A parameter called the scavenging coefficient Λ is widely used in aerosol chemical transport models (CTMs) to describe below-cloud scavenging of aerosol particles by rain and snow. However, uncertainties associated with available size-resolved theoretical formulations for Λ span one to two orders of magnitude for rain scavenging and nearly three orders of magnitude for snow scavenging. Two recent reviews of below-cloud scavenging of size-resolved particles recommended that the upper range of the available theoretical formulations for Λ should be used in CTMs based on uncertainty analyses and comparison with limited field experiments. Following this recommended approach, a new semi-empirical parameterization for size-resolved Λ has been developed for below-cloud scavenging of atmospheric aerosol particles by both rain (Λ<sub>rain</sub>) and snow (Λ<sub>snow</sub>). The new parameterization is based on the 90th percentile of Λ values from an ensemble data set calculated using all possible "realizations" of available theoretical Λ formulas and covering a large range of aerosol particle sizes and precipitation intensities (<i>R</i>). For any aerosol particle size of diameter <i>d</i>, a strong linear relationship between the 90th-percentile log<sub>10</sub> (Λ) and log<sub>10</sub> (<i>R</i>), which is equivalent to a power-law relationship between Λ and <i>R</i>, is identified. The log-linear relationship, which is characterized by two parameters (slope and <i>y</i> intercept), is then further parameterized by fitting these two parameters as polynomial functions of aerosol size <i>d</i>. A comparison of the new parameterization with limited measurements in the literature in terms of the magnitude of Λ and the relative magnitudes of Λ<sub>rain</sub> and Λ<sub>snow</sub> suggests that it is a reasonable approximation. Advantages of this new semi-empirical parameterization compared to traditional theoretical formulations for Λ include its applicability to below-cloud scavenging by both rain and snow over a wide range of particle sizes and precipitation intensities, ease of implementation in any CTM with a representation of size-distributed particulate matter, and a known representativeness, based on the consideration in its development, of all available theoretical formulations and field-derived estimates for Λ (<i>d</i>) and their associated uncertainties.