DRY DEPOSITION CALCULATIONS FOR THE CLEAN-AIR STATUS AND TRENDS NETWORK

The National Dry Deposition Network (NDDN) was established in 1986 to document the magnitude, spatial variability, and trends in dry deposit ion across the United States. Currently, the network operates as a com ponent of the Clean Air Status and Trends Network (CASTNet). Dry depos ition is not measured directly in CASTNet, but is determined by an inf erential approach (i.e. fluxes are calculated as the product bf measur ed ambient concentration and a modeled deposition velocity). Chemical species include ozone, sulfate, nitrate, ammonium, sulfur dioxide, and nitric acid. The temporal resolution for the ambient concentration me asurements and dry deposition flux calculations is hourly for ozone an d weekly for the other species. This paper describes the 50-station CA STNet dry deposition network, discusses dry deposition calculation pro cedures and presents dry deposition data for sulfur dioxide and nitric acid for 1991. Sources of uncertainty in dry deposition estimates are also discussed. (C) 1997 Elsevier Science Ltd.