Below-cloud rain scavenging of atmospheric aerosols for aerosol deposition models

Abstract

Below-cloud aerosol scavenging is generally estimated from field measurements using advanced instruments that measure changes in aerosol distributions with respect to rainfall. In this study, we discuss various scavenging mechanisms and scavenging coefficients from past laboratory and field measurements. Scavenging coefficients derived from field measurements (representing natural aerosols scavenging) are two orders higher than that of theoretical ones for smaller particles (Dp<2μm). Measured size-resolved scavenging coefficients can be served as a better option to the default scavenging coefficient (e.g. a constant of 10−4s−1 for all size of aerosols, as used in the CALPUFF model) for representing below-cloud aerosol scavenging. We propose scavenging correction parameter (CR) as an exponential function of size-resolved scavenging coefficients, winds and width in the downwind of the source–receptor system. For a wind speed of 3ms−1, CR decrease with the width in the downwind for particles of diameters Dp<0.1μm but CR does not vary much for particles in the accumulation mode (0.1<Dp<2μm). For a typical urban aerosol distribution, assuming 3ms−1 air-flow in the source–receptor system, 10km downwind width, 2.84mmh−1 of rainfall and using aerosol size dependent scavenging coefficients in the CR, scavenging of aerosols is found to be 16% in number and 24% in volume of total aerosols. Using the default scavenging coefficient (10−4s−1) in the CALPUFF model, it is found to be 64% in both number and volume of total aerosols.