Applications of lagrangian dispersion modeling to the analysis of changes in the specific absorption of elemental carbon J. C. Doran1, J. D. Fast1, J. C. Barnard1, A. Laskin1, Y. Desyaterik1, and M. K. Gilles2 1Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington, 99354, USA 2Lawrence Berkeley National Laboratory, Berkeley, CA, USA
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 experiment, which was a
component of the MILAGRO campaign. 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, T1 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, αABS,
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 T1 and T2 are substantially larger during the day than at night.
Values of αABS 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 scanning electron microscopy images of aerosols collected at the
sampling sites. During daylight hours the values of αABS 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 T1 to T2 but the statistical significance of that result is not strong.
Citation: Doran, J. C., Fast, J. D., Barnard, J. C., Laskin, A., Desyaterik, Y., and Gilles, M. K.: Applications of lagrangian dispersion modeling to the analysis of changes in the specific absorption of elemental carbon, Atmos. Chem. Phys., 8, 1377-1389, doi:10.5194/acp-8-1377-2008, 2008.