INTRODUCTION Sulfur dioxide emissions are a major contribution to air pollution in the United States. Effective and economic control of such emissions must be based on sufficient knowledge of the behavior and manner of dispersal of SO2 within the boiler@ the stack@ and the plume. Ideally@ of course@ the most positive SO2 control is accomplished by burning low-sulfur fuels. Current availability and technology makes this generally uneconomical. To provide adequate and economical regulation and control therefore@ it would be desirable to know the maximum level of sulfur that can realistically remain in a fuel for which sufficiently low-cost removal of SO2 by other control techniques can be achieved. To provide the basis for such decisions@ information is needed on the fate of SO2 from the time the fuel is burned to its exit and dispersal in the atmosphere. It is generally accepted that@ in combustion of pulverized coal and fuel oil. most of the sulfur in the fuel leaves the stack as SO2. About 1 to 2 percent of the sulfur is converted to SO3(1)*. and another 5 percent or less is claimed to be absorbed by the fly ash during transit through the boiler up to the base of the stack(2). In an earlier program conducted by GCA. the evidence indicated rapid decay of SO2 either within the stack or in the early plume.(3) Recent work at a Florida power station indicated less rapid consumption of the SO2 except when the relative humidity of the surrounding atmosphere was greater than 78-80 percent.(4) In both of these studies@ sampling of plumes was conducted using fixed-wing aircraft. This is an extremely difficult experiment to conduct with meaningful accuracy@ especially at distances of one mile or less from the stack. The purpose of the current program was to provide additional data to support and on clarity these observation through the use of a laboratory-scale system deigned to similar the' lime-temperature profile of a power-generation observation of the program included. included 1. Identification of sections at the system wherein major depletion in SO2 could be observed 2 Determination of the effect of fuel type on SO2 depletion 3 Determination of the significance of the contribution at fly ash to the overall SO2 depletion 4 Investigation of the rate at oxidation of SO2 in the early stages of plume development@ e g . equivalent to effective ranges of less than one-halt mile from the slack exit 5. Assessment of the effects of relative humidity@ temperature and process variables on the rate of SO2 loss in the early plume. *References are listed at the end of the report.