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Surface ozone (O 3) 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 O 3 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 O 3 pollution affected by the Western Pacific Subtropical High (WPSH). Results indicate that O 3 pollution episodes were characterized by higher air temperature, lower relative humidity, lower wind speed, and higher concentrations of precursors (including volatile organic compounds (VOCs), NO x , and CO) compared with non-pollution episodes (p < 0.05). Stronger atmospheric oxidation capacity (AOC), OH reactivity, and RO x production rates during the O 3 pollution episodes led to the remarkable increases in O 3. Additionally, the photolysis of HCHO and other carbonyl compounds (48.6%), as well as O 3 photolysis (41.3%) contributed the largest to RO x radical production, promoting the rapid formation of O 3 during the pollution episodes. Reducing VOC concentrations not only decreased the net O 3 production rates and AOC, but also weakened RO x self-reactions (Self-Rxns). Notably, Self-Rxns were weakened when the reduction percentage of NO x was from 10% to 20%, but decreasing NO x concentrations intensified Self-Rxns in other reduction percentages of NO x. This study provides a better understanding of photochemical O 3 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. • The field observation combing with model simulation were used to explore in situ O 3 photochemistry. • Elevated levels of precursors and enhanced atmospheric oxidation capacity accelerated O 3 formation. • VOC reductions decreased atmospheric oxidation capacity and weakened the RO x self-reactions. • HCHO greatly promoted RO x radical and O 3 formation. [ABSTRACT FROM AUTHOR]

Continuous in situ measurements of optical properties of fine aerosols (PM 2.5 ) were conducted in the urbanized coastal city Xiamen in Southeast China from November 2013 to January 2014. PM 2.5 samples were also collected and chemical compositions including organic carbon (OC), elemental carbon (EC) and water-soluble inorganic ions were determined to investigate the impacts of chemical compositions on aerosol optical properties. Average values of scattering coefficient (b scat ), absorption coefficient (b abs ), extinction coefficient (b ext ) and single scattering albedo (SSA) were 164.0 Mm − 1 , 22.4 Mm − 1 , 187.0 Mm − 1 and 0.88, respectively. b scat , b abs and b ext showed obvious bi-modal diurnal variations with high values in the morning and at night while low value in the early afternoon, whereas SSA exhibited an opposite diurnal variation. Average b scat and b abs were largest in the wind direction of southwest and were larger with slower wind. b abs was mainly affected by EC, while b scat was affected by ammonium, sulfate, nitrate and OC. The IMPROVE formula was applied to estimate b ext based on the chemical species. Results shows that ammonium sulfate was the largest contributor, accounting for 36.4% of b ext , followed by organic matter (30.6%), ammonium nitrate (20.1%), EC (9.0%) and sea salt (3.9%). The deterioration in visibility was mainly led by increases in secondary aerosols including sulfate and nitrate. Backward trajectories analysis showed that during the sampling period Xiamen was significantly affected by the air masses originating from the Northern and Northeastern areas. Air masses from the Northern associated with relative higher b ext and less relative contribution from ammonium sulfate and more relative contribution from ammonium nitrate, organic matter and sea salt. [ABSTRACT FROM AUTHOR]