Scavenging of aerosols and their chemical species by rain

Washout or scavenging coefficients have been widely used to study the wet deposition processes quantitatively. In the present theoretical study, the washout coefficients are computed for the aerosols of diameters in the range of 0.02–10 μm having various densities in accordance with their chemical compositions for heavy rain regime. The theoretical scavenging rates are applied to the observed average particle size distributions of pre-monsoon months of the year 1998 and 1999 for Pune and 1999 for Himalayan regions. The evolution of particle size distributions at different time intervals for the non-hygroscopic particles of CaCO3, MgCO3, Zn and Mn indicates that the inertial impaction mechanism is the dominant one in removing particles of all sizes for the heavy rain regime. The size dependence of aerosols as a function of relative humidity is considered for the estimation of washout coefficients of hygroscopic particles such as NaCl and (NH4)2SO4. The washout coefficients are found to be highly dependent on relative humidity for hygroscopic particles. The rainwater concentrations are predicted as a function of rainfall depth and a comparison is made with the observed rainwater concentrations of sequential samples collected on 27 June 2001 in a single rain event to support the results of this theoretical work. The predicted rainwater concentrations for RH=50% are about two times larger than that for RH=95% in the case of hygroscopic particles.