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

摘要

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 ROsubix/i/sub (ROsubix/i/sub?=??OH+HOsub2/sub+ROsub2/sub) and NOsub3/sub radicals. Highly abundant radical precursors (i.e. Osub3/sub, HONO and carbonyls), nitrogen oxides (NOsubix/i/sub) and volatile organic compounds (VOCs) facilitate strong production and efficient recycling of ROsubix/i/sub 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 ROsubix/i/sub with average daytime contributions of 34–47?%. HONO photolysis is the largest contributor to OH and the second-most significant source (19–22?%) of ROsubix/i/sub. Other considerable ROsubix/i/sub sources include Osub3/sub 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, NOsub3/sub became an important oxidant and the NOsub3/sub-initiated VOC oxidation presented another significant ROsubix/i/sub source (6.2?%) even during daytime. This study suggests the possible impacts of daytime NOsub3/sub chemistry in the polluted atmospheres under conditions with the co-existence of abundant Osub3/sub, NOsub2/sub, VOCs and aerosols, and also provides new insights into the radical chemistry that essentially drives the formation of photochemical smog in the high-NOsubix/i/sub environment of Hong Kong and the PRD region.