This report describes field measurement and modeling of the washout of SO2 and sulfate from plumes. Field measurements of precipitation washout were conducted in conjunction with both controlled test sources and actual power plant plumes. A primary achievement of this work has been the formulation of an SO2 washout model, which predicts rain-borne SO2 concentrations that agree favorably with those observed. An approximate theoretical analysis of sulfate washout in conjunction with field observations indicates that sulfate formation and scavenging exhibit a strong inverse dependence on acidity levels in the background rain. (Modified author abstract)

"Interagency Agreement no. IAG-025 (D) Program element no. 1A1009." -- title page Contract/Grant No. EPA-IAG-00104 (D). "contract/grant no. EPA-IAG-00104 (D)." -- form at back of volume EPA Project Officer: Herbert Viebrock "June 1973" -- title page and cover Title from title page. Includes bibliographical references (pages 131-133).

"A model has been developed for prediction of the reversible washout of SO2 emitted form power-plant plumes and other sources. Predictions of this computer-code model compare favorably with washout measurements made during fifteen controlled-source experiments and four power plant experiments. An application of the model to previous experimental conditions of high background rain acidity shows that "negative washout" can and has occurred as a result of desorption of SO2 from the rain below the SO2 plume. The resulting washdown effect was analyzed mathematically, and shown to be not a significant pollution problem on the distance scale of current interest, but could be important for gases that are more strongly dissolved that SO2. The effect of dry deposition upon the experimental results of the study appears to be unimportant for rainfall rates over about one mm hr-1. Sulfate washout measurements were made during the power plant study, and an approximate reaction-washout analysis indicates that a rapid initial oxidation occurs, which slows at increasing downwind distance. The sulfate washout coefficient appears to be about 0.05 hr-1 for background pH ≥ 5.2, but could be much smaller for more acid rains." -- Abstract, page iv.