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

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 (NOx), 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 (SO2) 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 SO42− and NO3− 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 SO2 and NOx were compiled out to 600, 1000, and1600 km, and these values were compared with the annual means (warm season only) of normalized SO42− and NO3− 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 SO2 emissions and showed the largest decline in SO2 emissions after the CAAA was implemented. These same quadrants which had the largest decrease in emissions showed significant declines in cloud water SO42− over the time period. NOx 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 NOx emissions decrease during the study period (1987–1999). Cloud water NO3− showed no consistent pattern at either mountain site with some quadrants having higher cloud water NO3− values after Phase 1 of the CAAA and other quadrants having little change in cloud water NO3− values.