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2                           jour nal o f e nvironmental sciences 93 (2020) 1 e12

                                                              constant. Wang et al. (2016) conducted a related study on the
                                                              variation characteristics and chemical reactivity of atmo-
                                                              spheric VOCs in a suburb of Beijing. The results show that
          With the rapid development of the world economy and the
                                                              aromatics and alkenes are the main contributors to ozone
          surge in population, environmental issues have become a  generation. Hui et al. (2018) have reported that Wuhan belongs
          persistent problem that cannot be ignored. Air pollution has
                                                              to a VOC limited area in terms of ozone generation by using
          become a common problem that has received widespread  the Empirical Kinetic Modelling Approach. Therefore, deter-
          attention in recent years in China (Huang et al., 2016; Hui et al.,
                                                              mining the contribution of VOCs to O 3 formation is important
          2019; Wei et al., 2018; Zhong et al., 2019). The occurrence of  for local governments to control photochemical pollution.
          haze, a common type of air pollution, is more frequent in
                                                                VOCs not only play an important role in atmospheric
          Beijing, an international metropolis with more than 20 million
                                                              chemistry, but also are harmful to human health (Parra et al.,
          people and nearly 6 million motor vehicles, owing to the rapid  2006). Toxic VOCs are mostly benzenes and halocarbons.
          regional industrialization and urbanization. Studies have
                                                              Different species, different exposure times, and exposure
          confirmed that short- or long-term exposure to haze pollution  pathways have different health effects on humans. Extensive
          is associated with cardiovascular and respiratory diseases,
                                                              studies have confirmed that VOCs are harmful to human health
          premature death, and increased risk of cancer (Gao et al., 2017;  owing to their toxicity, mutagenicity, and carcinogenicity (Gong
          Lu et al., 2013). Thus, devising strategies to effectively control
                                                              et al., 2017; Tuet et al., 2017; Tyagi et al., 2016). The health risks
          and prevent air pollution is pressing. Recently, studies have  to the human body exposed to VOCs can be divided into
          increasingly shown that volatile organic compound (VOC)
                                                              carcinogenic and non-carcinogenic effects (Jiang et al., 2017).
          emissions play an important role in air pollution.  Some VOCs, such as dichloromethane, trichloroethylene, and
             VOCs refer to organic matters with boiling points below
                                                              BTEX, are classified as hazardous air pollutants. A great deal of
          250 C at atmospheric pressure (Xu et al., 2016). These, including

                                                              research has been conducted on the health risk assessment of
          non-methane hydrocarbons, halocarbons, BTEX (benzene,  PM2.5 in Beijing (Song et al., 2019; Xu et al., 2018). In contrast,
          toluene, ethylbenzene, and meta-, para-and ortho-xylenes), and
                                                              the health risk assessments for VOCs require further work.
          oxygenated volatile organic compounds (OVOCs), which are  In the present study, the concentrations of 99 VOCs were
          emitted from various natural and anthropogenic sources. Ac-
                                                              measured using online instruments at an urban site in Beijing
          cording to the atmospheric photochemical reactivity, VOCs can  during winter from 15 December 2015 to 17 January 2016 and
          be divided into two types: strong and weak photochemical ox-
                                                              summer from 21 July to 25 August 2016. All values were
          ides. The former type can quickly participate in atmospheric  measured at 10 am. The sampling period was divided into
          chemical processes and is eliminated by reaction with atmo-
                                                              polluted days (air quality index (AQI) > 100) and “normal” days
          spheric oxidants such as OH radicals and O 3 . Its lifetime in the  (AQI  100). The variations in the type of VOCs, SOA formation
          troposphere is short and its overall lifespan generally lasts from
                                                              potential, and ozone formation potential were analyzed dur-
          several hours to one day. Therefore, such compounds are not  ing the normal or polluted days of winter and summer. In
          easy to grow. Distance transmission is mainly concentrated  addition, a health risk assessment (carcinogenic risk and non-
          near pollution sources. The latter is relatively stable and has a
                                                              carcinogenic risk) owing to inhalation exposure to specific
          relatively long residence time in the atmosphere. Many studies  VOCs was performed. Finally, the effects of local sources and
          have shown that VOCs with a higher photochemical reactivity
                                                              long-distance sources on VOC pollution were assessed using
          are generally important precursors to the formation of sec-  the potential source contribution function (PSCF) during both
          ondary organic aerosol (SOA) and ozone (Sun et al., 2016; Hui
                                                              sampling periods.
          et al., 2018). VOCs can react with OH radicals, ozone, and ni-
          trogen oxides (NO x ) in the atmosphere to form SOA. The sub-
          sequent accumulation of SOA accelerates the formation of haze  1.  Materials and methods
          (Rao et al., 2016). Huang et al., 2014 have reported that the for-
          mation of haze is greatly promoted by secondary aerosol for-  1.1.  Site description
          mation, contributing 30e77% and 44e71% of PM2.5 and organic
          aerosol, respectively. Therefore, studying the formation pro-  The sampling site was selected in Chaoyang District, Beijing,
          cess of SOA is expected to further our understanding of the  China (40 02 N, 116 24 E), which was near residential areas,

          formation of haze and reduce air pollution.         schools, parks, and road junctions. Samples were collected at
             VOCs are also of interest because of their participation in  10 am in winter from 15 December 2015 to 17 January 2016 and
          tropospheric ozone formation, stratospheric ozone depletion,  in summer from 21 July to 25 August 2016. Due to the appro-
          and indirect global greenhouse effects (Jaars et al., 2018; Jiang  priate temperature and solar radiation, the VOC concentration
          et al., 2017). In the presence of solar radiation, VOCs, NO x , and  of Beijing at 10 am was close to the daily average concentra-
          CO can form O 3 through a series of chemical reactions (Kumar  tion and was good to represent the daily concentration,
          et al., 2014). Numerous studies have been carried out on the  compared with the result of Gu et al., 2019.
          potential contribution of VOCs to the formation of O 3 .
          Bowman and Seinfeld (1994) and Carter (1996) have shown  1.2.  Sampling and chemical analysis
          that the composition of VOCs in the atmosphere is complex
          and variable, and the ozone generation potential varies greatly  VOCs were monitored by a gas chromatograph with flame
          because of the difference in concentration of each component,  ionization detector which was coupled to a mass spectrom-
          concentration of free radicals, and photochemical reaction  eter (6890/5973, Agilent Technologies; Santa Clara, CA, USA)
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