Megacities are important sources of fine aerosols and trace gases. These emissions can have significant roles in determining the radiative balance of urban, regional, and global-scale atmospheres. Black carbon is an important aerosol contributor to radiative effects on regional and global scales. One of the major sources of black carbon is the diesel engines used in light- and heavy-duty vehicles. We will compare our data for Chicago and Mexico City with results reported by other workers for other megacity environments. The potential importance of megacities as sources of black carbon will be discussed, and the implications for effects on both the shortwave and the longwave radiative balance will be reviewed.

This work was supported by the DOE Atmospheric Science Program.