CASTNET Monitoring in Sensitive Ecosystems

摘要

In existence since 1991, EPA’s Clean Air Status and Trends Network (CASTNET) provides a nationwide, long-term monitoring platform designed to estimate dry atmospheric deposition. It was created to answer the mandate of the Clean Air Act Amendments for long-term monitoring of 1990 and incorporated the approximately 50 sites that made up EPA’s National Dry Deposition Network (NDDN), which began operation in 1987. Since 1991, many sites have been added to the network, frequently through partnerships with other organizations such as the National Park Service (NPS). Currently, there are 85 CASTNET sites across the United States. The CASTNET database includes dry deposition data for a seventeen year period, 1987 through 2003. The values are produced using the Multi-layer Model (MLM), which estimates deposition velocity based on meteorological and site vegetation profile inputs. The product of deposition velocities and atmospheric concentrations is then calculated to provide an estimate of dry deposition flux. CASTNET data can be obtained from the EPA CASTNET data access web. CASTNET includes numerous sites in sensitive ecosystems such as the southern Appalachian Mountains and coastal/estuarine environments along the east coast of the United States. This paper presents analyses of data from these sites, which can be used to address the issue of critical loads to these ecosystems. The southern Appalachian group includes two sites in Great Smoky Mountains National Park: a standard CASTNET site and a specialized site designed to measure the contribution of cloud water impaction to total deposition at high elevations. Data from the cloud water site show that total deposition to high elevation sites is approximately 4 to 5 times higher than levels at lower elevations with the difference due presumably to cloud water deposition. High elevation ecosystems are often sensitive to deposition because they lack the ability to neutralize and/or assimilate atmospheric inputs. CASTNET sites along the east coast of the United States, including 3 sites representing estuaries in EPA’s National Estuary Program (NEP), show significant differences in deposition with higher levels in the mid-Atlantic region. At the site at Florida’s Indian River Lagoon, modeling has shown that atmospheric deposition contributes approximately 10% of the total nitrogen load to the lagoon, which can lead to nutrient enrichment and eutrophication of sensitive water bodies.