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

Recently, severe summertime ozone (O) 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-O pollution levels. Field observations indicated that Yuncheng had lower volatile organic compound (VOC) and NO concentrations but higher background O levels. The model simulation verified that both photochemical reactions and net O generation were stronger in Beijing. Ultimately, faster net O generation rates (8.4 ppbv/h) plus lower background O values in Beijing and lower net O generation rates (6.2 ppbv/h) plus higher background O values in Yuncheng caused both regions to reach similar O peak values in July 2020. However, different O control measures were appropriate for the two cities according to the different simulated O-VOCs-NO responses. Additionally, as surface O levels are greatly affected by the ongoing O production/depletion process that occurs in three dimensions, exploring the effects of spatially distributed O on surface O should be high on the agenda in the future.