Surface O3 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-3 and its precursors (such as volatile organic compounds (VOCs), nitrogen oxides (NOx = 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 O-3 photochemistry between non-episode and episode days, in terms of Os-precursor relationship, atmospheric photochemical reactivity and O-3 production. The simulation results revealed that, from non-O-3 episode days to episode days, 1) O-3 production changed from both VOC and NOx-limited (transition regime) to VOC-limited; 2) OH radicals increased and photochemical reaction cycling processes accelerated; and 3) both O-3 production and destruction rates increased significantly, resulting in an elevated net O-3 production over the SCS. The findings indicate the complexity of O-3 pollution over the SCS. (C) 2017 Elsevier Ltd. All rights reserved.