Oxidative capacity and radical chemistry in the polluted atmosphere of Hong Kong and Pearl River Delta region: analysis of a severe photochemical smog episode

Abstract. We analyze a photochemical smog episode to understand the oxidative capacity and radical chemistry of the polluted atmosphere in Hong Kong and the Pearl River Delta (PRD) region. A photochemical box model based on the Master Chemical Mechanism (MCM v3.2) is constrained by an intensive set of field observations to elucidate the budgets of RO_x (RO_x =  OH+HO₂+RO₂) and NO₃ radicals. Highly abundant radical precursors (i.e. O₃, HONO and carbonyls), nitrogen oxides (NO_x) and volatile organic compounds (VOCs) facilitate strong production and efficient recycling of RO_x radicals. The OH reactivity is dominated by oxygenated VOCs (OVOCs), followed by aromatics, alkenes and alkanes. Photolysis of OVOCs (except for formaldehyde) is the dominant primary source of RO_x with average daytime contributions of 34–47 %. HONO photolysis is the largest contributor to OH and the second-most significant source (19–22 %) of RO_x. Other considerable RO_x sources include O₃ photolysis (11–20 %), formaldehyde photolysis (10–16 %), and ozonolysis reactions of unsaturated VOCs (3.9–6.2 %). In one case when solar irradiation was attenuated, possibly by the high aerosol loadings, NO₃ became an important oxidant and the NO₃-initiated VOC oxidation presented another significant RO_x source (6.2 %) even during daytime. This study suggests the possible impacts of daytime NO₃ chemistry in the polluted atmospheres under conditions with the co-existence of abundant O₃, NO₂, VOCs and aerosols, and also provides new insights into the radical chemistry that essentially drives the formation of photochemical smog in the high-NO_x environment of Hong Kong and the PRD region.