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Aerosol and Air Quality Research, 17: 1660–1671, 2017
Copyright © Taiwan Association for Aerosol Research
ISSN: 1680-8584 print / 2071-1409 online
doi: 10.4209/aaqr.2017.05.0179
Ozone and Secondary Organic Aerosol Formation of Toluene/NOx Irradiations
under Complex Pollution Scenarios
Linghong Chen, Kaiji Bao, Kangwei Li, Biao Lv, Zhier Bao, Chao Lin, Xuecheng Wu,
Chenghang Zheng*, Xiang Gao, Kefa Cen
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China
ABSTRACT
Toluene is one of the most important precursor contributors to ozone and secondary organic aerosol (SOA), both of
which greatly affect the air quality and human health. In this study, the effects of toluene on ozone and SOA formation
were investigated in the presence of NOx in the CAPS-ZJU (Complex Air Pollution Study-Zhejiang University) smog
chamber. Three comparison groups of experiments were conducted under the gas-phase environments of toluene/NOx,
toluene/isoprene/NOx, and toluene/ethylene/NOx. The ozone concentration and physical properties of SOA such as mass
concentration, aerosol yield, effective density, extinction and scattering were measured simultaneously. A toluene-
dependent mechanism of ozone formation was found at ratios of toluene and NOx between 3.1 and 11.3 with the initial
NOx concentration about 30 ppb. With further increase of the toluene concentration, the maximum value of ozone
concentration remained almost stabilized. The maximum SOA yields decreased with increase of toluene, while the SOA
effective density was concentrated at 1.3–1.4 g cm–3. The presence of isoprene or ethylene can promote the formation of
ozone and SOA. The SOA nucleation was delayed under different initial toluene concentrations and the yield was reduced
at the same mass concentration. A linear increase of extinction and scattering was observed with the increase of SOA mass
concentration in both the toluene/isoprene/NOx and toluene/ethylene/NOx systems. A rapid increase of single scattering
albedo reflects the process of SOA nucleation and growth. In addition, organic aerosol oxidation products tend to
carboxylic acids in toluene/isoprene/NOx system according to Van Krevelen.
Key words: Smog chamber; Toluene; Ozone; Secondary organic aerosol; NOx.
INTRODUCTION
Ozone and secondary organic aerosol (SOA) are of great
importance in the atmosphere due to their strong impacts
on air quality (Tong et al., 2016a, b), climate change and
public health (Srebot et al., 2009; Lv et al., 2015; Yin et
al., 2015; Briggs et al., 2016; Kanaya et al., 2016; Tong et
al., 2017). Toluene, as one of the dominant aromatic
hydrocarbons, is a potential source of photo-oxidation of
ozone and SOA formation in urban air (Cheng, 2016; Sahu
et al., 2016; Yang et al., 2016; Deng et al., 2017).Ozone
formation potential (OFP) and aerosol formation potential
(AFP) are used to assess the ability of VOCs to generate
ozone and SOA, reflecting the relative contribution of ozone
and SOA precursors. According to analysis of the AFP and
OFP during August and September of 2016 in Hangzhou,
toluene is the most contributor not only for ozone formation,
* Corresponding author.
Tel: +86-571-87952243; Fax: +86-571-87951616
E-mail address: zhengch2003@zju.edu.cn
but also for SOA as well (the average of OFP and AFP of
toluene is 12.4 µg m–3 and 0.9 µg m–3, respectively).
Many studies have shown that the VOCs or NOx
concentrations have a great influence on the formation of
ozone and SOA. Johnson et al. (2004) observed a decreasing
trend of toluene SOA yield with decreasing toluene to NOx
ratio, with the NOx concentration ranging from 45 ppb to
1300 ppb and toluene concentration ranging from 500 ppb
to 6600 ppb. Meanwhile, the decreasing trend of toluene
SOA yield with increasing NOx concentration is less obvious
in Xu et al. (2015), which might be due to the different
initial reactant concentrations. A similar phenomenon was
also found in other VOCs, such as m-xylene, isoprene and
propylene (Song et al., 2005; Hu et al., 2011; Beardsley and
Jang, 2016; Ge et al., 2017). The previous studies mainly
focus on the impact of the toluene/NOx ratio on SOA
formation under different initial NOx concentrations in the
photo-oxidation reaction (Ng et al., 2007; Sato et al., 2007),
and chamber experiments were conducted under non-
atmospherically relevant conditions (see Table 1). The
influence of the toluene/NOx ratio with different initial
toluene concentrations at atmospheric conditions on ozone
and SOA formation has been little studied.
Chen et al., Aerosol and Air Quality Research, 17: 1660–1671, 2017 1661
Table 1. Summary of initial concentration setting of references in smog chamber.
VOC NO2 NO NOx T RH
References
Species ppm ppb ppb ppb °C %
Toluene 1.38 -- -- 370 28.7 30 Offenberg et al., 2007
Toluene 0.18–0.95 -- -- < 5, 270–1300 11–32 4–22 Hildebrandt et al., 2009
Toluene 0.088–0.27 524–568 373–421 -- 23–25 3.8–5.9 Ng et al., 2007
Toluene 3.8–4.0 -- 200–1000 210–1027 25 ± 1 -- Sato et al., 2007
α-pinene/Toluene 0.1/1.6 -- -- 378 24 30 Jaoui et al., 2008
The ozone and SOA produced from individual VOCs
precursor irradiations can be significantly affected by the
presence of other VOCs (Jaoui et al., 2008). The presence
of toluene and other atmospheric VOCs can affect the gas
phase chemistry of toluene and influence the oxidation
products (Chen and Jang, 2012). Jaoui et al. (2008) found
that adding isoprene to α-pinene/toluene/NOx system
significantly lowered the reacted amount of toluene, and
increased the ozone concentration. Zhou et al. (2011) found
that the HC mix system (o-, p-xylene and toluene with NOx)
under natural sunlight showed the highest SOA yield in the
o-xylene system compared to toluene and p-xylene. These
studies show that the effects of various VOCs precursors
on ozone and SOA generation are not simply linearly
superimposed. Thus, it is expected that the mixed VOCs
will affect the formation of ozone and SOA in toluene
photo-oxidation reactions.
In this study, the progress of the photo-oxidation of
toluene was investigated in a smog chamber. The influence
of the toluene/NOx ratio with different initial toluene
concentrations and mixed VOCs on the ozone and SOA
formation was studied.
EXPERIMENT SECTION
The photo-oxidation of toluene was performed in the
absence of seed aerosol in the Complex Air Pollution Study-
Zhejiang University (CAPS-ZJU) smog chamber, which
has been previously described (Li et al., 2017a). The volume
of the Teflon bag reactor of the experiments was 3 m3,
which had a surface/volume of 4.32 m–1. The experiments
were performed for around 8 h at a light intensity equivalent
to JNO2 of 0.17 min–1 with 20 black lamps (GE F40BLB, peak
intensity at 365 nm) opened. Prior to each experiment, the
chamber was flushed continuously by zero-air, which was
provided through an Aadco 737 series air purification system.
The background particles number concentration of the
reactor was below 10 # cm–3 and the concentration of NOx
was below 2 ppb. The temperature was relatively constant
at 25°C before the experiments began and increased to a
stable value (approximately 30–32°C) after the lamps were
turned on. The relative humidity (RH) inside the chamber
stabilized at approximately 15–20%. A fan inside the
chamber was used to help the components in the reactor to
mix well after injection.
Standard gases such as toluene, isoprene, ethylene, NO
and NO2 were injected into the chamber precisely by mass
flow meters. The standard concentration of each standard
gases is 100 ppm, and nitrogen was used as the background
gas. The concentrations of gas-phase NOx and ozone were
measured with an interval of 1 min by gas analyzers
(Thermo 42i, 49i). The rate constants for the wall loss of
NO2 and ozone were (1.642 ± 0.004) × 10–4 min–1 and (8.987
± 0.067) × 10–4 min–1, respectively. Toluene, isoprene and
ethylene were measured by an automated Preconcentrator
(Entech 7100A), coupled with a gas chromatography-mass
spectrometry instrument (GC-MS, Agilent 7980B-5977A).
The aerosol particle size distributions and number
concentration were monitored by using a Scanning Mobility
Particle Sizer (SMPS, TSI 3936), with a long Differential
Mobility Analyzer (DMA, TSI 3081) in combination with
a butanol Condensation Particle Counter (CPC, TSI 3776).
Effective density was determined by the SMPS coupled
with an Aerosol Particle Mass Analyzer (APM, Kanomax
3601). The wall loss constant of particles was obtained to
be (3.52 ± 0.031) × 10–3 min–1. Meanwhile, the chemical
composition of SOA and the non-refractory submicron
aerosol mass was measured using a high resolution time-
of-flight aerosol mass spectrometry (Aerodyne, HR-TOF-
AMS). The optical properties of SOA were measured with
a Cavity Attenuated Phase Shift equipment (Aerodyne,
CAPS PMssa).
RESULTS AND DISCUSSION
According to analysis of AFP and OFP during August
and September of 2016 in Hangzhou, toluene, isoprene and
ethylene are the important contributors for ozone and SOA
formation. The average concentration of NOx in August to
September from 2013 to 2015 at 11sites in Hangzhou was
about 25–30 ppb. Based on the atmospheric environments
of Hangzhou, a series of toluene photo-oxidation reactions
was conducted with different initial toluene concentrations
and mixed VOCs to evaluate the ozone and SOA formation
from the toluene/NOx system. The temperature in the
progress of the reaction was controlled at 303–305 K and
relative humidity was maintained at 15–20%. The initial
experimental conditions are listed in Table 2.
The Effect of Initial Toluene Concentration on Ozone
Formation
In order to investigate the influence of the initial
concentration ratios of toluene/NOx (ppbC:ppb) on the
peak ozone concentration, a series of experiments were
conducted (Exps. 1–8). The results in Fig. 1 show that the
maximum ozone concentration has two different trends
with the increasing toluene/NOx ratio (increasing the toluene
concentration at relatively constant NOx concentrations).
Chen et al., Aerosol and Air Quality Research, 17: 1660–1671, 2017
1662
Table 2. Initial conditions for each experiment.
Expt. VOCs NO2 NO VOCs/NOx
Species ppb ppb ppb ppbC:ppb
1 Toluene 14.2 27.5 4.8 3.1
2 24.5 26 4.9 5.5
3 37.2 24.5 4.5 8.9
4 49.7 25.7 4.9 11.3
5 58.6 24.7 5.3 13.6
6 66 25.3 4.6 15.5
7 84 24.4 5.3 19.8
8 107 24.5 5.7 24.8
9 Toluene/isoprene 17.6/14.4 24.6 6.4 6.3
10 Toluene/ethylene 17.5/14.4 26.1 5.4 4.8
* Each experiment used preconcentrator, GC-MS and gas analyzers. Expt. 3, 5, 8 did not use SMPS and APM, CAPS is
used in Expt. 9, 10, and AMS is used in Expt. 9.
Fig. 1. Effects of toluene/NOx ratios on the maximum
ozone concentration in toluene/NOx system (under toluene
limited conditions, NOx ~30 ppb, NO2:NO ~5:1)
At toluene/NOx ratio values from 3.1 to 11.3, the maximum
ozone concentration increases linearly from 35.1 ppb to
136.2 ppb (the maximum ozone concentration = 5.3 + 11.6
× toluene/NOx ratio, R2 = 0.97), which is controlled by the
initial toluene concentration. Meanwhile, the maximum ozone
concentration remained unchanged when the toluene/NOx
ratio was from 15.5 to 24.8, this regime being controlled
by the initial NOx concentration. However, the maximum
ozone concentration is different from these two ranges
when the ratio is 13.6. Jiménez (2004) and Mazzuca et al.
(2016) observed that there is a transition regime, in which
the ozone concentration is controlled by VOCs and NOx at
the same time. This inspires us, and a plausible explanation
may be like this: when toluene/NOx ratio is between 11.3
and 15.5 (the concentration of toluene was from 49.7 ppb
to 66 ppb, and NOx was constant), the ozone concentration
is influenced by toluene and NOx, which leads to the
different trend of maximum ozone concentration.
Meanwhile, the appearance of the maximum ozone
concentration was advanced to 225 min with the increase
of the toluene/NOx ratio. 5.6%–58.3% of the initial toluene
is consumed, and the attenuation rate of the toluene
increases from 1.1 × 10–4 to 1.6 × 10–3 min–1. When the
ratio is greater than 13.6, the decay rate of toluene tends to
stabilize (see Table 3). Such phenomena might mean that
more toluene is oxidized by OH radicals at the high
toluene/NOx ratio. At the same time, the organo-peroxide
radicals (RO2) concentration increased due to the restriction
of conversion of NO to NO2 (Jia et al., 2010), leading to
the accumulation of ozone concentration. In addition, the
initial toluene concentration is excessive with the increase
of the toluene/NOx ratio, which causes the attenuation rate
of toluene to become gradually gentler.
The Effect of Initial Toluene Concentration on SOA
Formation
Fig. 2 shows the typical photo-oxidation of toluene and
NOx. In this experiment, with the initial toluene/NOx ratio
~3:1 (ppbC:ppb), 0.8 ppb of toluene is reacted, and the
initial NO2 and NO concentrations are 27.5 ppb and 4.8
ppb, respectively. In the absence of initial HONO, OH
radicals are likely to drive the photolysis of HONO, which
is formed from the heterogeneous reaction of NO2 on the
chamber wall, or generated through recycling via NOx/HOx
chemistry (Ng et al., 2007). Aerosol growth did not occur
immediately, but was observed after the semi-volatile or
nonvolatile organic matter formed and accumulated to a
certain concentration by photo-oxidation.
The particle size distributions from five toluene/NOx
photo-oxidation experiments are shown in Fig. 3 (initial
toluene concentrations in Figs. 3(a)–3(e) are 14.2 ppb,
24.5 ppb, 49.7 ppb, 66.0 ppb, and 84.0 ppb, respectively.).
It was observed that the aerosol formation increases with
increasing of the initial toluene concentration, the particles
size grows further, and the total number concentration of
our chamber experiments reached the level of 103–104 # cm–3.
The toluene/NOx system shows a 50–100 min induction
period (no significant SOA growth), the time to reach the
saturation of the gas-phase toluene oxidation products, this
suggesting that SOA is further oxidized from toluene photo-
oxidation products (Song et al., 2005; Chen and Jang, 2012).
Li et al. (2017b) used two methods of Log Normal and
Max Concentration to derive the particle size evolution.
The asymptotic fit with merging data was used to decrease
Chen et al., Aerosol and Air Quality Research, 17: 1660–1671, 2017 1663
Table 3. Summary of the chamber experiment results.
Expt. ΔTola O
3max Time of O3max
b
Ntotal max M
0max
k
toluene Yield _max
µg m–3 ppb min # cm–3 µg m–3 min–1
1 3.1 35.1 480 1.9 × 103 1.6 1.1 × 10–4 0.38
2 11.3 75.9 480 2.3 × 103 7.8 2.5 × 10–4 0.37
3 61.5 104.5 480 -- -- 1.1 × 10–3 --
4 73.4 136.2 480 1.3 × 104 14.3 1.0 × 10–3 0.22
5 126.8 128.9 450 -- -- 1.5 × 10–3 --
6 137.2 120.3 324 2.1 × 104 17.5 1.5 × 10–3 0.21
7 190.6 117.7 306 2.7 × 104 20.6 1.6 × 10–3 0.19
8 234.9 120.5 225 -- -- 1.6 × 10–3 --
a: ΔTol is calculated at the end of each experiment, and each experiment is irradiated for 6–8 h;
b: In the experiment, if the ozone concentration does not reach the maximum, we take 480 min as the appearance time of
maximum ozone concentration.
Fig. 2. Reaction profile of a typical toluene/NOx photo-oxidation experiment (initial condition: 14.2 ppb of toluene, 27.5 ppb
of NO2, and 4.8 ppb of NO).
the uncertainties of particle size growth, and the particle
size evolution obtained by the two methods is consistent
with the contour plot of the particle size distribution. These
experimental results show that the particles condensation
grows with the progress of the reaction, while the peak size
growth rate increases first and then tends to flatten. This
indicates that the increase of the initial toluene concentration
is useful to generate SOA, and rapidly promotes the reaction.
The SOA density measured by the SMPS-APM is
displayed as a function of diameter in Fig. 4(a). Before the
experiments, a calibration curve was obtained to correct
the decay time for data collection between APM and CPC,
using spherical Polystyrene Latex (PSL) with known sizes
(100 nm) and material density (1.05 g cm–3) (Li et al., 2017b).
A Gauss model was applied in fitting to determine the peak
effective density in different particle sizes. The density of
SOA under different initial toluene concentrations fluctuates
by about 1.1–1.5 g cm–3, and no obvious change trend was
observed. According to the frequency of particle counts in
different effective density ranges, Fig. 4(b) shows that the
effective density range of 1.3–1.4 g cm–3 (average is 1.35
g cm–3) has the highest frequency of 50.8%. This density
range was consistent with other previous studies (Nakao et
al., 2013). Therefore, a constant effective density of 1.35
g cm–3 has been used to obtain the mass of the particles in
the toluene/NOx system, and the density of 1.3 g cm–3 was
used in the mixed VOCs system, which is based on
Offenberg et al. (2007).
The SOA yield was estimated using the total organic
aerosol mass concentration and consumption of toluene.
Fig. 5 shows the curves of SOA yield (Y) versus organic
aerosol mass concentration (M0) under different initial
toluene concentrations. It is noticeable that the high initial
toluene concentration tends to have lower yields at the
same mass concentration (see Table 3, where the maximum
yield of SOA decreased from 0.38 to 0.19). The SOA yield
shown in Exps. 6–7, which increased first and then decreased,
might be because the deposition of particles in the reactor
wall is greater than the generation of SOA, so SOA would
decline after reaching the maximum in the experiment, yet
toluene was still oxidized, which contributed to the different
yields.
The yields measured in our experiments are higher than
previous measurements (Takekawa et al., 2003; Song et
al., 2005; Song et al., 2007). According to the SOA growth
curve under different initial toluene concentrations, when
Chen et al., Aerosol and Air Quality Research, 17: 1660–1671, 2017
1664
Fig. 3. Contour plot of particle size distribution for 5 experiments (Expt. 1, 2, 4, 6, 7). Red scatters and white scatters
represent data using Log. Nor. and Max. Conc., respectively. Red line represents asymptotic fit for merging data of two
approaches.
the SOA yields are consistent, the experiment with a high
initial toluene concentration produces more aerosol mass
concentration and consumes a greater amount of toluene. If
M0 is the same, the lower the initial toluene concentration
(lower toluene/NOx ratio), the higher the SOA yields, which
was also observed by Chen et al. (2016), who found that the
SOA yields were 0.49 and 0.28 when 2-methylnaphthalene
was 20.0 ppb and 29.1 ppb, respectively. This is different
from the phenomenon that the yield increased with the
increase of the VOCs/NOx ratio. Possible reasons maybe
as follows. (1) The SOA yields measured under low-NOx
(< 100 ppb) conditions are higher than high-NOx conditions
in toluene/NOx system or m-xylene/NOx system (Ng et al.,
2007; Chen et al., 2016). In our experiments, the initial NOx
Chen et al., Aerosol and Air Quality Research, 17: 1660–1671, 2017 1665
Fig. 4. Diameter variations of SOA effective densities under different initial toluene concentrations (a) and frequency
distributions of particle counts in different effective density ranges (b).
Fig. 5. SOA yields from toluene photo-oxidation as a function
of M0 in different toluene regimes. Black line represents
allometric fit (initial toluene concentration is 14.2 ppb,
24.5 ppb, 49.7 ppb, 66.0 ppb, 84.0 ppb, respectively.).
concentration is about 30 ppb, which is low-NOx condition;
(2) The initial toluene concentrations change the oxidation
pathways, which leads to different yields. In high initial
toluene concentration conditions, the reaction rate is higher,
which means more oxidation products are generated and may
stay more in gas-phase, while the photo-oxidation products
may be more in the particle-phase in low toluene/NOx ratio
conditions; (3) Ng et al. (2007) suggest that the VOCs/NOx
ratio may be a useful metric of photochemistry for
experiments with similar oxidation conditions, while it is
less useful when comparing systems in different oxidative
conditions. The relationship between the VOCs/NOx ratio
and SOA yields, on which most previous studies report,
were based on the NOx level or changing VOCs and NOx
at the same time. In our study, the initial NOx concentration
of each experiment is the same, and the initial toluene
concentration level has a significant effect on the SOA yields.
The Effect of Multiple VOCs Components on Ozone
Formation
In the toluene/NOx, toluene/isoprene/NOx and
toluene/ethylene/NOx systems, the maximum ozone
concentration significantly increases due to the photo-
oxidation of isoprene and ethylene. Detailed results of
ozone and SOA formation under mixed precursor are listed
in Table 4.
In these three comparison experiments, 5.6%, 32% and
25% of the initial toluene were consumed, respectively. NO
was completely oxidized in the toluene/isoprene/NOx system
at the end of the experiment. As shown in Fig. 6, isoprene
and ethylene could promote the formation of ozone, and it
is clear that isoprene has a greater effect on ozone formation
than that of ethylene in the toluene/NOx system.
Farmer et al. (2011) and Valin et al. (2014) reported that
when the photochemical reaction system is converted from
a high NOx condition ([NOx]0/[VOCs]0 > 1) to a low NOx
condition ([NOx]0/[VOCs]0 < 1), it can promote the photo-
oxidation reaction and increase the concentration of organo-
peroxide radicals (RO2). So adding isoprene or ethylene in
the toluene/NOx system significantly increases the initial
ratio of VOCs/NOx, enhancing the formation of ozone.
Carter (1994) detected that the dynamic reaction of isoprene
(reaction rates of VOCs and OH radicals) is higher than
ethylene in 12 cities of the United States, while the ozone
concentration in the photochemical reaction system would
not rise very high with the presence of NO (Kiros et al.,
2016). Therefore, adding isoprene or ethylene has a positive
effect on ozone, and isoprene is more favorable than ethylene
for the formation of ozone under the same conditions.
The Effect of Multiple VOCs Components on SOA
Formation
The SOA formation from the three comparison
experiments is compared in Fig. 7. The peak particle size
and peak number concentration is still growing in these
three systems. This might be due to the low initial reactants
concentration in the three systems, which caused the slow
Chen et al., Aerosol and Air Quality Research, 17: 1660–1671, 2017
1666
Table 4. Summary of the chamber experiment results.
Mixture ΔTol a ΔIsoa ΔEthya O
3max
N
total max M
0max
Yield _max NO2
b
NO
b
µg m–3 µg m–3 µg m–3 ppb # cm–3 µg m–3 ppb ppb
Toluene 3.1 -- -- 35.1 1.9 × 103 1.6 0.38 19.4 4.6
Toluene/Isoprene 22.2 51.1 -- 106.5 6.9 × 103 15.7 0.17 17.5 0.2
Toluene/Ethylene 14.8 -- 6.6 72.2 5.0 × 103 7.1 0.27 19.7 1.4
a: Reactants consumption is calculated at the end of each experiment.
b: The concentration of NO2 and NO at the end of experiments.
Fig. 6. Temporal profiles of ozone concentration during
experiments.
reaction rate. The fact that the mass concentration and
peak size growth curve shown in Fig. 8 still maintain a
growth trend in the experiment period also proved this
point. Besides, the maximum mass concentrations at the end
of the experiments on toluene/NOx, toluene/isoprene/NOx
and toluene/ethylene/NOx are 1.6 µg m–3, 15.7 µg m–3 and
7.1 µg m–3, respectively. The presence of isoprene or
ethylene delays the nucleation time of SOA, which indicates
that isoprene or ethylene inhibits the SOA nucleation of
the toluene/NOx system.
The addition of isoprene or ethylene in the toluene/NOx
system increases the initial VOCs concentration, promotes
the photochemical reaction, and produces a large number
of organic products. The reason that isoprene is more
favorable than ethylene for the formation of SOA might be
that two of the isomers produced by the reaction of isoprene
with OH radicals are polyhydroxy compounds, which are
highly hygroscopic and can improve the nucleation ability
of aerosols (Claeys et al., 2004). For hydrocarbon that has
a lower carbon number, Eq. (1) > Eq. (2), while if C ≥ 4, the
reaction Eq. (2) > Eq. (1) (Johnson et al., 2004). Therefore,
in the case of more isoprene being reacted (97% of the
isoprene is consumed and 46% of ethylene is reacted), the
toluene/isoprene/NOx system is more capable of generating
more SOA than the toluene/ethylene/NOx system.
RO2·+ NO → RO + NO2 (1)
RO2·+ NO → RONO2 (2)
The growth curve of SOA yields from these three
systems is focused on in Fig. 9(b). When the SOA yields
are consistent, the aerosol mass concentration and organic
products consumed are the highest in the
toluene/isoprene/NOx system. If M0 is the same, the SOA
yields in the toluene/isoprene/NOx and toluene/ethylene/NOx
system are lower than that in the toluene/NOx system. The
reasons for this phenomenon may be as follows: (1) Isoprene
and ethylene possess few carbon atoms and produce highly
volatile oxides, so organic products produced by photo-
oxidation reactions are more likely to stay in gas-phase. (2)
The oxidation pathways are changed when various VOCs
coexist. For example, isoprene, ethylene and their gas-phase
products compete for the oxidants (e.g., OH, O3, NO3) with
toluene and toluene gas-phase products, which could produce
different amounts of oxidants and radicals (Chen and Jang,
2012).
Furthermore, we find extinction and scattering of SOA
using CAPS in the toluene/isoprene/NOx and
toluene/ethylene/NOx systems. Fig. 10(a) shows that the
aerosol mass concentration has a high correlation with
extinction and scattering, suggesting that the chemical and
physical properties of the particles are stable. The slope of
linear regression between the mass concentration and
extinction (scattering) is defined as the mass extinction
(scattering) coefficient, and the larger the slope, the stronger
the mass extinction (scattering) coefficient (Moise et al.,
2015; Wang et al., 2015). The mass extinction coefficient and
mass scattering coefficient in toluene/isoprene/NOx are 5.4
and 4.4, respectively, and in toluene/ethylene/NOx systems
are 6.2 and 5.1, respectively. This means that the SOA
generated in the toluene/ethylene/NOx system has a stronger
mass extinction coefficient and mass scattering coefficient.
The single scattering albedo (SSA) reflects the proportion
of the scattering and absorption of aerosol (Paredes-Miranda
et al., 2009; Han et al., 2016). It is observed that the SSA
value was lower at the beginning of the experiment, and
the main component of SOA was aerosol with absorbability.
With the progress of the reaction, the SSA value rises
rapidly, reaching 0.8 and 0.73 at the end of the experiments,
respectively. This may be caused by the growth of the
particle size and the change of particle composition, and
further studies are needed to clarify the possible causes.
The temporal profiles of the mass concentration results
of the toluene/isoprene/NOx system from SMPS and AMS
are compared in Fig. 9(a), and both have a high consistency.
Moreover, we collected the data of H:C, O:C, f43 and f44
to explore the degree of oxidation of SOA. According to
the element ratio, the slope of H: C vs. O: C of organic
Chen et al., Aerosol and Air Quality Research, 17: 1660–1671, 2017 1667
aerosol in Van Krevelen in Fig. 11(a) is about –1.3, which is
steeper, and close to (~–2). The possible reason may be that
organic aerosol oxidation products tend to be carboxylic
acids and with less cracking reactions in a low oxidation
condition (Lambe et al., 2012). The research shows that
the generation of organic aerosol changes in the order:
Fig. 7. Temporal profiles of particle size distribution in toluene/NOx, toluene/isoprene/NOx and toluene/ethylene/NOx
systems.
Fig. 8. Total number concentration and diameter evolution for toluene/NOx, toluene/isoprene/NOx and toluene/ethylene/NOx
systems. The time-dependent diameter uses the fitting curve from merging data of Log. Nor. and Max. Conc.
Chen et al., Aerosol and Air Quality Research, 17: 1660–1671, 2017
1668
Fig. 9. Mass concentration and SOA yields for toluene/NOx, toluene/isoprene/NOx and toluene/ethylene/NOx systems.
Fig. 10. Extinction, scattering (a) and single scattering albedo (b) for toluene/isoprene/NOx and toluene/ethylene/NOx
systems.
HOA → SV-OOA → LV-OOA, from the bottom to the
top of the triangle in the diagram of f44 vs. f43 (Jimenez et
al., 2009). As shown in Fig. 11(b), the data points were
mainly concentrated in the middle of the triangle, which is
lower than aging SOA in the actual environment, which
may be caused by a lower concentration of precursor or a
shorter reaction time (Hu et al., 2012), further studies of
the detailed reasons for the oxidation of organic aerosols
are needed. Besides, it is necessary to further study the
aerosol composition measurements with AMS and the
comparison between AMS and SMPS under complex
pollution condition in detail, and providing effective data
for the complex atmospheric pollution.
CONCLUSIONS
Three comparison groups of experiments were conducted
under the gas-phase environments of toluene/NOx,
toluene/isoprene/NOx, and toluene/ethylene/NOx in a smog
chamber. Experimental results show that under toluene
limited conditions, the effect of the toluene/NOx ratio on
the maximum ozone concentration shows two different
trends. When the ratio is within 3.1–11.3, the maximum
ozone greatly increases with the increasing ratio, whereas
for the ratio of 15.5–24.8, it remains unchanged as the ratio
increases. The same phenomenon was observed in the
propene photochemical reaction (Hu et al., 2011). The
appearance of the maximum ozone concentration was
advanced to 225 min, and the decay rate of toluene
increased from 1.1 × 10–4 to 1.6 × 10–3 min–1 with increase
of the toluene concentration. The toluene SOA was greatly
influenced by the initial toluene concentrations. The particle
size and total number concentrations increased with increase
of the initial toluene concentration. The SOA effective
density was concentrated around 1.3–1.4 g cm–3, which has
the highest frequency of 50.8%. Possibly due to the
difference of oxidative conditions and oxidation pathways,
the yield is higher at the same mass concentration under
lower initial toluene concentrations, and a similar case was
observed in Chen et al. (2016).
Chen et al., Aerosol and Air Quality Research, 17: 1660–1671, 2017 1669
Fig. 11. Scattering of Van Krevelen (H: C vs. O: C) (a) and f44 vs. f43 (b) in toluene/isoprene/NOx system.
In the mixed VOCs system, the presence of isoprene or
ethylene in the low initial concentration toluene/NOx system
will promote ozone formation and SOA. Isoprene has a
greater effect on ozone formation than that of ethylene, while
the nucleation time of SOA is delayed, and the maximum
SOA yield was reduced at the same mass concentration. In
the toluene/isoprene/NOx and toluene/ethylene/NOx systems,
the SSA rapidly increases with the progress of the reaction
and reaches 0.8 and 0.73 at the end of the experiments,
respectively. The mass extinction coefficient and mass
scattering coefficient are weaker in the toluene/isoprene/NOx
system. The slope of H/C and O/C of organic aerosol in
Van Krevelen (V-K) is about –1.3, which suggests that
organic aerosol oxidation products tend to carboxylic acids
in the toluene/isoprene/NOx system. Therefore, it is
concluded from our study that the different initial toluene
concentrations and coexisting VOCs are most important
for ozone and SOA formation from toluene.
ACKNOWLEDGEMENTS
This work was supported by the Project of Hangzhou
G20 Environmental Protection (2016-004), the Natural
Science Foundation of China (No. 51206144), the project
of Hangzhou Technology (20160533B85), the project of
Hangzhou Technology (20162013A06), the program of
Introducing Talents of Discipline to University (B08026),
the Innovative Research Groups of the National Natural
Science Foundation of China (51621005).
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Received for review, May 23, 2017
Revised, June 15, 2017
Accepted, June 15, 2017
