NOx (ppb)
21.75
22.00
22.25
22.50
22.75
Tula
20.50
20.75
21.00
21.25
21.50
21.75
22.00
22.25
22.50
22.75
20.50
20.75
21.00
21.25
21.50
21.75
22.00
22.25
22.50
22.75
20.50
20.75
21.00
21.25
21.50
21.75
22.00
22.25
22.50
22.75
20.50
20.75
21.00
21.25
21.50
21.75
22.00
22.25
22.50
22.75
20.50
20.75
21.00
21.25
21.50
21.75
22.00
22.25
22.50
22.75
80
40
40
0
0
16.00
16.25
16.50
16.75
17.00
17.25
17.50
17.75
18.00
18.25
1000
CO (ppb)
1000
CO (ppb)
21.50
Tula
0
16.00
500
500
0
0
16.00
16.25
16.50
16.75
17.00
17.25
17.50
17.75
18.00
18.25
40
volume
40
volume
21.25
50
80
20
20
0
0
16.00
16.25
16.50
16.75
17.00
17.25
17.50
17.75
18.00
18.25
1.00
SSA
1.00
SSA
21.00
Popocatepetl?
SO2 (ppb)
SO2 (ppb)
Popocatepetl?
50
0.85
0.70
0.85
0.70
16.25
16.50
16.75
17.00
17.25
17.50
15
17.75
OC
10
18.00
18.25
(mass ug m-3)
16.00
NO3
5
0
16.00
16.25
16.50
16.75
17.00
17.25
17.50
6
17.75
SO4
18.00
NH4
0
Fig. 3.
16.25
16.50
16.75
17.00
17.25
time (UTC)
17.50
17.75
18.00
18.25
OC
10
NO3
5
0
18.25
3
16.00
15
20.50
mass (ug m-3)
(mass ug m-3)
20.75
100
Tula
mass (ug m-3)
T2
100
O3 (ppb)
O3 (ppb)
100
20.75
21.00
21.25
21.50
21.75
22.00
6
22.25
SO4
22.50
22.75
NH4
3
0
20.50
20.75
21.00
21.25
21.50
21.75
time (UTC)
22.00
22.25
22.50
22.75
Observed (dots) and simulated (line) ozone, SO2, NOx, CO, particulate volume, single scattering albedo
(SSA), and mass of organic carbon, NO3, SO4, and NH4 along the G-1 flight paths during the morning (left)
and afternoon (right) of 20 March 2006.
between 0.1 and 3 μm. To compare the predicted
aerosol volume with the measurements, we used the
aerosol size bins between 0.04 and 2.5 μm. The
simulated aerosol volume was qualitatively consistent
with the data with peak values at the same locations as
the peak trace gases as shown in Fig. 2. Aerosol
composition measurements on the G-1 aircraft, for
aerosol diameters up to ~1 μm, were obtained from an
Aerodyne Mass Spectrometer (AMS). Predicted values
for bins 1 – 5 between 0.04 and 1.25 μm are compared
with the AMS measurements shown in Fig. 3. The
prediction composition was also consistent with the
AMS data, except that the model underestimated sulfate
in the vicinity of the SO2 plumes produced by the Tula
and Popocateptl sources. Most of the observed and
predicted aerosol mass was made up of organic carbon,
followed by nitrate (NO3), sulfate (SO4), and ammonium
The AMS does not measure all aerosol
(NH4).
composition, including black carbon that has a profound
effect on atmospheric radiation.
One of our objectives is to quantify the aerosol
radiative forcing associated with the particulates
downwind of Mexico City. Accurately simulating aerosol
radiative forcing requires that the aerosol optical
properties, in addition to particulate mass and
composition, also be represented well by models. Our
initial evaluation of the predicted aerosol optical
properties focuses on the single scattering albedo, o,
which is the fraction of light extinction resulting from
scattering. The observed o from the G-1 aircraft
shown in Fig. 3 were obtained from nephelometer and
particle soot aborbance photometer measurements.
While the magnitude of the simulated o of 0.80-0.85
was similar to the mean of the measurements, the
model failed to capture the observed spatial variability in
o. Our analyses will continue to explore this issue after
final measurement values become available.
The
predicted black carbon that influences the o and
aerosol optical depth (AOD) also needs to be evaluated.
Keeping this in mind, the effect of particulates on
the predicted AOD and shortwave radiation at 21 UTC
for 9, 15, and 20 March is shown in Fig. 4. For the
afternoons of 9 and 20 March, peak AOD was predicted
to be between 0.45 and 0.5 just northeast of central
Mexico City as a result of southwesterly winds
transporting the particle plume to the northeast. On 15
March, particulates were transported to the northwest by
the ambient southeasterly flow while a fraction of the
particle plume was also transported to the southwest
Shortwave Radiation, 21 UTC 9 March
over the mountains by upslope flows. In contrast with
Shortwave Radiation, 21 UTC 15 March
1530
Shortwave Radiation, 21 UTC 20 March
1530
1530
11.0
900 W m-2
1224
1224
1224
8.8
880
860
918
918
840
918
6.6
820
800
4.4
612
612
612
780
760
2.2
306
306
740
306
720
0.0
0.0
0
0
0
306
612
918
1224
1530
Aerosol Optical Depth, 21 UTC 9 March
306
612
918
1224
1530
0
Aerosol Optical Depth, 21 UTC 15 March
1530
0.2
700
0.4
0.6
0.8
1.0
0.6
0.8
1.0
0
0
306
612
918
1224
1530
Aerosol Optical Depth, 21 UTC 20 March
1530
1530
11.0
0.65
1224
1224
1224
8.8
0.60
0.55
918
918
0.50
918
6.6
0.45
0.40
4.4
612
612
612
0.35
0.30
2.2
306
306
0.25
306
0.20
0.0
0.0
0
0
0
306
612
918
1224
1530
0.2
0.15
0.4
0
0
306
612
918
1224
1530
0
306
612
918
1224
1530
Fig. 4. Simulated shortwave radiation (top) and aerosol optical depth (bottom) at 21 UTC on 8, 15, and 20 March
2006. Arrows depict dominant flow direction.
the other two periods, the AOD on 15 March was as
high as 1.0. Another interesting feature is the high AOD
east of Mexico City on 9 March that was produced by
sulfate from the SO2 emissions from the Popocateptl
volcano.
We have not yet examined why the
meteorological conditions were more favorable for
sulfate production on 9 March, given that the emission
rates for SO2 were the same for each simulation.
The predicted shortwave radiation in Fig. 4 includes
both the influence of clouds and aerosols; however,
significantly lower values are associated with the
regions of high aerosol optical depth. Aerosols reduced
-2
the shortwave radiation by as much as ~50 W m . The
lower shortwave values at the lower elevations closer to
the coast in the northeast corner of the domain results
from marine stratus clouds in the model. Small pockets
of thin clouds lead to reductions of shortwave radiation
in other parts of the domain.
5. Continuing Work
Global climate models still contain large
uncertainties in the predicted radiative forcing resulting
from aerosols. A part of this problem is comparing
coarse global climate model predictions with
measurements from field campaigns, such as
MILAGRO that contain large spatial and temporal
variations in particulates. Our strategy is to use the
multi-scale capabilities of WRF and the state-of-thescience aerosol model that is fully coupled with the
meteorology to better understand the uncertainties
associate with particulate evolution and aerosol
radiative forcing downwind of Mexico City.
The results from WRF-chem in this study are
preliminary and not all of the needed measurements
from MILAGRO are available yet. Evaluating the model
predictions with measurements of black carbon and
radiation will be critical. To accurate simulate aerosol
radiation forcing requires that the meteorology,
chemistry, particulate mass, particulate composition,
and aerosol optical properties to be simulated well.
This is a challenging, but necessary, task to make
improvements in aerosol formulations.
Acknowledgements: We thank Agustin Garcia (UNAM)
and Xuexi Tie (NCAR) for their assistance with the 2000
emissions inventory. This work is supported by the U.S.
Department of Energy’s (DOE) Atmospheric Sciences
Program through the Office of Biological and
Environmental Research.
The Pacific Northwest
National Laboratory is operated for the DOE by Battelle
Memorial Institute under contract DE-AC05-76RLO
1830.
6.
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