Wintertime Particulate Matter Decrease Buffered by Unfavorable Chemical Processes Despite Emissions Reductions in China
Danny M. Leung, Hongrong Shi, Bin Zhao, Jing Wang, Elizabeth M. Ding, Yu Gu, Haotian Zheng, Gang Chen, Kuo‐Nan Liou, Shuxiao Wang, Jerome D. Fast, Guangjie Zheng, Jingkun Jiang, Xiaoxiao Li, Jonathan H. Jiang
Abstract
Abstract 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.