A climatology of wet deposition scavenging ratios for the United States

A decade of weekly measurements of precipitation chemistry obtained in the US National Atmospheric Deposition Program (NADP) are combined with air concentration measurements from the Atmospheric Integrated Research Monitoring Network (AIRMoN) to estimate scavenging ratios (of concentrations in rain to like concentrations in surface air) for the ions that dominate precipitation chemistry—sulfate, nitrate, and ammonium. The intention is to provide a climatology of these ratios, for use in evaluating models that are now simulating precipitation chemistry as well as air quality. The scavenging ratios that are computed are found to be distributed approximately log-normally, with a standard deviation corresponding to about a factor of two. That is, if scavenging ratios are used to predict precipitation chemistry and wet deposition, then geometric means should be considered and the error for any given weekly prediction should be expected to be described by a standard deviation of a factor of two. (It follows that for daily predictions, the factor increases to about five.) Scavenging ratios appear to be similar for nitrate and ammonium, with those for sulfate being somewhat larger and displaying a behavior indicative of the contribution of in-cloud conversion from gaseous sulfur dioxide. Seasonal cycles are similar for most locations. The limited data available for western US stations indicate, however, that these locations yield scavenging ratios that can be significantly different from those characterizing the east. The data are in accord with the conventional understanding regarding the cloud scavenging of sulfur dioxide. For summer, each sulfur dioxide molecule appears to pass through an average of 1.3 clouds before being scavenged as sulfate. For winter, the average is 4.8 clouds.