Significant decrease in SO2 emission and enhanced atmospheric oxidation trigger changes in sulfate formation pathways in China during 2008-2016

Understanding the changes in the sulfate (SO42-) formation pathway is important to further control particle pollution owing to the significant reduction of SO2 emission and enhanced atmospheric oxidation. This study applied the Community Multiscale Air Quality (CMAQ) model coupled with the sulfur tracking model (STM) to quantify sources of atmospheric SO42- in China and their changes during 2008-2016. The results showed that the surface SO42- concentration decreased by 51.1%, 40.8%, 37.5%, and 36.2% for Beijing-Tianjin-Hebei (BTH), Yangtze River Delta (YRD), Pearl River Delta (PRD), and Sichuan Basin (SCB), respectively. In addition, the SO42- formation pathways also changed in a sustained manner. The gas-phase reaction of SO2 with OH (SO42- GAS) and the reaction catalyzed by Fe3+ and Mn2+ (SO4-FEMN) decreased significantly (63.4% and 61.7%), whereas the aqueous-phase reaction of SO2 with O-3 (SO4-O3 ) increased by 39.9% from 2008 to 2016. Primary SO42-- emission (SO4-EMIS) played an important role in autumn and winter with over 30% contribution, whereas SO(4-)GAS and the aqueous-phase reaction of SO2 with H2O2 (SO4-H2O2) dominated in spring and summer, respectively. The sensitivity simulation results showed that sharp SO2 reduction significantly decreased SO(4-)GAS (over 60%); however, it was less sensitive to SO4-O3 (8%-11%). In addition, the SO2 emission reduction resulted in a large increase in atmospheric NH3 concentrations due to less neutralization reaction, thus increasing NO3- concentration in some regions. These results suggest that multi-pollutants control for SO2-NOx-VOC should be implemented to prevent the enhanced atmospheric oxidation furtherly deteriorating secondary air pollution, particularly in spring and summer. In addition, limiting primary emissions from residential combustion in winter is important to achieve coordinated control of SO2 and SO42- in northern China.

Automated summary

Understanding the changes in sulfate formation pathways in China from 2008 to 2016 is crucial for controlling particle pollution. The study revealed a significant decrease in sulfate concentrations in various regions, accompanied by changes in formation pathways. It is suggested that multi-pollutant control measures should be implemented to prevent further deterioration of air pollution.