Ionic concentrations and initial S(IV)-oxidation rates in droplets during the condensational stage of cloud

Abstract

Concentrations of ions and the catalytic active trace metals iron and manganese, pH-values and S(IV)-oxidation rates are calculated in cloud droplets of different size in the beginning stage of a cloud. The calculations are based on size-dependent chemical composition of mixed-aerosol particles. The condensation process is modeled by using an adiabatic parcel model. High concentrations of all ions and metals are found in the interstitial particles. Sulfate, nitrate and ammonium have concentrations in the range 1.4-10-2 to 2.4 moles/1. The pH-values are generally between 1.3 and 1.7. In the small, activated cloud droplets of approximately 5 μm radius, ions and metals have the smallest concentrations in the order of 10-6 to 10-4 moles/1 for the ions and 10-8 to 10-7 moles/1 for Mn and Fe. Higher concentrations up to 7.5-10-2 moles/1 are found in the larger, growing droplets for the ions as well as for the metals. These concentrations are used to calculate the initial oxidation rates of sulfur(IV) to sulfur(VI) in the presence of hydrogen peroxide, ozone and the catalytic active metals Fe and Mn. Depending on the initial pH-value the oxidation rates can differ considerably in different sized cloud droplets. Regarding the whole parcel, the largest fraction of sulfate is produced in the large liquid water volume of the cloud droplets. In the interstitial particles, only a negligible fraction is produced. Basic droplets can increase the initial production rate of S(VI) considerably, but small droplets will become acidic after a few seconds by the given pressures of O3 and SO2. The oxidation rates in large, basic droplets would also be high, but they are limited by gas-phase diffusion of the reagents to the droplets. The initial oxidation rate of S(IV) calculated from summing the rates of the individual droplet classes is higher than the rate calculated for bulk cloud water with mean chemical composition.