Megacities are very important sources of fine aerosols and trace gases that can influence the radiative balance of the atmosphere on urban, regional, and global scales. Mexico City, one of the world’s largest megacities, is known for high levels of air pollution due to large emissions and topography. In April 2003, as part of the Mexico City Metropolitan Area 2003 air quality study, we measured the abundance of black carbon at time resolution of 2–5 min by using a seven-channel aethalometer (Thermo Anderson). Data were collected before and during the Easter holiday to assess changes in aerosol loadings as a function of vehicle traffic levels. We also took measurements in Chicago, Illinois, before, during, and after the July 4 holiday weekend in 2004 to examine diesel soot inputs from city traffic and from construction activity on the campus of the University of Chicago and on Lake Shore Drive. The data clearly indicate the presence of black carbon emitted from uncontrolled heavy diesel engines. Data for the two cities will be compared and discussed in light of potential impacts of black carbon on urban heating, air quality, and regional and global radiative balance.

This work was supported by the DOE Atmospheric Science Program and the EPA as part of the Center for Environmental Science.