Rates of Wintertime Atmospheric SO₂ Oxidation based on Aircraft Observations during Clear-Sky Conditions over the Eastern United States

Sulfur dioxide (SO₂) is emitted in large quantities from coal-burning power plants and leads to various harmful health and environmental effects. In this study, we use plume intercepts from the Wintertime INvestigation of Transport, Emission and Reactivity (WINTER) campaign to estimate the oxidation rates of SO₂ under wintertime conditions and the factors that determine SO₂ removal. Observations suggest that OH governs the rate SO₂ oxidation in the eastern United States during winter. The range of mean oxidation rates during the day from power plants were 0.22–0.71%/hr, producing SO₂ lifetimes of 13–43 days, if SO₂ consumption is assumed to occur during 10.5 hr of daylight in cloudless conditions. Though most nighttime rate measurements were zero within uncertainty, there is some evidence of nighttime removal, which suggests alternate oxidation mechanisms. The fastest nighttime observed SO₂ oxidation rate was 0.25±0.07%/hr, producing a combined day/night SO₂ lifetime of 8.5–21 days. The upper limit of the oxidation rate (the mean+1σ of the fastest day and night observations) is 16.5%/day, corresponding to a lifetime of 6.1 days. The analysis also quantifies the primary emission of sulfate from power plants. The median mole percentage of SO₄^-2 from observed plumes was 1.7% and the mean percentage sulfate was 2.8% for intercepts within 1 hr of transit to power plants. The largest value observed from close intercepts was over 7% sulfate, and the largest extrapolated value was 18%, based on intercepts further from their source and fastest observed oxidation rate.