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William C. Keene, James N. Galloway, Gene E. Likens, Frank A. Deviney, Kerri N. Mikkelsen, Jennie L. Moody, and John R. Maben

Abstract

Precipitation composition was characterized at 14 remote sites between 65°N and 51°S. Anthropogenic sources contributed to non-sea-salt (nss) SO4 2−, NO3 , and NH4 + in North Atlantic precipitation. Biogenic sources accounted for 0.4–3.3 μeq L−1 of volume-weighted-average (VWA) nss SO4 2− in marine precipitation. SO4 2− at the continental sites (2.9–7.7 μeq L−1) was generally higher. VWA NO3 (0.5–1.3 μeq L−1) and NH4 + (0.5–2.6 μeq L−1) at marine-influenced, Southern Hemispheric sites were generally less than those at continental sites (1.4–4.8 μeq L−1 and 2.3–4.2 μeq L−1, respectively). VWA pH ranged from 4.69 to 5.25. Excluding the North Atlantic, nss SO4 2−, NO3 , and NH4 + wet depositions were factors of 4–47, 5–61, and 3–39, respectively, less than those in the eastern United States during 2002–04. HCOOHt (HCOOHaq + HCOO) and CH3COOHt (CH3COOHaq + CH3COO) concentrations and depositions at marine sites overlapped, implying spatially similar source strengths from marine-derived precursors. Greater variability at continental sites suggests heterogeneity in terrestrial source strengths. Seasonality in deposition was driven by variability in precipitation amount, wind velocity, transport, and emissions. Between 1980 and 2009, nss SO4 2− at Bermuda decreased by 85% in response to decreasing U.S. SO2 emissions; trends in NO3 and NH4 + were inconsequential. Corresponding decreases in acidity, as reflected in the significant 30% decline in VWA H+, impacted pH-dependent chemical processes. Comparisons between measurements and models indicate that current predictive capabilities are uncertain by factors of 2 or more.

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