Applications of Lagrangian Dispersion Modeling to the Analysis of Changes in the Specific Absorption of Elemental Carbon Page: 1 of 13
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Atmos. Chem. Phys., 8, 1377-1389, 2008
Author(s) 2008. This work is distributed under
the Creative Commons Attribution 3.0 License.
_ and Physics
Applications of lagrangian dispersion modeling to the analysis of
changes in the specific absorption of elemental carbon
J. C. Doran', J. D. Fast', J. C. Barnard', A. Laskin', Y. Desyaterikl, and M. K. Gilles2
'Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington, 99354, USA
2Lawrence Berkeley National Laboratory, Berkeley, CA, USA
Received: 25 September 2007 - Published in Atmos. Chem. Phys. Discuss.: 19 October 2007
Revised: 17 January 2008 - Accepted: 12 February 2008 - Published: 7 March 2008
Abstract. We use a Lagrangian dispersion model driven by
a mesoscale model with four-dimensional data assimilation
to simulate the dispersion of elemental carbon (EC) over a
region encompassing Mexico City and its surroundings. The
region was the study domain for the 2006 MAX-MEX ex-
periment, which was a component of the MILAGRO cam-
paign. The results are used to identify periods when biomass
burning was likely to have had a significant impact on the
concentrations of elemental carbon at two sites, Ti and T2,
downwind of the city, and when emissions from the Mexico
City Metropolitan Area (MCMA) were likely to have been
more important. They are also used to estimate the median
ages of EC affecting the specific absorption of light, aABS, at
870 nm as well as to identify periods when the urban plume
from the MCMA was likely to have been advected over T1
and T2. Median EC ages at Ti and T2 are substantially
larger during the day than at night. Values of aABS at T1,
the nearer of the two sites to Mexico City, were smaller at
night and increased rapidly after mid-morning, peaking in
the mid-afternoon. The behavior is attributed to the coating
of aerosols with substances such as sulfate or organic carbon
during daylight hours, but such coating appears to be limited
or absent at night. Evidence for this is provided by scan-
ning electron microscopy images of aerosols collected at the
sampling sites. During daylight hours the values of aABS did
not increase with aerosol age for median ages in the range
of 1-4 h. There is some evidence for absorption increasing
as aerosols were advected from Ti to T2 but the statistical
significance of that result is not strong.
Qc J Correspondence to: J. D. Fast
It is generally acknowledged that as soot particles age, their
mixing state evolves from an external to an internal one
through processes such as heterocoagulation or condensation
(e.g., Jacobson and Seinfeld, 2004), and the specific absorp-
tion aABS (absorption per unit mass) will increase (Fuller
et al., 1999; Bond et al., 2006.) It is important to specify
appropriate values of aABS for use in global climate mod-
els, but there is still considerable variation in the values that
have been proposed. Bond and Bergstom (2006) concluded
that aABS for freshly emitted soot particles has a value of
7.5+ 1.2m2 g-1 at 550 nm, but direct measurements in the
field of the rates and effects of aging and coating have been
difficult to achieve.
One approach has been to make comparisons of optical
properties of soot at a single site at two different times of
day, e.g., in the early morning when fresh emissions from
traffic are likely to dominate the aerosol mix, and later in the
afternoon when a mixture of fresh and more aged particles
is likely to be present. The sources and ages of the various
components of the mixture may vary widely, however, and
efforts to explicitly characterize the ages of the mixture are
not usually made. Several investigations of this type have
been done in the Mexico City region, which is the location
of interest for this study. Johnson et al. (2005) showed that
soot can undergo significant coating and evolve into an inter-
nally mixed state in only a few hours. In contrast, Mallet et
al. (2004) concluded that single scattering albedo measure-
ments in an industrial region of France were more consistent
with externally mixed aerosols rather than internally mixed
ones. Baumgardner et al. (2000) measured time series of size
distribution, scattering and absorption coefficients, and bulk
chemical composition of aerosols at a mountain location near
Mexico City and related the variations to the meteorological
conditions. Baumgardner et al. (2007) measured variations
in aABS and in the coating of light absorbing carbon (LAC)
Published by Copernicus Publications on behalf of the European Geosciences Union.
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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/m1/1/: accessed September 25, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.