Applications of Lagrangian Dispersion Modeling to the Analysis of Changes in the Specific Absorption of Elemental Carbon Page: 4 of 13
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J. C. Doran et al.: Lagrangian dispersion modeling to study absorption by carbon
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Fig. 2. Measured (dots) and modeled (lines) wind speeds and direc-
tions at T1. Model values are shown without (red) and with (blue)
to the period of 18-21 March when winds from the south
southwest prevailed throughout the lowest 2km or more.
This time period appeared to be particularly favorable for
transport of the MCMA plume over TI and T2 (e.g., Fig. 9
in this paper and trajectories shown in Doran et al. (2007)),
and the modeled winds agree especially well with the data.
The comparisons between measured and modeled winds at
T2 (not shown) gave similar good agreement.
The transport and turbulent mixing of tracers was simu-
lated using the FLEXPART Lagrangian particle dispersion
model (Stohl, 2005). FLEXPART was originally designed to
use meteorological fields produced by global models, but we
adapted it to use meteorological fields produced by WRF and
to have turbulent mixing consistent with mesoscale applica-
In this study, meteorological fields at 30-min intervals
were used to drive FLEXPART. Passive tracer particles were
released to mimic the spatially and temporally varying emis-
sion rates of black carbon. Three types of sources were
defined: 1) anthropogenic black carbon released within
the Mexico Valley (MCMA sources), 2) anthropogenic
black carbon released outside of the Mexico Valley, and 3)
black carbon released from biomass burning sources. An-
I- I )s 0.600
- _ ", .J* 0.400
- _ - fl 0100
Fig. 3. Map of anthropogenic black carbon emission rates used
for simulations. White dots correspond to biomass burning sources
from MODIS during March and the white box outlines the MCMA
source region. Black contour intervals are 200 m.
thropogenic emission rates were estimated using the 1999
National Emission Inventory (NEI; http://www.epa.gov/ttn/
chief/net/1999inventory.html). In the NEI inventory, emis-
sion ratios of BC/PM2.5 averaged over urban and subur-
ban areas of Phoenix, Houston, and Dallas are given as
0.127, 0.222, and 0.124, respectively, with the high value
for Houston perhaps related to the petrochemical facilities
concentrated there. The NEI inventory contains emission
rates of PM2.5 over Mexico (http://www.epa.gov/ttn/chief/
net/mexico.html) that are not speciated, but an investigation
for Mexico City (Miguel Zavala, private communication) has
suggested a value of 0.18 for BC/PM2.5 in that region. We
settled on a value for the black carbon released of 0.15 of
the total PM2.5 mass, which is an intermediate value among
the various estimates we had available. Biomass burning
emission rates were derived from MODIS fire count data and
vegetation type as described by Wiedinmyer et al. (2006).
Biomass burning emission rates varied diurnally with the
lowest values around sunrise and peak values during the late
afternoon. Figure 3 shows a map of the study area with the
anthropogenic emission rates of black carbon for the MCMA
and non-MCMA area as well as the locations of biomass
burning sites from the MODIS data.
The median ages (from the time of release) of the tracer
particles in boxes 5km on a side at T1 and T2 were com-
puted, as were the fractions of the particles in each box
originating from each type of source. The output from the
model was then used to identify and select periods for anal-
ysis, based on factors such as median particle age, fraction
of the EC arising from MCMA sources, and transport of the
Atmos. Chem. Phys., 8, 1377-1389, 2008
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Doran, J. C.; Fast, Jerome D.; Barnard, James C.; Laskin, Alexander; Desyaterik, Yury; Gilles, Marry K. et al. Applications of Lagrangian Dispersion Modeling to the Analysis of Changes in the Specific Absorption of Elemental Carbon, article, March 7, 2008; Richland, Washington. (digital.library.unt.edu/ark:/67531/metadc894697/m1/4/: accessed November 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.