Wet sulfate and nitrate deposition patterns in eastern North America

Abstract

Four year (1982–1985) averaged acid deposition (sulfate and nitrate ion) data obtained at 77 stations of the Acid Deposition System in Eastern North America (ENA) are analyzed for seasonal and spatial variations. Average sulfate concentrations over most of ENA range from 1.5–3 mg l −1. Highest sulfate depositions (>35kg ha −1y −1) occur over west Pennsylvania, the Virginias, east Michigan, and southern Ontario between Lakes Huron and Erie. Sulfate isopleths generally stretch along a SW-NE axis. In the northeast quadrant of ENA, 65–70% of the annual sulfate depositions occur in the April–September half year; some regions in southern Canada experience as much as 75% in that period. Average nitrate concentrations range from 1–2mg l −1 throughout much of ENA. Peak nitrate depositions (>20kg ha −1 y −1) fall between Lakes Michigan and Ontario; 15 kg ha −1y −1 or more cover most of the Midwest, New York, New England and south Ontario. Nitrate depositions are more evenly distributed throughout the year, with 55–65% occurring in the April-September half year. The average annual molar ratio of SO 4 2− to NO 3 − varies from 0.8–1.5 over ENA. The ratio is highest at south-central receptors and lower at northern latitudes. Implications of these findings on strategies to mitigate acid deposition are discussed.