Atmospheric aerosols are now known to be important agents in the direct radiative forcing for both short- and long-wave radiation. Important factors for aerosol radiative forcing are the scattering and absorption of radiation as well as their atmospheric lifetimes. The lifetimes will depend upon the surface chemistries of the aerosols and their hygroscopicity. Measurements of light scattering and absorption were taken in Mexico City during April of 2003 and March of 2006 as part of the Department of Energy's investigations into megacities as major sources of important aerosols that would have impacts on regional and global scales. This work reviews the findings of these events and indicates that their are substantial amounts of fresh black carbon species that are not readily washed out below cloud nor grow to substantial sizes to be lost by deposition. The implications of these results will be discussed in terms of the aerosol radiative properties and their changes during one of these events.

This work was performed as part of the Department of Energy's Megacity Aerosol Experiment - Mexico City under the support of the Atmospheric Science Program.