The various forms of acidic components that may be deposited from the atmosphere are reviewed. These components are classified into three categories: wet deposition (rain and snow), dry deposition (particles and gases), and special events (dews, frosts, and fogs).

The wet deposition or precipitation component of the acid rain problem is emphasized, since most current monitoring activities are conducted in this area. Types of instruments and methods of collection for wet deposition are surveyed. A synopsis of current acid rain-related monitoring studies in North America is presented, detailing 71 recent-past and current monitoring studies. Tables are presented that describe: the name or title of the study; the organization or agency that funds each study; the chemical parameters monitored; the geographic extent and location of the study; the time period of operation; the types of samplers utilized; where samples are analyzed; and a contact for further information.

The phenomena of dry deposition is discussed with regard to deposition processes and the uncertainties inherent in collection and data interpretation. The acidity related to special events also is considered. Direct acidity associated with dews, frosts, and fogs is reviewed, followed by a discussion of the potential for the combination of dry deposition and subsequent dews or frosts to create strong localized acidities.

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