Oxidative capacity and radical chemistry in the polluted atmosphere of Hong Kong and Pearl River Delta region: analysis of a severe photochemical smog episode
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+HO 2+RO 2) and NO 3 radicals. Highly abundant radical precursors (i.e. O 3, 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 O 3 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, NO 3 became an important oxidant and the NO 3-initiated VOC oxidation presented another significant RO x source (6.2 %) even during daytime. This study suggests the possible impacts of daytime NO 3 chemistry in the polluted atmospheres under conditions with the co-existence of abundant O 3, NO 2, 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.