Atmospheric Chemistry and Physics
www.atmos-chem-phys.net
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9
1
2009
10.5194/acp-9-189-2009
http://www.atmos-chem-phys.net/9/189/2009/
http://www.atmos-chem-phys.net/9/189/2009/acp-9-189-2009.html
http://www.atmos-chem-phys.net/9/189/2009/acp-9-189-2009.pdf
189
206
2009-01-13
Measurements of aerosol absorption and scattering in the Mexico City Metropolitan Area during the MILAGRO field campaign: a comparison of results from the T0 and T1 sites
N. A. Marley
J. S. Gaffney
jsgaffney@ualr.edu
T. Castro
A. Salcido
J. Frederick
Graduate Institute of Technology, University of Arkansas at Little Rock, Little Rock, AR, USA
Department of Chemistry, University of Arkansas at Little Rock, Little Rock, AR, USA
Centro de Ciencias de la Atmósfera,Universidad Nacional Autónoma de México, México City, México
Instituto de Investigaciones Eléctricas, Gerencia de Sistemas de Calidad Ambiente y Seguridad, Cuernavaca, Morelos, México
Department of Geophysical Sciences, The University of Chicago, Chicago, IL, USA
In March 2006, a multiagency field campaign was undertaken in Mexico City
called the Megacities Initiative: Local and Global Research Observations
(MILAGRO). Two of the five field components of the MILAGRO study focused a
major part of their efforts on atmospheric particulate emissions from the
Mexico City basin and their effects on radiative balance as a function of
time, location and processing conditions. As part of these two MILAGRO
components, measurements of aerosol optical properties were obtained at a
site located in the northern part of Mexico City (T0) and also at a site
located 29 km northwest (T1) to estimate the regional effects of
aerosol emissions from the basin.
<br><br>
Measurements of aerosol absorption and scattering for fine mode aerosols
were obtained at both sites. Aerosol absorption at 550 nm was similar at
both sites, ranging from 7–107 Mm<sup>−1</sup> at T0 and from 3–147 Mm<sup>−1</sup> at T1. Aerosol scattering measured at 550 nm at T0 ranged from
16–344 Mm<sup>−1</sup> while the aerosol scattering values at T1 were much lower
than at T0 ranging from 2–136 Mm<sup>−1</sup>. Aerosol single scattering albedos
(SSAs) were calculated at 550 nm for the fine mode aerosols at both sites
using these data. The SSAs at T0 ranged from 0.47–0.92 while SSAs at T1
ranged from 0.35–0.86. The presence of these highly absorbing fine aerosols
in the lower atmosphere of the Mexico City area will result in a positive
climate forcing and a local warming of the boundary layer in the region.
<br><br>
Broadband UVB intensity was found to be higher at site T0, with an average
of 64 μW/cm<sup>2</sup> at solar noon, than at site T1, which had an average
of 54 μW/cm<sup>2</sup> at solar noon. Comparisons of clear-sky modeled UVB
intensities with the simultaneous UVB measurements obtained at sites T0 and
T1 for cloudless days indicate a larger diffuse radiation field at site T0
than at site T1. The determination of aerosol Ã…ngstrom scattering
coefficients at T0 suggests that this is due to the predominance of aerosols
in the size range of 0.3 micron, which leads to scattering of UVB radiation
peaked in the forward direction and to an enhanced UVB radiation observed at
ground level. This enhancement of the UVB diffuse radiation field would
explain the enhanced photochemistry observed in the Mexico City area despite
the reduction in UVB anticipated from light absorbing species.
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