Higher contribution of coking sources to ozone formation potential from volatile organic compounds in summer in Taiyuan, China

Highlights

• The analysis of VOCs/NO_x indicated that the O₃ formation was co-controlled by both VOCs and NO_x during the period of 12:00-18:00 LT.

• Coal and biomass combustion (33%), coking sources (28%) were the major sources to VOCs in Taiyuan.

• The control of local and southern coking source is conductive to reduce the peak of O₃ levels.

Abstract

Regional ozone pollution has become one of the most challenging environmental problems in China. In July 2019, hourly real-time monitoring of ozone (O₃) and nitrogen oxides (NO_x) and 3-hour off-line measurement of volatile organic compounds (VOCs) during a 10-day intensive campaign were conducted at four sites in Taiyuan, Shanxi Province, China. The average mixing ratio of total VOCs (including alkanes, alkenes, aromatics and acetylene) was 14.8 ± 2.8 ppbv and the dominant VOCs species to O₃ formation were alkenes in Taiyuan. According to China Ambient Air Quality Standard Grade II (hourly averaged mixing ratio of 103 ppbv), the studied periods were divided into O₃ attainment periods (EP1) and O₃ pollution periods (EP2). A continuous O₃ pollution event was captured during 12–15 July, with the maximum hourly O₃ mixing ratio of 131.7 ppbv. Higher temperature, lower relative humidity, weaker winds and local photochemical reaction were conducive to O₃ pollution during EP2. The analysis of VOCs/NO_x ratio indicated that the formation of O₃ was co-controlled by both VOCs and NO_x during the period of 12:00–18:00 LT (high value period of O₃), and it was controlled by VOCs during the remaining period. The abundances, compositions of typical VOCs and VOCs/NO_x ratio showed clear spatial and temporal variations. Six major sources of VOCs were identified by positive matrix factorization, including coal and biomass combustion (33%), coking sources (28%), vehicular emissions (14%), solvent usage (10%), industrial processes (8%) and biological sources (7%). Backward trajectory analysis found that higher concentration of O₃ in air masses from local (70%) and southern areas (22.5%) during EP2. Local (32%) and southern (30%) coking sources were the main contributors of ozone formation potential (OFP) during EP2.

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 PM_2.5 (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 PM_2.5, 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 (NO_x) in the atmosphere with free radicals under light (Sillman, 1995). The non-linear relationship between O₃ formation, VOCs and NO_x poses new challenges for reducing O₃ (Lin and Liu, 1988). The formation of O₃ is generally controlled by VOCs or NO_x or co-controlled by both VOCs and NO_x, the controlling factors mainly depend on the chemical compositions of the air, especially the ratio between OH reactivity of VOCs and NO_x. 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 NO_x (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 NO_x 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-NO_x 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 NO_x 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/NO_x; (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.