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

          and n-hexanal were the most abundant OVOCs, contributing  the order of summer normal days (1.99 ± 0.61) > summer
          30.8, 29.6, and 16.7%, respectively, to the total OVOC concen-  polluted  days  (1.48  ±  0.38)  >  winter  polluted  days
          tration. The mean concentration of OVOCs in summer was  (0.82 ± 0.15) > winter normal days (0.76 ± 0.31). As these values
          more than two times higher than that in winter. Halocarbons  were all below two, it can be deduced that vehicle exhaust
          are almost exclusively emitted from anthropogenic sources  emissions had a significant effect on the production of VOCs
          such as industry (Hui et al., 2018). Changes in the level of  in Beijing during the sampling periods studied. However, this
          halocarbon emissions can be attributed to the type of industry  effect was higher in winter than in summer, as the T/B values
          involved in the emissions. During all the observation period,  in summer were much higher than in winter.
          dichloromethane was the most abundant halocarbon com-  Another important diagnostic ratio is the m/p-xylene-to-
          pounds, followed by Freon-11 and chloroform. Among the  ethylbenzene (X/E) ratio (Yurdakul et al., 2018). It is often used as
          alkenes, ethylene was the most abundant at 59.5%. The alkyne  an indicator for the occurrence of photochemical reactions
          category comprised acetylene only, which contributed very  because the reaction rate of m/p-xylene and OH radicals is much
          little to TVOCs, similarly to acetonitrile, an important biomass  faster than that of ethylbenzene in the atmosphere (Atkinson,
          burning tracer (Yuan et al., 2010). In general, the contribution  1990; Vardoulakis et al., 2011). A low X/E value implies the pro-
          of each type of VOC to the TVOC concentration was compa-  pensity of reaction between m/p-xylene and OH radicals, shorter
          rable during the normal days or polluted days, irrespective of  VOC residence time in the atmosphere, and occurrence of air
          the season examined. The ratios of the concentrations of  mass aging (Han et al., 2017; Huang et al., 2015). In urban areas,
          alkane, alkene, acetylene, and aromatic to the concentrations  the X/E values are generally between 2.5 and 2.9. When X/E ratio
          of TVOC were higher in winter than in summer. For the  is considerably lower than 3, the degree of air mass aging is high
          remaining categories of VOC, the ratios were reversed.  and VOCs may propagate from other locations (Yurdakul et al.,
             The seasonal change in the VOC concentration can be  2018). In the present study, the mean X/E value during polluted
          attributed to the concentration of OH radicals, difference in  days in winter (2.65) was higher than that during normal days in
          emission source, and changes in the atmospheric meteorolog-  winter (2.39). In contrast, the mean X/E value during summer
          ical conditions (Hui et al., 2018). In summer, the temperatures  normal days (2.37) was higher than that during winter polluted
          are higher and solar radiation is more intense than in winter,  days (2.01). Overall, the average value of X/E in summer (2.18)
          thus exacerbating the photochemical reactions of VOCs in the  was lower than that in winter (2.52), but all less than 3. It indi-
          atmosphere. Therefore, the VOCs generated in summer are  cated that there was a stronger atmospheric photochemical
          consumed to a larger extent, resulting in lower concentrations  reaction and higher air mass aging in summer than in winter.
          of TVOC in summer than in winter. In contrast, the release of  And it can also thus be deduced that distant sources may
          more emission source such as coal combustion might also  contribute to VOCs presence in the study area studied.
          contribute to higher concentration of VOCs in winter. In addi-
          tion, unlike other VOC types, the concentration of OVOC in  2.3.  SOA formation potential
          summer is much higher than in winter. This result is mainly
          due to the enhancement of atmospheric photochemical re-  VOCs are important precursors to the formation of SOA and are
          actions in summer. OVOC is the product of VOC atmospheric  essential for the formation of complex air pollution such as
          photochemical reactions (Hui et al., 2019). Thus, the occurrence  urban haze. VOCs mainly generate SOA by oxidation reaction
          of amplified photochemical reactions in summer produces a  with OH radicals, NO 3 radicals, and O 3 in the atmosphere
          large amount of OVOCs. And OVOCs are typically released from  (Tajuelo et al., 2019). The rate of reaction depends on the differ-
          plants at higher concentrations in summer than winter (Hakola  ences in the chemical properties of the different classes of VOCs.
          et al., 2017; Yoshino et al., 2012).                  In many studies, the secondary organic aerosol potential
                                                              (SOAP) is used to estimate SOA formation. Derwent et al.
          2.2.   Diagnostic ratios                            (2010) first used the concept of SOAP to calculate the contri-
                                                              bution of VOCs to the formation of SOA. The SOAP represents
          Diagnostic ratios are often used to estimate the sources of  the propensity of each organic compound to form SOA on an
          VOC in the atmosphere and the aging degree of air masses  equal mass-emitted basis relative to toluene, and it is typically
          (Tiwari et al., 2010; Niu et al., 2012). The toluene-to-benzene  expressed as an index relative to that of toluene (considered
          (T/B) ratio is commonly used for preliminary estimation and  as 100) (Derwent et al., 2010). Toluene is chosen as the base
          assessment of sources of VOCs. Both benzene and toluene are  compound for determining the SOAP because of its well
          mainly derived from the emission of automobile exhausts and  characterized emissions and wide consensus that it can serve
          the volatilization of gasoline. In addition, toluene emission  as an important anthropogenic precursor to SOA formation
          can arise from the volatilization of organic solvents in paints.  (Hu et al., 2008; Johnson et al., 2006; Kleindienst et al., 2007).
          Therefore, a low T/B value indicates an increase in the effect of  The SOAPi is the SOA formation potential parameter for
          vehicle exhausts on the sources of VOCs (Barletta et al., 2008).  species i which can be calculated using Equation (1) below
          For instance, many studies have shown that T/B values of less  (Derwent et al., 2010):
          than 2 are indicative that the main source of VOCs in a given
                                                                    Increment in SOA mass concentration with species; i
          area is vehicle exhaust (Nelson and Quigley, 1984). T/B values  SOAP i ¼
          above 2 are indicative of the contribution of other sources and    Increment in SOA with toluene
          T/B values above 10 infer the contribution of strong industrial
          sources nearby a given study area (Niu et al., 2012; Kumar                                        (1)
          et al., 2018). In the present study, the T/B value decreased in
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