FLAHigher contribution of coking sources to ozone formation potential from volatile organic compounds in summer in Taiyuan, China
Introduction
With the rapid economic development, the air pollutants emission has been gradually increased in China and much attention has been paid to pollution control. Since September 2013, the Chinese government has adopted a series of control measures to alleviate air pollution. Some studies have indicated the annual average concentration of PM2.5 (particulate matter with aerodynamic diameter less than or equal to 2.5 μm) in major cities of China has decreased from 2013 to 2017, while the tropospheric O3 levels showed an increased trend (Zhang et al., 2015b, 2017). O3 pollution is less visible than PM2.5, but it also is harmful to human health (Wang et al., 2020a, 2020b). Especially in the summer, O3 pollution episodes have occurred frequently in many cities in China, like Beijing, Guangzhou and Shanghai (Gao et al., 2017; Ou et al., 2016; Shao et al., 2009).
Understanding the current status of O3 pollution in various regions of China is of great significance for formulating reasonable and effective prevention measures. Tropospheric O3 is a pollutant produced by the photochemical reaction of volatile organic compounds (VOCs) and nitrogen oxides (NOx) in the atmosphere with free radicals under light (Sillman, 1995). The non-linear relationship between O3 formation, VOCs and NOx poses new challenges for reducing O3 (Lin and Liu, 1988). The formation of O3 is generally controlled by VOCs or NOx or co-controlled by both VOCs and NOx, the controlling factors mainly depend on the chemical compositions of the air, especially the ratio between OH reactivity of VOCs and NOx. Due to different meteorological conditions and industrial structure, different regions exhibited different O3 sensitive characteristics. As the capital of China, most areas of Beijing were VOCs-controlled (Wei et al., 2019). As a transportation hub in northwestern China, Lanzhou was controlled by NOx (Liu et al., 2010; Xue et al., 2014). In Southwest China, Chengdu was VOCs-controlled due to the large amount of pollutants emitted by the petrochemical industry (Tan et al., 2018). Wuhan (Hui et al., 2018) and Zhengzhou (Li et al., 2019) in central China were controlled by VOCs. As the largest industrial city in the Yangtze River Delta (YRD), the formation of O3 in Shanghai was also controlled by VOCs (Xue et al., 2014). In the North China Plain (NCP), the formation of O3 in Jinan was co-controlled by both VOCs and NOx when maximum hourly O3 mixing ratio exceeds the China Ambient Air Quality Standard Grade II (100 ppbv), and controlled by VOCs when it reaches the standard (Lyu et al., 2019). Thus it is necessary to carry out refined research on determining the controlling factors of O3 formation in the specific regions.
At present, most studies were mainly concentrated on meteorological factors, source apportionment of VOCs and process analysis of O3 concentration in developed areas such as the YRD, the Pearl River Delta (PRD) and the NCP (Chen et al., 2018; He et al., 2019; Li et al., 2019; Liu et al., 2019; Lyu et al., 2019). To the best of our knowledge, fewer studies were conducted in Shanxi. Shanxi Province is the coal base of China, coking production and coking export of Shanxi accounted for 40% and 60% of total coke production and export in China, respectively (He et al., 2015). The central and southern regions of Shanxi Province are the main areas of coke in China. As the capital city of Shanxi Province, Taiyuan's unique energy and industrial structure makes its pollution situation different from other areas. In recent years, the O3 concentration in Taiyuan has shown an upward trend year by year, the number of days with O3 as the main pollutant increased from 35 days in 2015 to 100 days in 2018, and the 90th percentile concentration of daily maximum 8h ozone increased from 120 μg/m3 in 2015 to 191 μg/m3 in 2018 (http://www.taiyuan.gov.cn/). Li et al. (2020) found that coking pollution (33%) was an important source of VOCs in Taiyuan, Shanxi. However, the spatial-temporal variations, the reactivity of VOCs species and source contributions of ozone formation potential (OFP) from VOCs as well as the spatial sensitivity of O3–VOCs-NOx in Taiyuan were unclear. To further reduce the concentration levels of O3, it is necessary to conduct more research on O3 during severe O3 pollution periods in Taiyuan.
For the above reasons, hourly real-time monitoring of O3 and NOx and 3-hour off-line measurement of VOCs during a 10-day intensive campaign were conducted at four sites in Taiyuan in July 2019. The main objectives of this study are to (1) analyze the spatial and temporal distribution of O3 and its precursors, and meteorological impacts on O3 formation during O3 attainment periods (EP1) and O3 pollution periods (EP2); (2) determine the sensitivity of O3 formation by VOCs/NOx; (3) compared the difference of VOCs sources based on positive matrix factorization (PMF) model, and their contributions to OFP during EP1 and EP2; (4) evaluate the impacts of regional transport and local emissions of VOCs sources to OFP.
Section snippets
Sampling site
Taiyuan is located in the central part of Shanxi Province, which is surrounded by mountains in the west, north and east. The specific terrain has been described in our previous study (Li et al., 2016). As an important energy and heavy industry base in China, Taiyuan has a thermal power plant and two heavy industrial plants in the urban area. Taiyuan Iron and Steel Group is the largest stainless steel production base in China. The coking industries in Shanxi Province are mainly concentrated in
O3 levels
Fig. 2 shows the time series of meteorological parameters and O3 mixing ratios at four sites. The range and average mixing ratio of O3 were 4.5–131.7 ppbv and 65.4 ± 5.5 ppbv, respectively. The average mixing ratio of O3 at SL (68.2 ± 10.4 ppbv) was the highest, followed by JY (66.1 ± 3.7 ppbv), XD (65.6 ± 3.8 ppbv) and TY (61.8 ± 5.6 ppbv). According to Chinese National Air Quality Standard Grade II (hourly averaged mixing ratio of 103 ppbv), the entire measurement period was divided into EP1
Conclusion
A 10-day intensive campaign during July 2019 was conducted to characterize O3, VOCs and NOx at four sites in Taiyuan, Shanxi Province, China, the O3 concentration on 5 days exceeded the China Ambient Air Quality Standard II. Some main conclusions could be drawn as follows:
- (1)
Higher T, lower RH, weaker WS and local photochemical reaction were conducive to O3 pollution during EP2. Based on the MIR and PE method, alkenes contributed the most to OFP. The results of the VOC/NOx ratio method showed that
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the funding of Joint Research Fund for Overseas Chinese Scholars and Scholars in Hong Kong and Macao Young Scholar (41728008); the National Natural Science Foundation of China (41472311), and Doctoral Scientific Research Foundation of TYUST (20202072).
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Peer review under responsibility of Turkish National Committee for Air Pollution Research and Control.