Abstract
Many urban regions around the US are plagued by problems caused by photochemical smog or ground-level ozone. Reductions in the amounts of sulfur and hydrocarbons emitted to the atmosphere may provide unexpected air quality benefits. Sulfate and certain organic aerosol particles are known to scatter solar radiation back into space, leading to local cooling. These pollutants can also alter the actinic flux of UV radiation. Results from monitoring programs and numerical modeling studies suggest that UV-scattering particles in the boundary layer may accelerate photochemical reactions and the production of smog. However, UV-absorbing aerosols (e.g., mineral dust and soot) may actually control the production of smog. The implications of these findings for air pollution control efforts are analyzed.