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 deposition across the United States. Currently, the network operates as a component of the Clean Air Status and Trends Network (CASTNet). Dry deposition is not measured directly in CASTNet, but is determined by an inferential approach (i.e. fluxes are calculated as the product of measured 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 measurements and dry deposition flux calculations is hourly for ozone and weekly for the other species. This paper describes the 50-station CASTNet dry deposition network, discusses dry deposition calculation procedures and presents dry deposition data for sulfur dioxide and nitric acid for 1991. Sources of uncertainty in dry deposition estimates are also discussed.