Parametric study of scavenging of atmospheric aerosols of various chemical species during thunderstorm and nonthunderstorm rain events

Abstract

The overall collision efficiencies between falling raindrops and particles of selected chemical composition are computed theoretically by considering the contributions of collection mechanisms due to Brownian diffusion, directional interception, inertial impaction and phoretic effects caused by thermal and concentration gradients for the rainfall associated with thunderstorm and nonthunderstorm events. Computations of collision efficiency are performed for the particles in the diameter range of 0.02–0.2 μm (in 0.02 μm steps), 0.2–2 μm (in 0.2 μm steps) and 2–10 μm (in 1 μm steps) and for raindrops in size range of 200–5800 μm (in 200 μm steps). The results show order of magnitude difference in collision efficiencies for the case of thunderstorm and nonthunderstorm rain, and it reduces with increase in particle size from 0.02 to 0.8 μm for the particles of KNO3 because of decrease in magnitude of electrical forces. The effect of rain associated with thunderstorms and nonthunderstorms on scavenging rates for the particles of CaCO3, KNO3, (NH4)2SO4 and aerodynamic size is evaluated in terms of mean mass scavenging coefficients for polydispersed aerosols lying in size regimes of 0.02–0.2 μm, 0.2–2 μm and 2–10 μm. Mean mass scavenging coefficients are found to exhibit maximum potential for the particles of CaCO3 as compared to those of KNO3, (NH4)2SO4 and aerodynamic size particles. Present results may be useful to evaluate the Pollutant Standard Index (PSI) or Air Quality Index (AQI), when the atmosphere undergoes rain during thunderstorm and nonthunderstorm conditions.