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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
100
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