ABSTRACT
Acid rain is the general and now popular term that pertains to both acid rain and acid snow. This article discusses the physical and chemical aspects of the acid rain phenomenon, presents results from a U.S. monitoring network to illustrate spatial and seasonal variability, and discusses time trends of acid rain during recent decades. A chemical equilibrium model is presented to emphasize that one cannot measure only pH and then expect to understand why a particular rain or melted snow sample is acidic or basic. Monitoring networks are now in operation to characterize the time trends and spatial patterns of acid rain. Definitions, procedures, and results from such measurement programs are discussed. The monitoring results are necessary to assess the effects of acid rain on the environment, a topic only briefly discussed in this article.
