Ambient volatile organic compounds and their effect on ozone production in Wuhan, central China
Graphical abstract
Introduction
Ambient VOCs are important in the formation of O3 and secondary organic aerosols (SOAs) (Guo et al., 2013, Cheng et al., 2010, Camredon et al., 2007, Tsigaridis et al., 2005). As key O3 precursors, VOCs react with nitrogen oxides (NOx) in the presence of sunlight, leading to photochemical pollution with elevated O3 concentration. Due to intense and uncontrolled emissions of VOCs, mega-cities and city clusters in China, such as Beijing (Yao et al., 2015, Wei et al., 2014), the Yangtze River Delta of eastern China (Li et al., 2014, Xue et al., 2014), and the Pearl River Delta (PRD) of southern China (Ling et al., 2011, Cheng et al., 2010) currently suffer from severe photochemical pollution.
Identifying the sources of VOCs is crucial for local government to formulate and implement emission reduction strategies, but the sources depend largely on energy consumption levels and the industrial structures of a city/region, and are often complex. In general, VOCs sources include emissions from gasoline-, diesel-, and liquefied petroleum gas (LPG)-fuelled vehicles, fuel evaporation, emissions from industries such as shoe making, furniture manufacturing and etc., solvents in paints and consumer products, and coal and biomass burning (Guo et al., 2004, Guo et al., 2011, Li et al., 2014, Wang et al., 2013, Cai et al., 2010). The source contributions to ambient VOCs vary between cities and regions. For example, Wang et al. (2013) found that coal burning contributed 26–39% and vehicular exhausts 31–45% to the total VOCs in Beijing in winter, while An et al. (2014) indicated that industrial emissions constituted a considerable proportion (45–63%) of the total VOCs in the Yangtze River Delta, and Cai et al. (2010) reported that industry accounted for 36% of the ambient VOCs in Shanghai. Guo et al. (2011) and Ling et al. (2011) found that solvent usage was the major source of ambient VOCs in the PRD region, whereas vehicular emissions made the most significant contribution in Hong Kong, although it is adjacent to the PRD region (Ou et al., 2015).
VOCs contribute to O3 formation through a series of photochemical reactions, including the hydroxyl radical (OH) initiated oxidation of VOCs, nitrogen cycling driven by peroxyl (RO2) and hydroperoxyl (HO2) oxidation and photolysis, and the combination of oxygen atom (O) with molecular oxygen (O2) (Ling et al., 2014, Cheng et al., 2013). The photochemical reactivity of the VOC species and the intermediates therefore significantly influences O3 production, along with the level of concentration. For instance, alkenes and aromatics are highly reactive in photochemical O3 formation (Ling and Guo, 2014, Ling et al., 2011). The relative incremental reactivity (RIR) method proposed by Carter and Atkinson (1989) is frequently used to evaluate the sensitivity of O3 production to the changes of VOC precursors, through the application of an observation-based model (OBM) using the carbon bond IV mechanism. Ling et al. (2011) recently developed a combined application of the positive matrix factorization (PMF) model and the OBM to study the VOC source contributions to the formation of photochemical O3 in Guangzhou and Hong Kong. Moreover, the maximum incremental reactivity (MIR), O3 formation potential (OFP), and photochemical O3 creation potential (POCP) have also been used to describe the contribution of VOC species to O3 production (Barletta et al., 2002, Jenkin and Hayman, 1999).
Wuhan is the largest megacity in central China and an important transport hub, and has experienced rapid development through growth in real estate and new technology industries such as optoelectronics, biological science, environmental science, etc. Accompanying this urbanization and industrialization is air pollution, characterized by haze and photochemical smog, which has been often observed in recent years. Even so, there are virtually no published studies on ambient VOCs in Wuhan. Obtaining first-hand information on VOC characteristics and sources and their association with photochemical smog formation is therefore an urgent task. To our best knowledge, this is the first study on VOC sources and their effects on the O3 formation in this region.
In this study, the characteristics of ambient VOCs and their effects on O3 production in Wuhan were investigated. The abundance and temporal patterns of VOCs were discussed, and the source contributions to ambient VOCs were quantified. The dominant factors affecting the photochemical O3 formation were determined using a photochemical box model incorporating the Master Chemical Mechanism (PBM-MCM), and the contributions of VOC sources to the O3 formation were also quantified. The O3-precursor relationships were also evaluated using RIR and RIR-weighted concentration. The outcomes will be of help to air scientists and local governments, for further study and to take measures to reduce VOC and O3.
Section snippets
Site description and chemical analysis
From February 2013 to October 2014, 99 VOCs consisting of 56 non-methane hydrocarbons, 28 halocarbons, and 15 oxygenated VOCs (OVOCs), the five trace gases SO2, NO2, NO, CO, and O3, and two meteorological parameters (temperature and humidity) were simultaneously monitored at an urban site (30.54 N, 114.37 E, < 50 m a.s.l.) in Wuhan. The site was set up on the rooftop of a six-story building (~ 18 m height) within the Hubei Provincial Environmental Monitoring Center, as shown in Fig. 1.
Ambient air was
Meteorological conditions
Table 1 shows the statistics of meteorological parameters simultaneously monitored with VOCs and trace gases. The prevailing wind was from the southwest, while the wind speed was very low (1.1 ± 0.02 m/s), implying the dominance of local air masses at the monitoring site. Obvious seasonal patterns were observed, with the maximums of wind speed, temperature, and relative humidity in summer and minimums in winter. Conversely, the sea level pressure was lowest in summer and highest in winter. The
Conclusions
In this study, VOCs were continuously measured at an urban site in Wuhan from February 2013 to October 2014. Of the Chinese megacities, Wuhan has a relatively low level of TVOCs. Ethane, ethene, and toluene were the most abundant VOC species. The most significant contribution to both VOCs (27.8 ± 0.9%) and O3 production (23.4 ± 1.3) was from vehicular exhausts. The contributions of LPG and solvent usage in coating/painting to O3 production were higher than their contributions to VOCs. RIR
Acknowledgements
This project was supported by the exchange project between Hong Kong and Mainland China Universities, the Research Grants Council of the Hong Kong Special Administrative Region (PolyU5154/13E and PolyU152052/14E), and the Hong Kong Polytechnic University Ph.D. scholarships (project #RTUP). This study is partly supported by the Public Policy Research Funding Scheme (2013.A6.012.13A) and the National Natural Science Foundation of China (No. 41405112).
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