EFFECT OF DROP-PHASE OXIDATION ON ACIDIFICATION OF GROWING CLOUD DROPLET DUE TO ABSORPTION OF SO2 (g)

Abstract

In order to investigate the acidification of cloud droplets due to rainout (in cloudscavenging) of gaseous air pollutants, the characteristics of acidification of growing single cloud droplet due to condensation of the atmospheric water vapor on cloud condensation necleus (CCN) and the rainout of SO2 (g) are simulated numerically with use of a mathematical model. Little is known about rainout compared with washout (below cloud scavenging).(NH4)2 SO4 is selected for CCN, which is produced from NH3 (g) and SO2 (g) via gas-phase chemical reaction in the atmosphere. The mathematical model is based on the equations of mass and energy conservation. Using the lumpedmodel, time variation of droplet radius, temperature and concentrations of chemicals are simulated. The major driving force for droplet growth, temperature rise and the absorption of SO2 (g) is the difference of the concentration of (NH4)2 SO4 between droplet and pure water. The larger the initial size of CCN, the smaller the contribution of oxidation (i. e., sulfate ion) on the acidification of cloud droplet becomes.