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Oxidative Capacity and Radical Chemistry in a Semi-arid and Petrochemical-industrialized City, Northwest China

Category: Aerosol and Atmospheric Chemistry

Volume: 18 | Issue: 6 | Pages: 1391-1404
DOI: 10.4209/aaqr.2017.11.0506
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Chenhui Jia1, Yanan Wang2, Yaojie Li1, Tao Huang1, Xiaoxuan Mao1, Jingyue Mo3, Jixiang Li3, Wanyanhan Jiang1, Xiaoxue Liang1, Hong Gao1, Jianmin Ma 1,4

  • 1 Key Laboratory for Environmental Pollution Prediction and Control, Gansu Province, College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, China
  • 2 Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong, China
  • 3 College of Atmospheric Science, Lanzhou University, Lanzhou 730000, China
  • 4 Laboratory for Earth Surface Processes, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China

Highlights

OH initiation was dominated by the photolysis of O3 and HONO in downtown.
Reaction of O3 with VOCs was important to OH in petrochemical industrial suburb.
The predominant oxidant was OH in daytime and NO3 in nighttime.


Abstract

Heavy ozone (O3) pollution is often observed in chemically industrialized Lanzhou and other capital cities located in the semi-arid and mountainous provinces of northwestern China. There are large knowledge gaps regarding the relationship between radical budgets and photochemistry in these cities. To gain insights into this relationship, a photochemical box model based on the Master Chemical Mechanism (MCM v3.3) was applied to investigate oxidative capacity and radical chemistry in the city of Lanzhou. The budgets of ROx (OH + HO2 + RO2) radicals were quantified, and the initiation, propagation, and termination process of the diurnal variation in the ozone chemistry were examined. The MCM model was constrained by in situ measurements at two sampling sites in the city, one located in the City downtown area (S1) and the other in the heavy petrochemical industrial area (S2) in the western suburb of the City, characterized by significant differences in volatile organic compounds (VOCs) and NOx concentrations between the two sites. Results showed that during the high O3 episodes in summer, OH initiation was dominated by the reaction of excited oxygen atoms O(1D) with water and the photolysis of nitrous acid (HONO) at the S1 site. At the S2 site, the most important production of OH was the reaction of O(1D) + H2O, followed by the reaction of O3 with VOCs. HONO photolysis was mostly identified at 7:00–13:00 local time at the S2 site, which is less important than that at the S1 site during the daytime. The photolysis of HCHO and OVOCs (except for HCHO) were the primary sources contributing to the initiation of HO2 and RO2 radicals at both sites. Results also revealed that the ROx termination could be attributed to the reactions of ROx with NO and NO2. The self-reactions between radicals also played an essential role at the S2 site. Overall OH was found to be the predominant oxidant, and NO3 was a major oxidant in the nighttime chemistry in the city.

Keywords

ROx radical Source Oxidative capacity Lanzhou MCM model


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