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

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 O and its precursors (such as volatile organic compounds (VOCs), nitrogen oxides (NO = NO + NO) 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 O photochemistry between non-episode and episode days, in terms of O-precursor relationship, atmospheric photochemical reactivity and O production. The simulation results revealed that, from non-O episode days to episode days, 1) O production changed from both VOC and NO-limited (transition regime) to VOC-limited; 2) OH radicals increased and photochemical reaction cycling processes accelerated; and 3) both O production and destruction rates increased significantly, resulting in an elevated net O production over the SCS. The findings indicate the complexity of O pollution over the SCS.