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ESTIMATES OF THE ATMOSPHERIC DEPOSITION OF SULFUR AND NITROGEN SPECIES: CLEAN AIR STATUS AND TRENDS NETWORK. 1990 THROUGH 2000
Citation:
Baumgardner, R E., S. Isil, T. L. Lavery, AND C. Rogers. ESTIMATES OF THE ATMOSPHERIC DEPOSITION OF SULFUR AND NITROGEN SPECIES: CLEAN AIR STATUS AND TRENDS NETWORK. 1990 THROUGH 2000. ENVIRONMENTAL SCIENCE & TECHNOLOGY 36(12):2614-2629, (2002).
Impact/Purpose:
Our main objective is to assess the exposure of selected ecosystems to specific atmospheric stressors. More precisely, we will analyze and interpret environmental quality (primarily atmospheric) data to document observable changes in environmental stressors that may be associated with legislatively-mandated emissions reductions.
Description:
The Clean Air Status and Trends Network (CASTNet) was established by EPA in response to the requirements of the 1990 Clean Air Act Amendments. To satisfy these requirements CASTNet was designed to assess and report on geographic patterns and long-term, temporal trends in ambient air pollution and acid deposition in order to gauge the effectiveness of current and future mandated emission reductions. This paper presents an analysis of the spatial patterns of deposition of sulfur and nitrogen pollutants for the period 1990 through 2000. Estimates of deposition are provided for two four-year periods: 1990-1993 and 1997-2000.
These two periods were selected to contrast deposition before and after the large decrease in S02 emissions that occurred in 1995.
Estimates of dry deposition were obtained from measurements at CASTNet sites combined with deposition velocities that were modeled using the Multi-Layer Model (MLM), a 20-layer model that simulates the various atmospheric processes that contribute to dry deposition. Estimates of wet deposition were obtained from measurements at sites operated by the National Atmospheric Deposition Program (NADP). The estimates of dry and wet deposition were combined to calculate total deposition of atmospheric sulfur (dry S02, dry and Wet S04) and nitrogen (dry HN03, dry and wet N03, dry and wet NH4+)-
An analysis of the deposition estimates showed a significant decline in sulfur deposition and no change in nitrogen deposition. The highest rates of sulfur deposition were observed in the Ohio River Valley and downwind states. This region also observed the largest decline in sulfur deposition. The highest rates of nitrogen deposition were observed in the Midwest from Illinois to southern New York State. Sulfur and nitrogen deposition fluxes were significantly higher in the eastern U.S. as compared to the western sites. Dry deposition contributed approximately 3 8% of total sulfur deposition and 3 0% of total nitrogen deposition in the eastern U. S. Percentages are similar for the two four-year periods. Wet sulfate and dry S02 depositions were the largest contributors to sulfur deposition. Wet nitrate, wet ammonium, and dry HN03 depositions were the largest contributors to nitrogen deposition.