Status and trends in atmospheric deposition and emissions near Atlanta, Georgia, 1986–99

Abstract

Wet and dry atmospheric deposition were investigated from weekly data, 1986–99 (1986–97 for dry deposition) at the Panola Mountain Research Watershed (PMRW), a forested research site 25 km, southeast of Atlanta, Georgia. Furthermore, the wet deposition was compared to that at three adjacent National Atmospheric Deposition Program's National Trends Network (NTN) sites (GA41, 50 km south of PMRW; AL99, 175 km northwest; NC25, 175 km north-northeast) and dry deposition was compared to that at adjacent Clean Air Status and Trends Network (CASTNET) sites, co-located at the NTN sites. The pH of precipitation is acidic and the dominant acid anion is SO 4; the pH (derived from the volume-weighted mean H concentration) averages 4.44 for 1986–99, and varies seasonally with average lowest values in summer (4.19) and highest in winter (4.63). From 1986–99, the annual wet deposition of sulfur (S) and nitrogen (N) averaged 400 and 300 eq ha −1 (6.4 and 4.2 kg ha −1), respectively. Inferential model estimates of annual dry S and N deposition from 1986–97 averaged 130 and 150 eq ha −1 (2.1 and 2.1 kg ha −1), respectively. From 1993–99, net S deposition (dry deposition plus canopy interactions) for coniferous and deciduous throughfall (throughfall minus wet-only deposition) averaged 400 and 150 eq ha −1 (6.4 and 2.1 kg ha −1), respectively. The annual wet deposition of S and N species at PMRW was comparable to that at NTN sites, with the exception of higher N species deposition at AL99 and relatively lower H, SO 4 and NO 3 deposition at GA41. Dry S deposition at PMRW differed markedly from the CASTNET sites despite similarity in S concentrations for all but NC25; the differences are attributed to differences in model parameters associated with the landscape and vegetation characteristics at the sites. At PMRW, atmospheric deposition trends were not detected for the entire sampling period, but were detected for shorter periods (4–5 yr). Annual S and N deposition increased from 1986 to 1991, decreased to 1995 and then increased to 1999. SO 2 emissions from seven major point sources within 120 km of PMRW decreased markedly from the late 1980s to 1995 and have remained relatively constant or increased slightly from 1995 to 1998 Annual wet and dry S deposition at PMRW significantly correlates ( p<0.01) with SO 2 emissions, and the correlation is dominated by the large SO 2 emissions decreases in the early 1990s, consistent with the implementation of Phase I of Title IV of the 1990 Clean Air Act Amendments.