Abstract
Abstract. In recent years, ozone pollution has become one of the most severe environmental problems in China. Evidence from observations have showed increased frequency of high O3 levels in suburban areas of the Yangtze River Delta (YRD) region. To better understand the formation mechanism of local O3 pollution and investigate the potential role of isoprene chemistry in the budgets of ROx (OH+HO2+RO2) radicals, synchronous observations of volatile organic compounds (VOCs), formaldehyde (HCHO), and meteorological parameters were conducted at a suburban site of the YRD region in 2018. Five episodes with elevated O3 concentrations under stagnant meteorological conditions were identified; an observation-based model (OBM) with the Master Chemical Mechanism was applied to analyze the photochemical processes during these high O3 episodes. The high levels of O3, nitrogen oxides (NOx), and VOCs facilitated strong production and recycling of ROx radicals with the photolysis of oxygenated VOCs (OVOCs) being the primary source. Our results suggest that local biogenic isoprene is important in suburban photochemical processes. Removing isoprene could drastically slow down the efficiency of ROx recycling and reduce the concentrations of ROx. In addition, the absence of isoprene chemistry could further lead to a decrease in the daily average concentrations of O3 and HCHO by 34 % and 36 %, respectively. Therefore, this study emphasizes the importance of isoprene chemistry in the suburban atmosphere, particularly with the participation of anthropogenic NOx. Moreover, our results provide insights into the radical chemistry that essentially drives the formation of secondary pollutants (e.g., O3 and HCHO) in suburban areas of the YRD region.
Highlights
The hydroxyl radical (OH), hydroperoxy radical (HO2) and organic peroxy radical (RO2), collectively known as ROx, dominate the oxidative capacity of the atmosphere and govern the removal of primary contaminants (e.g., volatile organic compounds (VOCs)) and the formation of secondary pollutants (e.g., ozone (O3), secondary organic aerosols (SOAs)) (Liu et al, 2012; Xue et al, 2016)
The results suggest that high O3 concentrations (> 131 ppb) were usually observed when the site was influenced by weak south wind
It is worth noting that NO2 and O3 concentrations were high even during nighttime, suggesting that the atmospheric oxidative capacity (AOC) remained high at nighttime
Summary
The hydroxyl radical (OH), hydroperoxy radical (HO2) and organic peroxy radical (RO2), collectively known as ROx, dominate the oxidative capacity of the atmosphere and govern the removal of primary contaminants (e.g., volatile organic compounds (VOCs)) and the formation of secondary pollutants (e.g., ozone (O3), secondary organic aerosols (SOAs)) (Liu et al, 2012; Xue et al, 2016). ROx radicals can undergo efficient recycling (e.g., OH → RO2 → RO → HO2 → OH) and produce O3 and oxygenated VOCs (OVOCs) (Liu et al, 2012; Tan et al, 2019; Xue et al, 2016). The photolysis of OVOCs can in turn produce primary RO2 and HO2 radicals, and further accelerate the recycling of ROx (Liu et al, 2012). The reaction rates of different VOCs with ROx vary significantly (Atkinson and Arey, 2003; Atkinson et al, 2006). Among the hundreds and thousands of VOC species, isoprene (C5H8, 2-methyl-1,3-butadiene) is among the most active and abun-
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