Field studies of scavenging of aerosols by rain events

The effect of rain during thunderstorms on the concentrations of atmospheric particulates was evaluated, and these measurements were compared with theoretical calculations of washout coefficients. Number concentrations of eight particle sizes (0.013, 0.042, 0.075, 0.133, 0.237, 0.422 and 0.75 μm) were measured right up to start of rainfall and immediately after the rainfall in total 17 rainfall episodes to derive washout coefficients. Precipitation intensities were obtained to establish the theoretical washout coefficients during thunderstorm conditions. The experimental washout coefficients for particles sizes considered in the present study lies between 1.08×10−5 and 7.58×10−4 S−1 except for two occasions of high values for particles of 0.013 μm. These values are considerably greater than theoretical estimates that considered the raindrop collection mechanisms of Brownian diffusion, directional interception, inertial impaction and electrostatic charge effects. Variation of washout coefficients with rainfall rate is also studied during thunderstorm days.