Wintertime Particulate Matter Decrease Buffered by Unfavorable Chemical Processes Despite Emissions Reductions in China

Extreme and persistent haze events frequently occur during wintertime China. While recent emissions reductions reduced annual mean fine particulate matter (PM_2.5) concentrations over eastern China, their effectiveness on wintertime PM_2.5 trend remains uncertain. We use observations and model simulations to quantify seasonal differences in PM_2.5 trends and investigate the underlying chemical mechanisms driving such differences. We find a much slower decrease in observed wintertime PM_2.5 (−3.2% yr⁻¹) since 2014, in contrast to a drastic summertime decrease (−10.3% yr⁻¹). Simulations show two previously underappreciated mechanisms buffering wintertime PM_2.5 decrease, including an increase in oxidation capacity due to nitrogen oxides (NO_x) reductions under wintertime volatile organic compound (VOC)-limited chemistry, and an enhanced conversion of nitric acid to nitrate by ammonia due to sulfur dioxide reductions. Our findings suggest that control policies targeting VOC and deep NO_x reductions are needed to improve wintertime PM_2.5 air quality over China.