Abstract
Heavy ozone (O-3) 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 O-3 episodes in summer, OH initiation was dominated by the reaction of excited oxygen atoms O(D-1) 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(D-1) + H2O, followed by the reaction of O-3 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.
Highlights
The hydroxyl radical (OH) and hydro/organic peroxy radicals (HO2 and RO2) collectively known as ROx, are crucial factors in atmospheric chemistry reactions and air pollution processes (Stone et al, 2012)
The reaction of O(1D) + H2O and the photolysis of nitrous acid (HONO) were the dominant OH sources, while at the petrochemical industrial site, the reactions of O(1D) + H2O and O3 + volatile organic compounds (VOCs) made the main contributions to OH
At both the S1 and S2 sites, the photolysis of HCHO and reaction of O3 + VOC dominated the initiation of HO2 radicals in the daytime and nighttime, respectively
Summary
The hydroxyl radical (OH) and hydro/organic peroxy radicals (HO2 and RO2) collectively known as ROx, are crucial factors in atmospheric chemistry reactions and air pollution processes (Stone et al, 2012). These radicals can oxidize other atmospheric species which strongly affect the quality of ambient air as well as climate (Hofzumahaus et al, 2009; Vullo et al, 2016; Chen et al, 2017). In Hong Kong and the Pearl Region Delta (PRD) region (Xue et al, 2016), ROx sources during the daytime have been identified to be the photolysis of OVOCs (34–47%), HONO photolysis (19–22%), and O3 photolysis (11–20%)
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