There is little doubt that acids deposited from the troposphere contribute to the corrosion of outdoor equipment, buildings and works of art, particularly in urban areas-costing tens of billions of dollars each year for repairs and equipment replacement in the U.S alone. Particles containing sulfate (SO42) have other effects as well. By scattering light efficiently, they decrease visibility by influencing cloud albedo, they may have important implications for climate [see "The Changing Climate," by Stephen H. Schneider, page 38].