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jo urna l o f e nviro n mental scie nces 93 (2020) 1 e12              5

              In the present study, the SOAP i value of each VOC species is  the opposite trend was observed in summer. In summary,
           directly derived from Derwent et al. (2010). The SOAP (mg/m )  evaluating aromatic sources and controlling aromatic emis-
           of a species i can be calculated using Eq. (2) as follows:  sions are important for reducing the formation of SOA in
                   ðE i  SOAP i Þ
           SOAP ¼            FAC toluene                (2)     Fig. 3 shows the top 10 SOAP species across the seasonal
                                                               days studied. As observed, aromatics contributed mostly to
           where, E i (mg/m ) represents the emission mass contribution  the SOAP. The top 10 SOAP species accounted for 95.90%e
           of VOC species i and FAC toluene (%) is the fractional aerosol  97.21% of the total SOAP. In winter, the most abundant SOAP
           coefficient of toluene. In the present study, FAC toluene was set  species was benzene, accounting for 30.83 and 31.84% of the
           to 5.4 (Zhang et al., 2017).                        total SOAP on normal days and polluted days, respectively,
              Using the FAC coefficient method to estimate the genera-  followed by toluene, m/p-xylene, ethylbenzene, and o-xylene.
           tion potential of SOA, only reactions between VOCs and OH  In summer, the most abundant SOAP species was toluene,
           radicals are considered, whereas reactions between VOCs and  accounting for 29.69% and 30.33% of the total SOAP on normal
           NO 3 radicals or O 3 are not considered. These considerations  days and polluted days, respectively, followed by other types
           can thus lead to lower estimates. However, this approach can  of aromatics. The different result obtained in summer and
           still give an approximate value associated with the formation  winter may be due to differences in the source of pollution.
           of SOA as well as indicate the relative contribution of each  Other anthropogenic sources, such as industry, can also affect
           SOA precursor, thereby enabling determination of key pre-  the formation of SOA. Thus, determining the source of VOCs is
           cursors involved (Dechapanya et al., 2004). In the present  beneficial for achieving better control on the formation of
           study, the SOAP decreased in the order of winter polluted  SOAs.
           days > summer polluted days > winter normal days > summer
           normal days. In winter, the total SOAP on polluted days  2.4.  Contribution to ozone formation
           (199.70 ± 15.05 mg/m ) was much higher than that on normal
           days (51.00 ± 3.73 mg/m ) because of the considerable increase  VOCs are also important precursors to the formation of O 3 ,
           in the concentration of aromatics i.e., from 10.13 to 39.19 mg/  and many cities suffer from severe O 3 pollution. The various
           m . This result indicates the strong correlation between the  components in VOCs can photochemically react with free
           formation of haze in winter in Beijing and SOA. In summer,  radicals in the atmosphere to form ozone, but their contri-
           the SOAP on polluted days (61.17 ± 4.14 mg/m ) was only  bution is different. Determining the ozone formation potential
           slightly higher than that on normal days (51.90 ± 3.36 mg/m ).  (OFP) has become a common method to estimate the forma-
              Generally, aromatics have high SOAPs and are the main  tion of ozone. Maximum incremental reactivity (MIR), as
           source of SOA formation (Kroll and Seinfeld, 2008). Fig. 2  defined by Carter (1994), is used to calculate the OFP of each
           shows the contribution of alkanes, alkenes, and aromatics to  component in VOCs, as described by Eq. (3):
           the SOAP across the seasonal days (winter/summer normal
           and polluted days) studied. These VOCs were chosen as they  OFP i ¼ E i  MIR i                   (3)
           are the main contributors to SOAP (relative to the other VOCs).  where, OFP i is the ozone formation potential of VOC species i
           Among these three classes of VOCs, aromatics contributed the  and MIR i is the ozone formation coefficient for VOC species i in
           most to SOAP (>97%) irrespective of the seasonal days studied.  the maximum increment reactions of ozone (Carter, 1994).
           This is consistent with the result reported by Hui et al., 2019.In  The total OFP was the highest during the polluted days in
           winter, alkenes contributed more to SOAP than alkanes, but  winter (497.29 mg/m ) in Beijing, followed by summer pollution
                                                               days (271.36 mg/m ), summer normal days (220.95 mg/m ), and
                                                               winter normal days (129.85 mg/m ). The total OFP was higher
                                                               during the polluted days than that during the normal days.
                                                               Thus, haze days are often accompanied with high ozone
                                                               concentrations. Fig. 4 shows the contribution of VOCs to OFP.
                                                               The OFP results for halocarbons and acetonitrile are not
                                                               shown owing to their low photochemical reactivity. The low
                                                               reactivity of halocarbons is due to the strong interaction be-
                                                               tween the carbon and chlorine atoms (Kumar et al., 2018). In
                                                               winter, alkenes contributed the most to OFP, reaching more
                                                               than 40%, followed by aromatics and OVOCs. In summer,
                                                               however, the OFP of OVOCs accounted for the highest pro-
                                                               portion of total OFP, close to 50%, followed by alkenes and
                                                               aromatics. The high contribution of OVOCs to OFP is due to the
                                                               surge in the concentration of OVOCs in summer (from
                                                               476.50 mg/m 3  in winter up to 1149.02 mg/m 3  in summer).
                                                               However, the low environmental concentration of VOCs does
                                                               not necessarily mean low OFP. And the contribution of al-
           Fig. 2 e Variations in the contribution of VOC components  kanes and acetylene to OFP is stable across the different
           to SOAP.                                            seasons.
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