Mechanisms for the removal of sulfur dioxide from the atmosphere



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The removal of sulfur dioxide from the atmosphere by photochemical oxidation and by absorption into rain or fog droplets with possible subsequent catalytic oxidation in solution has been simulated. Photochemical oxidation of sulfur dioxide is represented by a first order reaction mechanism with a maximum rate constant of 10 percent per hour. Absorption of sulfur dioxide into rain or fog droplets is characterized using conventional mass-transfer mechanisms which account for the reversibility of the absorption and for the liquid phase mass-transfer resistance. Catalytic oxidation is represented by a mechanism relating oxidation rate to sulfur dioxide and metal oxide concentration in solution. The initial sulfur dioxide distribution in the atmosphere was determined using the binomial continuous plume equation and Holland’s equation for plume rise. Simulation of these mechanisms was made by use of a digital computer. Results of the simulation show that absorption of sulfur dioxide into rain droplets is a more efficient sulfur dioxide removal process than is photochemical oxidation at all rainfall rates and raindrop sizes studied. Absorption of sulfur dioxide into liquid fog droplets was found to be an ineffective sulfur dioxide removal process due to the small mass of liquid water present in fogs. Oxidation of sulfur dioxide in solution in the presence of metal oxide catalysts is found to increase the overall removal of sulfur dioxide from the atmosphere and to be a reasonable mechanism to explain the production of sulfates in precipitation. It is proposed that the reported reduction in the rate of catalytic oxidation in solution with time is due to the reduction in aqueous sulfur dioxide solubility caused by the sulfates produced in the catalytic oxidation reaction lowering the pH of the solution.