Wet acid deposition episodicity in eastern North America and the influence of deposition episodes on annual deposition amounts

Abstract

A large fraction of acidic pollutant wet deposition during annual or longer time periods is contributed by relatively few events. Wet deposition is, therefore, episodic. In this paper, precipitation chemistry observations between 1979 and 1990 collected by three different daily/event monitoring networks are combined to study the behaviour of this episodicity in eastern North America. The two main factors examined are (1) the spatial pattern in episodicity and its relationship to emissions and mean concentrations and (2) the temporal pattern in episodicity and episode days (events with large depositions) and their influence on annual depositions. On average, ammonium, sulphate and hydronium depositions are found to be the most episodic with 30% of the deposition between 1985 and 1989 coming from 4.5, 5.1 and 5.3% of the events, respectively. Nitrate is the least episodic with 6.9% of the events contributing 30% of the deposition, which is very similar to the episodicity of precipitation. Variability in episodicity among sites is found to exhibit a spatial pattern with more episodic sites located farther from the main pollutant source regions. This leads to relationships between mean pollutant concentration and episodicity across sites since concentrations tend to be lower at sites farther from the sources. There is also a greater de-coupling of precipitation and deposition episodes at sites farther from the main pollutant source regions (i.e. at sites with lower mean concentrations), with deposition episode days contributing a greater percentage of the total precipitation amount at sites with higher mean concentrations and vice versa. Annual wet deposition totals of H + and SO 4 2− tend to be larger in years with a greater frequency of episode days (frequency is defined as the number of episode days in a year divided by the total number of episode days during a site's operation). This suggests, assuming that the occurrence of episodes depends mainly on meteorology, that decreases in annual deposition amounts due to future emission reductions could be offset by increases in the frequency of meteorological conditions conducive to episodes.