Acidic deposition distribution and episode statistics from the MAP3S network database

Data for precipitation and acidic pollutant concentration from the MAP3S network in the eastern United States were analyzed to assess the probability distributions of acidic deposition and the spatial and temporal character of high deposition events (or ‘episodes’). About 20% of the precipitation events account for about 55% of the deposition of the major species (sulfur, nitrate, free hydrogen, and ammonium). The mean ‘episodicity’, as defined by Smith and Hunt (Atmospheric Environment12, 461–477, 1978), is 7%; this is not considered highly episodic. The distribution characteristics are spatially quite uniform over the MAP3S region, suggesting a homogeneous pollution-exposure environment. Occurrences of the leading deposition events generally follow the seasonal trends in concentration, and the major episodes usually account for about 10% or less of a given year's total deposition. Most episode events are characterized by above-normal values of both precipitation amount and concentration.