Ozone formation and key VOCs in typical Chinese city clusters

Abstract

China, especially Beijing-Tianjin-Hebei (BTH), Yangtze River Delta (YRD), Pearl River Delta (PRD), and Sichuan Basin (SCB), suffers from severe ozone pollution problems during summer and autumn. Ground-level ozone is formed via a series of complex photochemical reactions between nitrogen oxides (NO x ) and volatile organic compounds (VOCs), which are one of the most concerning pollutants in the four megacities and have adverse impacts on human health. In recent years, O3 concentrations that exceed the national ambient air quality standard have been frequently observed during summer and autumn. However, the mechanism of O3 formation and the key factors that regulate this mechanism have not been studied in detail. To evaluate ozone pollution episodes in Chinese megacities, field observation data of O3 and its precursors at 20 sites in the four major urban regions during summer and autumn from 2014 to 2016 were collected in this study. We found that all the four regions suffered from serious O3 pollution. The daily maximum 8-hour average O3 concentrations ranged from 59–146 (10−9 V/V). Moreover, the hourly maximum O3 concentrations were in the range of 73–187 (10−9 V/V). High values of both these parameters were measured at the same site, Dingling, which is a rural area of Beijing. Furthermore, O3 concentrations in urban areas were lower than those in suburban regions. Ozone non-attainment days occurred more frequently in BTH and YRD than in the other regions. Although the concentrations of NO x and VOCs, which are ozone precursors, varied largely across different sites, they were generally in ranges of ~4–22 (10−9 V/V) and 11–53 (10−9 V/V), respectively. Additionally, isoprene reactivity significantly contributed to total the VOC reactivity ( k VOC) in rural sites. However, in urban sites, the reactivities of alkenes and aromatics were significant contributors. To evaluate the effectiveness of various ozone precursor control strategies in reducing ozone pollution, we combined the observation-based model (OBM) with the relative incremental reactivity (RIR) method. The sensitivity of O3 production was studied by varying the emission of ozone precursors at each site and the key indicators that affect the local ozone production rate were identified. The ozone precursors were divided into four groups: anthropogenic hydrocarbons (AHC), which consist of alkanes, alkenes, and aromatics, natural hydrocarbons (NHC, only isoprene), NO x , and CO. It was found that most of the sites in these four regions were in a VOC-limited regime or transition regime (VOC and NO x sensitive) and the key VOC precursors were alkenes and aromatics. Additionally, an empirical kinetic modeling approach (EKMA) was used to determine the cooperative control scheme for NO x and VOCs. Most of the urban sites were under VOC-limited conditions while the suburban site was in a NO x -limited regime. These results are consistent with the results derived with the RIR method. To further explore the role of anthropogenic emissions in ozone pollution, we used the positive matrix factorization (PMF) model to identify the major sources contributing to VOCs. This model identified five to seven VOCs sources in different urban sites. The major sources for these sites were vehicular exhaust and gasoline evaporation, accounting for 30%–50% of the observed VOCs. According to the source profile, a series of sensitivity tests were performed by reducing each source by 20% to determine the RIR of the individual source. Vehicular exhaust and solvent utilization were the major contributors to O3 formation in Dongsi, Pudong, Modiesha, and Chaozhan, which are located in BTH, YRD, PRD, and SCB, respectively. In summary, by regulating VOCs emissions, especially vehicular emissions and emissions from solvent use, the O3 concentration in China can be efficiently regulated.