Measurements of aerosol absorption and scattering were obtained in Mexico City as part of the Megacity Aerosol Experiment: Mexico City (MAX-Mex) component of the MILAGRO (Megacity Initiative: Local and Global Research Observations) field campaign during March 2006. A comparison of these measurements will be presented for site T0 located in the northern part of Mexico City at the IMP (Instituto Mexicano de Petroleo) Laboratories and for site T1 located at the Technical University of Tecamac, 18 miles northwest of T0. Hourly averages of aerosol scattering at T0 ranged from 27-344 with an average of 128 Mm-1 while the scattering values at T1 were in general much lower ranging from 6-136 with an average of 62 Mm-1. However, except for periodic impacts of local emissions at T1, the values of aerosol absorption were very similar at both sites ranging from 6 – 93 Mm-1 with an average of 31 Mm-1 at T0 and from 2 – 160 with an average of 27 Mm-1 at T1. The aerosol single scattering albedo at T1 ranged from 0.35-0.87 with an average of 0.69 compared to that at T0 of 0.57-0.93 with an average of 0.77. These results are discussed along with the impacts of transport and rain events on the overall aerosol optical properties in the area.

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 and Mexican Government through FOSEMARNAT-2004-01-116. We also wish to thank Mexican Scientists and students for their assistance from the Instituto Mexicano de Petroleo (IMP) and CENICA in particular to M. I. Saavedra from CCA, UNAM.