Trends in cloud water sulfate and nitrate as measured at two mountain sites in the Eastern United States:: Regional contributions and temporal changes compared with regional changes in emissions, 1986–1999

Abstract

Air pollutant emissions in the US generally peaked in 1970, the year that the Clean Air Act was passed, and have declined since, except for oxides of nitrogen (NO x ), which have remained steady or slowly increased. In 1995 Phase 1 of the Clean Air Act Amendments (CAAA) of 1990 went into effect resulting in a sharp drop in sulfur dioxide (SO 2) emissions in most areas. Pollutant concentrations measured in precipitation and ambient air generally reflected the changes in emissions in most areas of the eastern US. Only the southern Appalachian Mountain region did not see an appreciable improvement in precipitation acidity over the last decade. Previous studies of cloud chemistry in the eastern US found no pattern of temporal trends, possibly because of large year-to-year variation in meteorology. In this paper, we present spatial and temporal trends in SO 4 2− and NO 3 − concentrations of cloud water samples collected in warm seasons only at two mountain sites (Whiteface Mountain, NY and Whitetop Mountain, VA). This analysis is based on a combined cloud chemistry data-set collected by EPA's Mountain Cloud Chemistry Program (MCCP) (1986–1989) and Mountain Acid Deposition Program (MADPro) (1994–1999). Sample concentrations were (1) normalized by liquid water content (to reduce within-cloud variation) and (2) segregated into 90° arrival quadrants based on 36 h back trajectory analysis (to diminish between-cloud or meteorological variation). For each quadrant at the two sites, annual (12 month) county emissions of SO 2 and NO x were compiled out to 600, 1000, and1600 km, and these values were compared with the annual means (warm season only) of normalized SO 4 2− and NO 3 − concentrations in hourly samples of cloud water (segregated by back-trajectory) collected at each site. For the period 1987–1999, Quadrant 3 (SW) for Whiteface Mt. and Quadrants 3 (SW) and 4 (NW) for Whitetop Mt. had the highest SO 2 emissions and showed the largest decline in SO 2 emissions after the CAAA was implemented. These same quadrants which had the largest decrease in emissions showed significant declines in cloud water SO 4 2− over the time period. NO x emissions were highest in Quadrant 3 for Whiteface Mt. and in Quadrants 1 and 4 for Whitetop Mt. Only in Quadrant 1 at Whitetop Mt. did NO x emissions decrease during the study period (1987–1999). Cloud water NO 3 − showed no consistent pattern at either mountain site with some quadrants having higher cloud water NO 3 − values after Phase 1 of the CAAA and other quadrants having little change in cloud water NO 3 − values.