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2                          JOUR NAL O F E NVI R ONM E NT AL S C I E NC ES 79 (201 9) 1 – 10



          (O 3 ) and secondary organic aerosol (SOA) in different environ-  would  cause  the  observed  ratios  of  [MVK]/[isoprene]
          ments (Dreyfus et al., 2002; Makar et al., 1999). Therefore,  and [MACR]/[isoprene] to deviate from those calculated by
          investigating the roles of BVOCs in atmospheric photochem-  the reaction yields of isoprene and OH (Grosjean et al., 2001).
          istry is important for air quality improvement on local,  The Pearl River Delta (PRD) region, one of the highly
          regional, and global perspectives.                  developed regions in China, experiences severe photochem-
             Isoprene, with global emission budget of 500–750 Tg/year  ical pollution characterized by a continuous increase in the O 3
          and a chemical lifetime (due to oxidation by OH radical)  abundance due to rapid urbanization and industrialization. To
          shorter than an hour, is the most important BVOC for the  quantify the role of BVOCs such as isoprene, in photochemical
          formation of O 3 and SOA formation at the boundary layer  O 3 formation, many efforts have been made in this region and
          (Guenther, 2008; Atkinson et al., 2006; Guenther et al., 2006;  include the development of emission inventories (Wang et al.,
          Carter and Atkinson, 1996). In recent years, knowledge of the  2011; Zheng et al., 2009), source identification (Cheung et al.,
          mechanisms for oxidation cycles of isoprene has advanced  2014; Leung et al., 2010; Liu et al., 2009) and model simulation
          significantly based on a number of laboratory and simulation  of potential contribution of isoprene to photochemical O 3
          studies (Atkinson et al., 2006; Pinho et al., 2005; Jenkin and  (Guo et al., 2012; Zheng et al., 2009). These studies have not
          Clemitshaw, 2000; Chew et al., 1998). During daytime, iso-  only quantified the source attributions of some typical VOCs
          prene experiences sequential oxidation initiated by the OH  and oxygenated VOCs (OVOCs), but also highlighted the
          radical in NO x -rich environments to produce methyl vinyl  significance of BVOCs in photochemical O 3 formation. As
          ketone (MVK), methacrolein (MACR) and formaldehyde  important intermediate products of isoprene oxidation, the
          (HCHO) as oxidation products. Furthermore, oxidation of  abundance and evolution of MACR and MVK could provide
          MACR and MVK by the OH radical in NO x -rich environments  important insights into the relationships between isoprene
          leads to the formation of glycolaldehyde, methylglyoxal and  and its secondary products. However, the contributions of
          hydroxyacetone (Spaulding et al., 2003), which could subse-  primary emissions and secondary formation to MACR and
          quently participate in O 3 and SOA formation.       MVK abundances as well as their subsequent photooxidation
             The evolution of isoprene and its oxidation products in  are still poorly understood in the PRD region which is
          the atmosphere is different from that determined in the  characterized by diverse sources of MACR and MVK (Cheung
          laboratory studies due to continuous emissions of isoprene,  et al., 2014; Liu et al., 2009). Therefore, in this study, we
          radical variations, and anthropogenic emissions of MACR and  presented an intensive field measurement of isoprene, MACR
          MVK in the atmosphere (Karl et al., 2009). Therefore, to  and MVK at a receptor site (Heshan site, HS) in the PRD region.
          advance the understanding of the behaviors of isoprene and  Additionally, the source contributions of MACR and MVK as
          its oxidation products in the real ambient air, many field  well as their contributions to subsequently oxidation products
          studies have been conducted to investigate the evolution  were quantified. To the best of our knowledge, this is the first
          of isoprene and its contributions to O 3 formation in different  study on the source apportionments of MACR and MVK and
          environments (e.g., Jones et al., 2011; Park et al., 2011; Roberts  the evaluation of their oxidized products in the PRD.
          et al., 2006; Spaulding et al., 2003). The most frequently used
          method for studying the contributions of secondary formation
          to the observed MACR and MVK concentrations is based on  1. Materials and methods
          the ratios of isoprene and its oxidation products, i.e., [MACR]/
          [MVK], [MACR + MVK]/[isoprene], [MVK]/[isoprene] and  1.1. Site description
          [MACR]/[isoprene]. This method relies on differences in the
          photochemical reactivities of isoprene and its oxidation  The field measurement was conducted at Guangdong Atmo-
          products in the atmosphere (Table 1). However, ratio methods  spheric Supersite of China, Heshan (112.93°E, 22.73°N) (Fig. 1),
          are based on the assumption that no fresh emissions of  Jiangmen City, Guangdong Province. The urban centers of
          isoprene, MACR and MVK are introduced into the atmosphere  Jiangmen City were ~30 km to the east of the study site, while
          or, alternately, isoprene emissions are constant during the  the urban centers of Foshan and Guangzhou cities were ~50
          processes and physical processes do not influence the  and 80 km, respectively, to the northeast. The measurement
          observed ratios of isoprene and its oxidation products  was conducted from 22 October to 20 November 2014 since
          during transport. Therefore, any other emissions of MACR  photochemical smog and air masses from the PRD were
          and MVK (in addition to photochemical oxidation of isoprene)  frequently observed during this season (Zhou et al., 2013;
                                                              Zheng et al., 2010; Zhang et al., 2008). Detailed description of
                                                              the sampling site and its surrounding environments has been
                                                              provided in the previous study (Zhou et al., 2013).
            Table 1 – Reaction rate constants for isoprene, MVK and
                    3
            MACR (cm /(molecule. sec)
                                                              1.2. Chemical analysis of VOCs
            Species   k O3  a   k OH  a   k NO3     k Cl  b
            Isoprene  1.28× 10 −17  1.10× 10 −10  6.16× 10 −12  4.0× 10 −10  In this study, hourly VOC concentrations, including 58 non-
            MVK     4.56× 10 −18  1.88× 10 −11  < 6.00× 10 −16  2.2× 10 −10  methane hydrocarbons (NMHCs) and 19 OVOCs were deter-
            MAC     1.14× 10 −18  3.35× 10 −11  3.30× 10 −15  2.4× 10 −10  mined and analyzed by an automated online GC-mass
                                                              spectrometer (MS)/flame ionization detector (FID) system
            a  Carter and Atkinson, 1996
            b  Apel et al., 2002.                             (Hewlett Packard 7890/5975). These target compounds were
                                                              identified based on their retention times and mass spectra,
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