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 multi-day 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 ROX (ROX=OH+HO2+RO2) and NO3 radicals. Highly abundant radical precursors (i.e., O3, HONO and carbonyls), nitrogen oxides (NOX) and volatile organic compounds (VOCs) facilitate strong production and efficient recycling of ROX 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 ROX with an average daytime contribution of 47 %. HONO photolysis is the largest contributor to OH and the second most significant source (19 %) of ROX. Other considerable ROX sources include O3 photolysis (11 %), formaldehyde photolysis (10 %), and ozonolysis reactions of unsaturated VOCs (6.2 %). In one case when solar irradiation was attenuated by the high aerosol loadings, NO3 became an important oxidant and the NO3-initiated VOC oxidation presented another significant ROX source (6.2 %) even during daytime. Sensitivity studies show that controlling aromatics is the most efficient way to reduce the atmospheric oxidative capacity and mitigate photochemical pollution in Hong Kong. This study suggests the possible impacts of daytime NO3 chemistry in 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 Hong Kong and the PRD region.