Samples of fine atmospheric aerosols (<0.1micron) were collected in Mexico City in April of 2003 during the MCMA 2003 field campaign and in March 2006 during the MAX-Mex component of the MILAGRO campaign. The samples were collected on quartz fiber filters with high volume impactor samplers. Continuous absorption spectra of the aerosol samples have been obtained in the laboratory from 280 to 900nm with the use of an integrating sphere coupled to a Beckman DU spectrometer. The integrating sphere allows the detector to collect and spatially integrate the total radiant flux reflected from the sample and therefore allows for the measurement of absorption on highly reflective or diffusely scattering samples. The wavelength dependence of the mass specific absorption is obtained from these spectra and total carbon measurements. The wavelength dependence of the aerosol complex refractive index (k) is determined by application of the Kramers Kronig and Kubelka Munk functions. Results from the continuous spectra are compared to absorbance measurements obtained in the field at 7 wavelengths with an Aethalometer (Magee Model AE3). Differences between the field and laboratory measurements are attributed to the presence of multifunctional absorbers (e.g. acyl-, nitro-, etc.) and aromatic compounds that have narrow band absorbances not covered by the 7 wavelengths of the Aethalometer.

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.