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(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,