CASTNET methodology for modeling dry and total deposition

Abstract

Gaseous and particulate pollutants are deposited to the environment through dry, wet, and occult atmospheric processes. The US Environmental Protection Agency (USEPA), in conjunction with the National Park Service (NPS), the Bureau of Land Management (BLM), and other partners, has established the Clean Air Status and Trends Network (CASTNET) to provide estimates of the dry deposition component of total deposition of sulfur and inorganic nitrogen across the United States. CASTNET began operation in 1991 and currently features 90 active sites with many partners including multiple federal agencies, tribal, state, and local entities, and educational institutions. Most CASTNET locations are rural and intended as long term monitoring sites. Previously, CASTNET used an inferential method to estimate dry deposition by combining measured pollutant concentrations and modeled deposition velocities. Until recently, deposition velocities were modeled using the NOAA/USEPA Multi-layer Model (MLM), which incorporated meteorological measurements and information on the vegetative cover within 1 km of each site. These values were combined with wet deposition values provided by the National Atmospheric Deposition Program’s National Trends Network (NADP/NTN) to obtain total deposition. Recent changes to the methodology have improved data completeness.