Submicron fly ash particles can accumulate sulfate by surface oxidation of SO2 in amounts between 9% and 45% of their mass at residence times between 0.3 and 12.5 hours at low humidities in the plume of a coal-fired power plant. The accumulation is size dependent and rapid decrease in the amount of fly ash due to fallout and dispersion prevents efficient removal of sulfur from the atmosphere by this mechanism.