Articles | Volume 16, issue 15
Atmos. Chem. Phys., 16, 9891–9903, 2016
Atmos. Chem. Phys., 16, 9891–9903, 2016

Research article 08 Aug 2016

Research article | 08 Aug 2016

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

Likun Xue1, Rongrong Gu1, Tao Wang2,1, Xinfeng Wang1, Sandra Saunders3, Donald Blake4, Peter K. K. Louie5, Connie W. Y. Luk5, Isobel Simpson4, Zheng Xu1, Zhe Wang2, Yuan Gao2, Shuncheng Lee2, Abdelwahid Mellouki1, and Wenxing Wang1 Likun Xue et al.
  • 1Environment Research Institute, Shandong University, Ji'nan, Shandong, China
  • 2Department of Civil and Environmental Engineering, Hong Kong Polytechnic University, Hong Kong, China
  • 3School of Chemistry and Biochemistry, University of Western Australia, WA, Australia
  • 4Department of Chemistry, University of California at Irvine, Irvine, CA, USA
  • 5Environmental Protection Department, the Government of Hong Kong Special Administrative Region, Hong Kong, China

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.

Short summary
The chemical budgets and principal sources of ROx and NO3 radicals during a multi-day photochemical smog episode in Hong Kong are elucidated by an observation-constrained MCM model. NO3 was shown to be an important oxidant even during daytime in a pollution case when high aerosol loading attenuated the solar irradiation. This study suggests the potential important role of daytime NO3 chemistry in polluted atmospheres under conditions with the co-existence of abundant O3, NO2, VOCs, and aerosols.
Final-revised paper