Divergent summertime surface O3 pollution formation mechanisms in two typical Chinese cities in the Beijing-Tianjin-Hebei region and Fenwei Plain

Abstract

Recently, severe summertime ozone (O3) pollution has swept across most areas of China, especially the Beijing-Tianjin-Hebei (BTH) region and Fenwei Plain. By focusing on Beijing and Yuncheng, which are two typical cities in the BTH region and the Fenwei Plain, we intended to reveal the neglected fact that they had disparate emission features and atmospheric movements but suffered from similar high-O3 pollution levels. Field observations indicated that Yuncheng had lower volatile organic compound (VOC) and NOx concentrations but higher background O3 levels. The model simulation verified that both photochemical reactions and net O3 generation were stronger in Beijing. Ultimately, faster net O3 generation rates (8.4 ppbv/h) plus lower background O3 values in Beijing and lower net O3 generation rates (6.2 ppbv/h) plus higher background O3 values in Yuncheng caused both regions to reach similar O3 peak values in July 2020. However, different O3 control measures were appropriate for the two cities according to the different simulated O3-VOCs-NOx responses. Additionally, as surface O3 levels are greatly affected by the ongoing O3 production/depletion process that occurs in three dimensions, exploring the effects of spatially distributed O3 on surface O3 should be high on the agenda in the future.