Chemically Induced Decline in Wintertime SO₂ Emission Control Efficacy

The reduction of SO2 emissions from 2004 to 2023 has resulted in significant decreases in SO2 and sulfate concentrations across the Rust Belt and Southeast of the United States, regions with coal-combustion power plant emissions. While the observed sulfate-to-total-sulfur ratio remained relatively constant at ∼30% in summer, this fraction increased from ∼10% to ∼20% in winter from 2004 to 2013 and then remained at ∼20%. The rise in sulfate-to-total-sulfur ratio resulted in a slower decrease of sulfate in 2004–2013 compared to 2013–2023 despite a greater reduction of SO2 in the earlier decade, reflecting a significant decline in the efficacy of SO2 emission reduction in improving air quality in winter. The decrease in efficacy is attributed to the increased oxidation of SO2 in winter because of the diminishing oversupply of SO2 compared to the availability of atmospheric oxidant, H2O2. Consequently, the seasonal differences in SO2 and sulfate concentrations between summer and winter have narrowed. This chemical damping effect, caused by limited oxidant availability, is likely to delay the reduction in sulfate concentrations in other polluted regions where the transition from coal to natural gas in power plants or alternative energy sources has not progressed as rapidly as in the United States.