Evolution and variations of atmospheric VOCs and O-3 photochemistry during a summer O-3 event in a county-level city, Southern China

Studies on ambient volatile organic compound (VOC) and ozone (O-3) pollution in county-level cities in China are scarce. The variations of atmospheric VOCs and O-3 photochemistry on O-3 pollution and non-O-3 pollution period were explored in summer 2019 in a typical county-level city, Southern China. The average O-3 concentration was 84.2 ppbv during an O-3 episode, which was more than 1.6 times higher than that on non-O-3 pollution days. The VOC concentration was 20.0 +/- 4.5 ppbv during the sampling period, which was about x 1.5 lower than megacities and suburban areas. The OH, HO2, and RO2 radicals and O-3 photochemistry are simulated using a box model. O-3 formation was dominated by the reaction of HO2+NO. OH + NO2 was the major contributor to O-3 destruction. The net O-3 production rates were higher on O-3 episode days as compared to non-O-3 episode periods. O-3 formation regimes were explored using the empirical kinetics modelling approach (EKMA). The shift of wind directions led to the change from transition regimes to NOX-limited regimes. Industrial source was a critical element in the increasing VOC concentrations, compositions, the transition of O-3 formation regimes, and formation of O-3 episodes. The results revealed that O-3 formation in the Yangxin County was governed by interplay of meteorological conditions, O-3 precursors, emissions sources, air mass transport, photochemical production conditions, and O-3 formation regimes. There were distinct chemical features in the atmosphere of county-level cities and megacities.