Composition and Changes in Atmospheric Deposition near Atlanta, Georgia, 1986-99
Peters, Norman E.
Aulenbach, Brent T.
Meyers, Tilden P.
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Trends in dry and wet deposition were investigated using data from a weekly sampling network at the Panola Mountain Research Watershed (PMRW), a forested research site 25 km, southeast of Atlanta, Georgia. Wet and dry atmospheric deposition has been monitored using various methods at PMRW since 1985, as a site of National Oceanic and Atmospheric Administration (NOAA) Atmospheric Integrated Monitoring Network (AIRMoN-dry) and following protocols of the National Trends Network (NTN). These data were compared for overlapping collection periods and analyzed for temporal trends. 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 230 and 160 eq ha⁻¹ (3.7 and 2.2 kg ha⁻¹), respectively. From 1993-99, net S deposition (dry deposition plus canopy interactions) for coniferous and deciduous throughfall (throughfall minus precipitation) averaged 400 and 150 eq ha⁻¹ (6.8 and 2.3 kg ha⁻¹), respectively. The pH of precipitation is acidic, the volume weighted mean (VWM) pH (from H concentration) is 4.44 for 1986-99. Coniferous throughfall also is acidic having an annual volume-weighted mean VWM pH of 4.42 from 1993-99. The inferential model estimates are within this range and the variation in net S deposition of throughfall is attributed to variations in the leaf area index above the collectors and the presentativeness of the collectors of throughfall for each canopy type. Temporal variations in precipitation SO₄concentrations are similar to the atmospheric SO₄concentrations, and are highest in summer and lowest in winter. In contrast, atmospheric SO₂concentrations are negatively correlated with the atmospheric SO₄concentrations. Atmospheric deposition trends were not detected for the entire sampling period, but were detected for shorter periods (four to five year). Annual S and N deposition increased from 1986 to 1990, decreased from 1991 to 1994, and increased from 1995 to 1999. The recent S and N deposition increase does not reflect the expected emission reductions associated with the January 1, 1995, implementation of Phase I of Title IV of the 1990 Clean Air Act Amendments.