Contribution to the reaction kinetics of water soluble aerosols and SO 2 in air at PPM concentrations

Abstract

A laboratory study of the heterogeneous catalysis of sulfur dioxide in the atmosphere involved the determination of the rate of atmospheric oxidation of sulfur dioxide to sulfuric acid in the presence of aerosols which were selected metal salts previously reported to act as catalysts. A new aerosol stabilizing technique was developed in which aerosol particles were deposited on inert supporting Teflon beads. The deposition was carried out in a fluidized-bed to ensure discrete aerosol deposition and to achieve a uniform distribution of aerosol concentration on the supporting beads. Because neither the physical shape of the aerosol nor their chemical properties were altered by the stabilizing process, kinetic data obtained in the laboratory by using stabilized aerosols can be extrapolated to describe reactions which may occur in the atmospheric environment. Teflon beads with deposited aerosol particles were packed into a flow reactor. The catalytic oxidation of sulfur dioxide occurred within the reactor as sulfur dioxide gas in humid air mixture flowed through it. The reactor influent sulfur dioxide concentrations ranged from 3 to 18 ppm. Progress of the chemical reaction was continuously monitored by determining the sulfur dioxide concentrations in the reactor effluent with a microcoulometer. Reaction products were identified and quantified by analyzing the reactor contents after the completion of an experimental run. It was observed that higher rates of sulfur dioxide oxidation were always associated with higher relative humidities of the air mixture. Because aerosol particles hydrated into solution drops at high relative humidities, the predominant mass transfer mechanism was absorption of sulfur dioxide by aqueous catalyst drops accompanied by chemical reaction in the liquid phase. Experimental results indicated that the over-all rate of reaction was controlled by the chemical reaction. Milligram for milligram, aerosols of MnSO 4, MnCl 2, and CuSO 4 were found to be 12.2, 3.5, and 2.4 times as effective as NaCl aerosol in promoting the oxidation of sulfur dioxide. CuCl 2 aerosol acted as a reactant in its reaction with sulfur dioxide; sulfur dioxide was oxidized to sulfuric acid while CuCl 2 was reduced CuCl. When MnSO 4 aerosol was used as catalyst, a reaction rate equation was derived which showed that the over-all reaction rate was first order with respect to the sulfur dioxide concentration in the gas phase. By extrapolating laboratory results to the industrial atmospheric environment, the rate of oxidation of sulfur dioxide in natural fog was estimated to be 2 per cent h /t-1.