Estimates of sulfate aerosol wet scavenging coefficient for locations in the Eastern United States

Scavenging of atmospheric aerosols by falling precipitation is a major removal mechanism for airborne particles. The process can be described by a wet scavenging coefficient (WSC), denoted L, that is dependent on the rainfall rate, R, and the collision efficiency between raindrops and aerosol particles, E. We report bulk average L values for location in the Eastern United States, estimated based on sulfate mass balance in the atmospheric domain of interest. Data used are taken from several observational networks: (a) the Atmospheric Integrated Research Monitoring Network (AIRMoN) which is part of the National Atmospheric Deposition Program/National Trends Network (NADP/NTN); (b) the Interagency Monitoring of Protected Visibility Environments (IMPROVE); and (c) the National Climatic Data Center (NCDC). The results are fitted relatively well by L values computed using a microphysical representation of the WSC process based on collision efficiency and precipitation size distribution. Such representation leads to a simple expression L=f(R) for soluble aerosols, suitable for WSC description in regional scale models. The agreement between the bulk method and the microphysical representation is due in part to the predominant widespread precipitation, well represented by Marshall and Palmer raindrop distribution, and in part due to assumptions made in the bulk model. Results indicate that high-resolution rainfall rates and realistic vertical cloud structure information are needed to improve the accuracy of aerosol wet scavenging modeling for pollution studies.