Abstract

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 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 average daytime contributions of 34–47 %. HONO photolysis is the largest contributor to OH and the second-most significant source (19–22 %) of ROx. Other considerable ROx 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 ROx 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-NOx environment of Hong Kong and the PRD region.

Highlights

  • The hydroxyl radical (OH) and hydro/organic peroxy radicals (HO2 and RO2), collectively known as ROx, play a central role in atmospheric chemistry and air pollution (Stone et al, 2012)

  • During 25–31 August 2011, Hong Kong was hit by a prolonged photochemical smog episode, with concentrations of various air pollutants exceeding the ambient air quality standard

  • Elevated concentrations of O3, NO2, HONO and volatile organic compounds (VOCs) were concurrently observed, which resulted in strong production of ROx and NO3 as well as efficient radical recycling

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Summary

Introduction

The hydroxyl radical (OH) and hydro/organic peroxy radicals (HO2 and RO2), collectively known as ROx, play a central role in atmospheric chemistry and air pollution (Stone et al, 2012). They dominate the oxidative capacity of atmosphere, and govern the removal of primary contaminants and formation of secondary pollutants such as ozone (O3) and secondary organic aerosol (Hofzumahaus et al, 2009). Xue et al.: Radical chemistry in Hong Kong–Pearl River Delta region

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