Ultra-fine particles (UFP) or aerosols with diameter d(p) < 100 nm are produced in the atmospheric boundary layer (BL) by nucleation or by direct emission from various sources. This study describes a method to calculate the UFP below-cloud scavenging coefficient (L), applicable in numerical modeling of environmental aerosols under rainy conditions. Results illustrate the UFP scavenging dependence on aerosol diameter, rainfall intensity and average electric charge. The electric charge effect on UFP scavenging is treated for strongly electrified clouds and the results are applicable to rain events associated mainly with thunderstorms. It is shown that for the UFP diameter range, L decreases significantly with aerosol diameter, increases with the rainfall rate R, and increases with the average raindrop and aerosol electric charge. Model estimations of the UFP 0.5-folding time and the fraction scavenged under a wide range of conditions are presented and discussed. (C) 2004 Elsevier Ltd. All rights reserved.