Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 8 3 2008 10.5194/acpd-8-10189-2008 http://www.atmos-chem-phys-discuss.net/8/10189/2008/ http://www.atmos-chem-phys-discuss.net/8/10189/2008/acpd-8-10189-2008.html http://www.atmos-chem-phys-discuss.net/8/10189/2008/acpd-8-10189-2008.pdf 10189 10225 2008-05-30 Estimation of the mass absorption cross section of the organic carbon component of aerosols in the Mexico City Metropolitan Area (MCMA) J. C. Barnard james.barnard@pnl.gov R. Volkamer E. I. Kassianov Pacific Northwest National Laboratory, Richland, WA, USA University of Colorado, Boulder, CO, USA Data taken from the MCMA-2003 and the 2006 MILAGRO field campaigns are used to examine the absorption of solar radiation by the organic component of aerosols. Using irradiance data from a Multi-Filter Rotating Shadowband Radiometer (MFRSR) and an actinic flux spectroradiometer (SR), we derive aerosol single scattering albedo, π_0,λ, as a function of wavelength, λ. We find that in the near-UV spectral range (250 to 400 nm) π_0,λ is much lower compared to π_0,λ at 500 nm indicating enhanced absorption in the near-UV range. Absorption by elemental carbon, dust, or gas cannot account for this enhanced absorption leaving the organic part of the aerosol as the only possible absorber. We use data from a surface deployed Aerodyne Aerosol Mass Spectrometer (AMS) along with the inferred π_0,λ to estimate the Mass Absorption Cross section (MAC) for the organic carbon. We find that the MAC is about 10.5 m²/g at 300 nm and falls close to zero at about 500 nm; values that are roughly consistent with other estimates of organic carbon MAC. These MAC values can be considered as "radiatively correct" because when used in radiative transfer calculations the calculated irradiances/actinic fluxes match those measured at the wavelengths considered here. For an illustrative case study described here, we estimate that the light absorption by the "brown" (organic) carbonaceous aerosol can add about 40% to the light absorption of black carbon in Mexico City. This contribution will vary depending on the relative abundance of organic carbon relative to black carbon. Furthermore, our analysis indicates that organic aerosol would slow down photochemistry by selectively scavenging the light reaching the ground at those wavelengths that drive photochemical reactions. Finally, satellite retrievals of trace gases that are used to infer emissions currently assume that the MAC of organic carbon is zero. 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