Atmospheric Environment 39 (2005) 6608–6619 Study of scavenging of submicron-sized aerosol particles by thunderstorm rain events D.M. Chate à Indian Institute of Tropical Meteorology, Pune-411 008, India Received 11 February 2005; accepted 25 July 2005 Abstract Observed scavenging coefficients for 0.013–0.75 mm particles are between 1.08  10 5 and 7.58  10 4 s 1 . Based on observed results a correlation between scavenging coefficient and rain intensity is obtained to study below thundercloud scavenging of atmospheric aerosols during thunderstorm rain events. When the rain intensity increases from 5.24 to 45.54mmh 1 , the corresponding scavenging coefficient increases from 0.5  10 5 to 4  10 5 s 1 for thunderstorm rain episodes. The overall scavenging coefficients for 0.02 – 10 mm particles at different rainfall rates are estimated from contributions of Brownian diffusion, directional interception, inertial impaction, thermophoresis, diffusiophoresis and electrical forces during thunderstorms. The evolutions of PSD are predicted at different time intervals with theoretical scavenging rates. Comparison of observed evolutions of PSD during thunderstorm rain events with predicted evolutions of PSD shows an order of discrepancy between the observed and model results. Possible causes for discrepancy are discussed in terms of uncertainties in observed data and shortcomings in theoretical approach. The present results are useful for recommendations for the type of experimental setup essential for the field study of precipitation scavenging and improvements in theoretical approach close to atmospheric conditions during thunderstorm rain events. r 2005 Elsevier Ltd. All rights reserved. Keywords: Wet scavenging; Electrical scavenging mechanism; Diffusiophoresis; Thermophoresis; Directional interception 1. Introduction A significant fraction of the earth’s rainfall in temperate climates comes from thunderstorms. Rainfall associated with electrified and lightning-producing storms plays an important role in the natural washout of atmospheric particles. Raindrops falling through the cloud of aerosol particles collect a fraction of these particles in their path. The fraction of particles in the cylindrical volume swept out by a falling raindrop that makes contact with the droplet is known as collision efficiency. The tendency for particles to be carried around the droplet by the flow, instead of making contact with it, results in collision efficiency less than unity. The collision efficiencies are even less than 10 3 for aerosol particles in the aerodynamic diameter (d ae ) range 0.2 – 2 mm(Horn et al., 1988; Garcia et al., 1994; Mircea et al., 2000; Androache, 2003; Ma et al., 2004). Particles in this size range are too large to have sufficient Brownian diffusivity and too small to get effectively collected by raindrops due to mechanisms of inertial impaction and directional interception. The region of the low collision efficiency for particles in the accumula- tion mode (0.2od ae o2.0 mm) is known as ‘‘Greenfield ARTICLE IN PRESS www.elsevier.com/locate/atmosenv 1352-2310/$-see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2005.07.063 à Corresponding author. Tel.: +912025893600; fax: +912025893825. E-mail address: chate@tropmet.res.in.