Acidity of raindrop by uptake of gases and aerosol pollutants

Below-cloud raindrops acidification simulated with a simple model incorporating gas–liquid equilibriums, gas-phase mass transfer, and catalyzed SO₂ oxidation in aqueous phase with uptake of gases and scavenging of particles. Ionic contents of various species in raindrops of different size and pH are computed using one-dimensional time-variant model. The model results are based on SO₂ and NH₃ absorption and collection of calcium aerosols by raindrops with various collection mechanisms. Aqueous concentrations of (SO₂)_l and (NH₃)_l and their ionic components in raindrops are found to be increased with the fall distance from cloud base and decrease of drop size. The overall magnitude of pH enhances with the increase in drop size and transient position of raindrops in the atmosphere below the cloud base. The elevated ionic calcium in raindrops by impaction of calcium aerosols of higher inertia neutralizes the acidic components. Acidic ion contents in smaller droplets are found to be significant and resulted pH of raindrop increases with the size and neutralizing potential of alkaline species. The pH values of rainwater contents of predominant size raindrops in bulk samples corresponding to various rainfall intensities are higher as against the individual non-evaporating smaller raindrops. Results are important in view of the impact of showers on earth surfaces during rain containing large number of smaller droplets as compared to the acidification studies of bulk rainwater.