Atmospheric photochemical reactivity and ozone production at two sites in hong kong: Application of a master chemical mechanism–photochemical box model

A photochemical box model incorporating the Master Chemical Mechanism (v3.2), constrained with a full suite of measurements, was developed to investigate the photochemical reactivity of volatile organic compounds at a semirural site (Mount Tai Mo Shan (TMS)) and an urban site (Tsuen Wan (TW)) in Hong Kong. The levels of ozone (O3) and its precursors, and the magnitudes of the reactivity of O3precursors, revealed significant differences in the photochemistry at the two sites. Simulated peak hydroperoxyl radical (HO2) mixing ratios were similar at TW and TMS (p = 0.05), while the simulated hydroxyl radical (OH) mixing ratios were much higher at TW (p < 0.05), suggesting different cycling processes between OH and HO2at the two sites. The higher OH at TW was due to high-NO mixing ratios, which shifted the HOx(OH + HO2) balance toward OH by the propagation of HO2and alkyl peroxy radicals (RO2) with NO. HOxproduction was dominated by O3photolysis at TMS, but at TW, both HCHO and O3photolyses were found to be major contributors. By contrast, radical-radical reactions governed HOxradical losses at TMS, while at TW, the OH + NO2reaction was found to dominate in the morning and the radical-radical reactions at noon. Overall, the conversion of NO to NO2by HO2dictated the O3production at the two sites, while O3destruction was dominated by the OH + NO2reaction at TW, and at TMS, O3photolysis and the O3+HO2reaction were the major mechanisms. The longer OH chain length at TMS indicated that more O3was produced for each radical that was generated at this site.