Seasonal and regional air quality and atmospheric deposition in the eastern United States

Abstract

The atmospheric concentration, wet deposition, and inferred dry deposition of selected air pollutants reported over two 5‐year periods in the 1990s at or near 34 rural Clean Air Status and Trends Network (CASTNET) sites located in the eastern United States (U.S.) are adjusted for known biases, composed into seasonal values, and examined. Several terms are defined for the current study, where OxN is the measured oxidized nitrogen (i.e., airborne OxN is the sum of airborne HNO3 and NO3−, expressed as nitrogen), NH4 is the measured reduced nitrogen (i.e., airborne NH4 is the aerosol NH4+, expressed as nitrogen), N is the sum of measured oxidized and reduced forms of nitrogen, expressed as nitrogen, and S is the measured oxidized sulfur (i.e., airborne S is the sum of airborne SO2 and SO42−, expressed as sulfur). The atmospheric NH3 concentration is not monitored in the current study. Similar patterns of seasonal and regional behavior are found consistently in both periods. In the east, atmospheric concentration, estimated deposition velocity, precipitation rate, inferred dry deposition, wet deposition, and total (dry plus wet) deposition estimates of each of the monitored chemical constituents display regular seasonal cycles of behavior. High and low seasonal values occur in summer and winter, respectively, for atmospheric concentration and dry deposition of SO42−, NH4+, O3, HNO3, and N; for dry OxN deposition; for wet S and H+ deposition; and for total OxN and N deposition. In contrast, high seasonal values of SO2 concentration and dry deposition, and atmospheric NO3− concentration occur in winter. In the east, SO2 composes a major portion (≈70%) of the atmospheric S concentration and is the dominant (>85%) contributor to dry S deposition. Although aerosol NH4+ represents a major portion of the measured atmospheric N concentration (≈67%), HNO3 dominates estimates of both dry OxN (>90%) and N (>75%) deposition. Dry deposition contributes ≈15%, 38%, and 43% to total deposition of NH4, OxN, and S, and these appear to be conservative estimates. Wet deposition is a major contributor to total deposition, generally peaking in summer or spring. Total S, OxN, and N deposition peak in summer. Although mean O3 concentration is ≈70% larger in summer than winter, dry O3 deposition estimates in the east are >5 times higher in summer. Within the uncertainty of current conservative estimates, dry deposition of SO42−, HNO3, OxN, N, and O3 appears to be highest at the high‐elevation subset of sites. This underscores the potential importance of dry deposition as a stressor to high‐elevation ecosystems in the eastern U.S.