Bulk or modal parameterizations for below‐cloud scavenging of fine, coarse, and giant particles by both rain and snow

Abstract

AbstractBulk or modal parameterizations for below‐cloud mass and number scavenging coefficients Λm (s−1) and Λn (s−1) of three aerosol modes—fine (PM2.5), coarse (PM2.5–10), and giant (PM10+)—for both rain and snow scavenging are developed for use in modal‐approach aerosol transport models. The new bulk parameterizations are based on the size‐resolved Λ(d) parameterization of Wang et al. (2014), using assumed lognormal mass and number size distributions for PM2.5, PM2.5–10, and PM10+. The resulting modal‐mean formulas for Λm and Λn follow power law relationships with precipitation intensity R, consistent with most existing studies. The empirical parameters in the power law relationships obtained in this study are also within the range of parameter values obtained in previous field and theoretical studies. Uncertainties in Λm due to the size distribution or size range assumed for each aerosol mode are generally smaller than 30% for PM2.5–10 and PM10+ but could be on the order of factor of 2 for PM2.5. These uncertainties, however, are much smaller than other known uncertainties in existing Λ formulations, which are typically larger than 1 order of magnitude. Moreover, the new bulk parameterizations are believed to be more representative than most existing schemes because the size‐resolved parameterization of Wang et al. (2014), which they are based on, was developed with consideration of all available theoretical formulations and field‐derived estimates for size‐resolved Λ and their associated uncertainties.