The wavelength dependence of fine aerosol absorption was obtained in Mexico City in April 2003 during the MCMA 2003 field study and in March 2006 during the MAX-Mex component of the MILAGRO campaign with a 7-wavelength Aethalometer (Magee Model AE3). These results have indicated an enhanced UV absorption over that expected from black carbon aerosols resulting in Angstrom absorption coefficients ranging from -0.7 to –1.5. Samples of fine atmospheric aerosols (<0.1micron) were also collected in Mexico City on quartz fiber filters with high volume impactors. Diffuse reflectance infrared spectra have been obtained on the samples to identify the organic components responsible for the variations in aerosol absorption wavelength dependence observed in the field. Attenuated total reflectance spectra have also been obtained on the water-soluble aerosol fraction to determine the contribution of humic-like substances (HULIS) to the aerosol absorption profiles. These results also serve as an indication of the longwave radiative forcing potential of these fine aerosol species.

This work was conducted as part of the Department of Energy's Atmospheric Science Program as part of the Megacity Aerosol Experiment – Mexico City during MILAGRO. This research was supported by the Office of Science (BER), U.S. Department of Energy Grant No. DE-FG02-07ER64329. We also wish to thank Mexican Scientists and students for their assistance from the Instituto Mexicano de Petroleo (IMP) and CENICA.