Monday, 14 January 2002: 11:00 AM
A Continental USA Climatology of Sulfate, Nitrate, and Ammonium Scavenging Ratios
The mechanism by which air pollutants affect the biosphere is atmospheric deposition, the transfer of chemicals from the air to surface receptors either living or otherwise. Moreover, it is this deposition that defines the sink term for many airborne trace chemicals. Wet deposition and dry deposition are the dominant pathways, the former associated with precipitation scavenging and the latter with surface gas exchange and particle settling. Models used for scenario development and trend prediction usually rely on the use of a scavenging ratio (Sr) to describe the wet deposition term. Sr is the ratio of concentrations of some chemical species in rain to concentrations of the same (or sometimes precursor) species in air. In the past, related understanding has been limited to either the results of intensive field studies, or of extensive monitoring at specific locations. Here, results from twelve sites spread across the continental USA are described, based on data from the NOAA Atmospheric Integrated Research Monitoring Network (AIRMoN) and the National Atmospheric Deposition Program (NADP). The results show encouraging uniformity among the sites, each of which shows summer maximums and winter minimums. The Sr data are highly variable for any particular sampling period, with a distribution that is closely log-normal (as expected). It is suggested that association between air and precipitation chemistry is sufficiently stochastic as to warrant use of probabilistic measures of deposition, rather than averages.