Photochemical pollution during summertime in a coastal city of Southeast China: Ozone formation and influencing factors

Surface ozone (O₃) has a significant impact on the air quality, and its concentrations have been increasing despite a distinct decrease in fine particulate matter (PM_2.5) since 2013 in China. However, the formation mechanisms of photochemical O₃ pollution in coastal regions are not fully understood. Therefore, an intensive field observation was conducted from July 8th to August 4th 2022 in Xiamen of Southeast China, combining with the observation-based model to reveal the formation and influencing factors of photochemical O₃ pollution affected by the Western Pacific Subtropical High (WPSH). Results indicate that O₃ pollution episodes were characterized by higher air temperature, lower relative humidity, lower wind speed, and higher concentrations of precursors (including volatile organic compounds (VOCs), NOx, and CO) compared with non-pollution episodes (p < 0.05). Stronger atmospheric oxidation capacity (AOC), OH reactivity, and ROx production rates during the O₃ pollution episodes led to the remarkable increases in O₃. Additionally, the photolysis of HCHO and other carbonyl compounds (48.6%), as well as O₃ photolysis (41.3%) contributed the largest to ROx radical production, promoting the rapid formation of O₃ during the pollution episodes. Reducing VOC concentrations not only decreased the net O₃ production rates and AOC, but also weakened ROx self-reactions (Self-Rxns). Notably, Self-Rxns were weakened when the reduction percentage of NOx was from 10% to 20%, but decreasing NOx concentrations intensified Self-Rxns in other reduction percentages of NOx. This study provides a better understanding of photochemical O₃ pollution mechanisms under the synoptic situation controlled by the WPSH, as well as the urgency of VOC reductions in the rapid development of coastal regions.