Surface O3 photochemistry over the South China Sea: Application of a near-explicit chemical mechanism box model

Highlights: • Polluted continental air caused elevated levels of O₃ precursors and high O₃ episodes over theSCS. • Ozone production changed from both VOC and NO_x-limited during non-episodes to VOC-limited on episodes. • During episodes, photochemical cycling accelerated and net O₃ production elevated. • Contributions of VOCs to O₃ formation pathways were all positive on episodes. A systematic field measurement was conducted at an island site (Wanshan Island, WSI) over the South China Sea (SCS) in autumn 2013. It was observed that mixing ratios of O₃ and its precursors (such as volatile organic compounds (VOCs), nitrogen oxides (NO_x = NO + NO₂) and carbon monoxide (CO)) showed significant differences on non-episode days and episode days. Additional knowledge was gained when a photochemical box model incorporating the Master Chemical Mechanism (PBM-MCM) was applied to further investigate the differences/similarities of O₃ photochemistry between non-episode and episode days, in terms of O₃-precursor relationship, atmospheric photochemical reactivity and O₃ production. The simulation results revealed that, from non-O₃ episode days to episode days, 1) O₃ production changed from both VOC and NO_x-limited (transition regime) to VOC-limited; 2) OH radicals increased and photochemical reaction cycling processes accelerated; and 3) both O₃ production and destruction rates increased significantly, resulting in an elevated net O₃ production over the SCS. The findings indicate the complexity of O₃ pollution over the SCS.