Introduction
Atmospheric aerosols profoundly impact the global climate directly by
scattering and absorbing solar radiation and indirectly by affecting cloud
formation and distribution via acting as cloud condensation nuclei (CCN) and
ice nuclei (IN). Moreover, atmospheric aerosols exert negative effects on
human health because of their toxicity. Due to fast urbanization and
industrialization, high levels of atmospheric fine particle (PM2.5)
pollution have been a persistent problem in many cities of China since the
1990s (van Donkelaar et al., 2010). As the capital of
China and one of the largest megacities in the world, Beijing has suffered
from frequent severe haze pollution, especially in winter, affecting more than
21 million people by the end of 2014 (Beijing Municipal Bureau of Statistics,
2015) and causing billions in economic losses (Mu and Zhang, 2013). To improve
the air quality, the Beijing government has made many efforts to reduce the
pollutant emissions (i.e., SO2, NOx, dust, and volatile organic
compounds (VOCs)) from a variety of sources. The 2014 Asia-Pacific Economic
Cooperation (APEC) summit was hosted in Beijing from 5 to
11 November. To ensure good air quality for the summit, a joint
strict emission control program was conducted from 3 November 2014 in
Beijing and its neighboring provinces including Inner Mongolia, Shanxi,
Hebei, and Shandong provinces. During this period thousands of factories and power
plants with high emissions were shut down and/or halted, all the construction
activities were stopped, and the numbers of on-road vehicles were reduced.
These strict emission controls resulted in the air quality of Beijing during
the APEC period being significantly improved, leading to a decrease in
PM2.5 concentration by 59.2 % and an increase in visibility by
70.2 % in Beijing during the summit compared with those before the APEC
(Tang et al., 2015; Z. Wang et al., 2015) and a term of “APEC blue” being
created to refer to the good air quality. Such strong artificial intervening
not only reduced PM2.5 and its precursors' loadings in Beijing and its
surrounding areas but also affected the composition and formation mechanisms
of the fine particles (Sun et al., 2016).
A number of field measurements have shown that particle compositions in
Beijing during wintertime haze periods are dominated by secondary aerosols
(Guo et al., 2014; Huang et al., 2014; Xu et al., 2015). Rapid accumulation
of particle mass in Beijing during the haze formation process is often
accompanied by continuous particle size growth (Guo et al., 2014; Zhang et
al., 2015), which is in part due to the coating of secondary organic
aerosols (SOA) on pre-existing particles (Li et al., 2010). Several studies
have found that SOA production during the 2014 Beijing APEC periods
significantly reduced and ascribed this reduction to the efficient regional
emission control (Sun et al., 2016; Xu et al., 2015). However, up to now
information on the SOA decrease on a molecular level has not been reported.
Dicarboxylic acids are the major class of SOA species in the atmosphere and
ubiquitously found from the ground surface to the free troposphere (Fu et
al., 2008; Myriokefalitakis et al., 2011; Sorooshian et al., 2007; Sullivan and Prather, 2007).
Previous studies have suggested that organic acids including
dicarboxylic acids could take part in atmospheric particle nucleation (Zhang
et al., 2004; Zhao et al., 2009) and growth processes (Zhang et al., 2012).
Furthermore, organic acids may play a central role in the aging of black carbon
particles (Xue et al., 2009; Ma et al., 2013), enhancing their roles in air
pollution accumulation, and direct radiative forcing (Peng et al., 2016). In
the current work we measured molecular distributions of dicarboxylic acids,
keto-carboxylic acids and α-dicarbonyls and stable carbon isotope
composition of oxalic acid in PM2.5 aerosols collected in Beijing
before, during, and after the APEC event in order to explore the impact of
the APEC emission control on SOA in Beijing. We first investigated the
changes in concentration and composition of dicarboxylic acids and related
compounds during the three periods, then recognized the difference in stable
carbon isotope composition of oxalic acid in different air masses in Beijing
during the APEC campaign. Finally we compared the differences in chemical
compositions of PM2.5 during two heaviest pollution episodes.
Results and discussion
Variations in meteorological conditions, gaseous pollutants, and
major components of PM<inline-formula><mml:math id="M310" display="inline"><mml:msub><mml:mi/><mml:mn>2.5</mml:mn></mml:msub></mml:math></inline-formula> during the Beijing 2014
APEC campaign
Based on the emission control implementation for the APEC, we divided the
whole study period into three phases: before APEC (8 October to 2 November),
during APEC (3 to 12 November) and after APEC (13 to 24 November). Temporal
variations in meteorological parameters and concentrations of gaseous
pollutants and major components of PM2.5 during the three phases are
shown in Fig. 1 and summarized in Table 1.
Temperature during the sampling campaign showed a continuous decreasing
trend with averages of 13 ± 2.6, 7.0 ± 1.7, and 4.3 ± 1.3 ∘C before, during, and
after APEC, respectively, while relative humidity (RH) did not show
a clear trend, with mean values of 62 ± 19, 47 ± 14, and
51 ± 16 % during the three periods (Fig. 1a and Table 1). SO2
showed a similar level before and during APEC (8.8 ± 4.6 µg m-3 versus
7.6 ± 3.9 µg m-3; Table 1 and Fig. 1b),
but increased dramatically to 23 ± 8.8 µg m-3 after APEC due
to domestic coal burning for house heating. NO2 concentration (45 ± 18 µg m-3) during the APEC reduced by about 30 %
compared to that in the before- and after-APEC phases (71 ± 27 µg m-3 versus 78 ± 29 µg m-3; Table 1),
mainly because of
the reduction of the on-road vehicle numbers, as well as the reduced
productivities of power plant and industry. O3 displayed a decreasing
trend similar to that of temperature (Fig. 1c). PM2.5 pollution
episodes in Beijing showed a periodic cycle of 4–5 days, which is caused
by the local weather cycles. Secondary inorganic aerosols (SIA, i.e.,
SO42-, NO3-, and NH4+) are major components of
PM2.5 and present a temporal variation pattern similar to that of the
fine particles (Fig. 1d). In the current work, the mass ratio of
NO3- / SO42- in PM2.5 during the whole study time is
1.8 ± 1.9 (Table 1), which is in agreement with the ratio (1.6–2.4)
for PM1 observed during the same time by using aerosol mass
spectrometry (AMS; Sun et al., 2016). OC and EC of PM2.5 are linearly
correlated each other (R2= 0.91) and varied periodically in a cycle
similar to SIA (Fig. 1e). The OC / EC ratio during the whole sampling period is
3.3 ± 0.6 (range: 2.2–4.7) with no significant differences among the
three APEC phases (Table 1), although the source emissions could be largely
different.
Chemical composition of PM2.5 during the 2014 APEC
campaign.
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Figure 2 shows the differences in chemical composition of PM2.5
before, during, and after APEC. PM2.5 is 98 ± 46 µg m-3 during APEC, about 50 % lower than that before and after APEC
(178 ± 122 µg m-3 versus 161 ± 100 µg m-3, respectively). Organic matter (OM) is the most abundant component
of the fine particles. Relative abundance of OM (1.6 times OC; Xing
et al., 2013) to PM2.5 continuously increases from 24 % before APEC
to 30 and 39 % during and after APEC, respectively, although the mass
concentration (19 ± 7.6 µg m-3) of OC during APEC is the
lowest compared to those before and after APEC (26 ± 16 µg m-3 versus 39 ± 23 µg m-3). Sulfate, nitrate, and
ammonium before APEC are 15 ± 13, 28 ± 26, and 9.0 ± 8.0 µg m-3 (Table 1) and account for 8, 16, and 5 % of
PM2.5, respectively (Fig. 2). Their concentrations decrease to 5.3 ± 2.8, 10 ± 8.1, and 3.1 ± 2.6 µg m-3 (Table 1)
with the relative contributions to PM2.5 down to 5, 10, and
3 % during APEC, respectively, while after APEC their concentrations
increased to 11 ± 10, 15 ± 13, and 6.9 ± 6.4 µg m-3 and accounted for 7, 9, and 4 % of PM2.5. Such
significant decreases in concentrations of OM and SIA during APEC
demonstrate the efficiency of the emission controls. The OC / EC ratio is almost
constant during the whole period, but the WSOC / OC ratio decreased by 20 % from
0.42 ± 0.13 before APEC, 0.38 ± 0.16 during APEC to 0.35 ± 0.17 after APEC (Table 1). Since WSOC in fine aerosols consists mainly of
SOAs (Laskin et al., 2015), the decreasing ratio
of WSOC / OC probably indicates reduced SOA production during the campaign.
Molecular distributions of dicarboxylic acids and related
compounds in PM2.5 of Beijing, China, during the 2014 APEC campaign. The
pie chart is the average composition of total detected organic compounds
(TDOC) and the top number is the average mass concentration of TDOC of the
whole study period.
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Oxalic acid and related SOA during the Beijing 2014 APEC
campaign
A homogeneous series of dicarboxylic acids (C2-C11),
keto-carboxylic acid and α-dicarbonyls in the PM2.5 samples
were detected. As shown in Table 2, total dicarboxylic acids during the whole
study period is 593 ± 739 ng m-3, which is lower than that
observed during the Campaign of Air Quality Research in Beijing 2006
(CAREBeijing; average 760 ng m-3) and 2007 (average
1010 ng m-3; Ho et al., 2010, 2015) and the averaged wintertime concentration reported by
a previous study on 14 Chinese cities (904 ng m-3; Ho et al.,
2007). Total keto-carboxylic acid is 66 ± 81 ng m-3, while total
dicarbonyl is 126 ± 115 ng m-3 (Table 2). These values are
higher than those during CAREBeijing 2006 and 2007 (Ho et al., 2010, 2015),
but close to the value observed for the 14 Chinese megacities (Ho et al.,
2007). Being similar to those previous observations, oxalic acid (C2)
is the most abundant diacid in the 2014 APEC samples, with an average of 334 ± 461 ng m-3 (range: 10–2127 ng m-3, Table 2) during the
whole campaign, followed by methylglyoxal (mGly), succinin acid (C4),
terephthalic acid (tPh), and glyoxal (Gly). These five species account for
43, 10, 9, 6 and 6 % of total detected organic compounds
(TDOC), respectively (Fig. 3).
Compositions of total detected organic compounds (TDOC)
in PM2.5 during the 2014 APEC campaign.
![]()
As seen in Fig. 4, TDOC in PM2.5 are 1099 ± 1104, 325 ± 220,
and 487 ± 387 ng m-3 before, during, and after APEC,
respectively. In comparison with those before APEC, TDOC during APEC
decreased by 71 %. Oxalic acid (C2) is the leading species among the
detected organic compounds and accounted for 46, 31, and 34 % of
TDOC during the three phases, respectively (Fig. 4). C2 is an end
product of precursors that are photochemically oxidized in aerosol aqueous
phase via either oxidation of small compounds containing two carbon atoms or
decomposition of larger compounds containing three or more carbon atoms.
Thus the mass ratio of C2 to TDOC is indicative of aerosol aging (Wang et
al., 2012; Ho et al., 2015). As shown in Fig. 4, the highest proportion of
C2 before APEC suggests that organic aerosols during this period are
more oxidized, compared to those during and after APEC. Gly and
methylglyoxal (mGly) are the precursors of C2. Mass ratios of both
compounds to TDOC are lowest before APEC (Fig. 4), further indicating
enhanced SOA production during this period.
Formation mechanism of oxalic acid
Correlation of oxalic acid with temperature, relative humidity
(RH), aerosol liquid water content (ALWC) and acidity and sulfate
Correlation analysis for oxalic acid (C2) and
sulfate in PM2.5 during the whole 2014 APEC campaign. (a–c) Concentrations of C2 with sulfate, relative humidity (RH), and aerosol
liquid water content (ALWC); (d, e) sulfate and C2 with
aerosol acidity [H+] and (f) temperature with mass ratio of
C2 to total detected organic compounds (C2 / TDOC).
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A few studies have pointed out that aerosol aqueous phase oxidation is a
major formation pathway for oxalic acid (Yu et al., 2005; van Pinxteren et
al., 2014; Bikkina et al., 2015; Tilgner and Herrmann, 2010). To explore the
formation mechanism of oxalic acid, we calculated ALWC and acidity (i.e.,
proton concentration, [H+]) of PM2.5 aerosols by using
ISOROPPIA-II model (Weber et al., 2016). As shown in Fig. 5, during the
entire period C2 showed a strong linear correlation with sulfate
(R2= 0.70 Fig. 5a), which is consistent with the measurements observed in Xi'an
(Wang et al., 2012) and other Chinese cities (Yu et al., 2005). Previous
studies on particle morphology showed that sulfate particles internally
mix with SOA in Beijing, especially on humid haze days (Li et al., 2010,
2011), which probably indicates that they are formed via similar aqueous
phase pathways (Wang et al., 2016b). In addition, a robust correlation was
also found for C2 with RH (R2= 0.64, Fig. 5b) and ALWC (R2= 0.61, Fig. 5c), indicating that humid
conditions are favorable for the aqueous phase formation of C2, which
is most likely due to an enhanced gas-to-aerosol aqueous phase partitioning
of the precursors (e.g., Gly and mGly; Fu et al., 2008; G. Wang et al.,
2015).
NH4+, NO3-, and SO42- are the dominant cation
and anions of fine particles in Beijing (Guo et al., 2014;
Zhang et al., 2015) and the molar ratio of [NH4+] to
[NO3-] + [SO42-] in this study is 1.1. Thus it is
plausible that SO42- during the APEC campaign largely existed as
ammonium bisulfate, resulting in a strong linear correlation between
[H+] and SO42- with a molar slope of 1.03 (Fig. 5d; Zhang et
al., 2007). In addition, [H+] shows a significant positive correlation
with C2 (R2= 0.84; Fig. 5e), possibly due to the fact that
acidic conditions are favorable for the formation of C2 precursors. For
example, Surratt et al. (2007, 2010) found that aerosol acidity can promote
the formation of biogenic SOA (BSOA) derived from isoprene oxidation, such as
2-methylglyceric acid, Gly and mGly. These BSOA precursors can be further
oxidized into C2 (Meng et al., 2014; Wang et al., 2009).
There is a significant positive correlation (R2= 0.58,
p < 0.001) between the mass ratios of C2 / TDOC and
ambient temperatures (Fig. 5f), which is similar to the results found by
previous researchers (Ho et al., 2007; Strader et al., 1999), indicating
that organic aerosols are more aged under a higher temperature condition
(Erven et al., 2011; Carlton et al., 2009). Thus, compared with those
before APEC, the lower C2 / TDOC ratios (31 and 34 % (Fig. 4) during and after APEC respectively) can be ascribed in part to the relatively
lower temperature conditions that are not favorable for oxidation of the
precursors to produce oxalic acid (13 ± 2.6, 7.0 ± 1.7 and 4.3 ± 1.3 ∘C in the before-, during- and
after-APEC periods, respectively; Table 1).
Linear correlation coefficients of δ13C of C2 with
C2 / ωC2, C2 / mGly, and TDOC / WSOC.
C2 / ωC2
C2 / mGly
TDOC / WSOC
δ13C
0.49**
0.35*
0.41*
** p < 0.01; * p < 0.05.
Temporal variation in stable carbon isotopic composition of
oxalic acid
(a) 72 h backward trajectories determined by the
National Oceanic and Atmospheric Administration Hybrid Single Particle
Lagrangian Integrated Trajectory (HYSPLIT) model arriving at the sampling
site to reveal the major air-mass flow types during the study period.
Northwesterly wind (light blue) was most frequent (64 %), followed by
northerly (21 %, pink) and southerly (15 %, black), and these are defined as
clean, mixed, and polluted types, respectively (see the definitions in the
text and the trajectories with a 6 h interval in the Supplement); (b) time series of δ13C values and
concentration of oxalic acid during the whole study period (colors in Fig. 6a are corresponding to those in Fig. 6b).
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To further discuss the formation mechanism of C2, we investigated the
temporal variations of concentration and stable carbon isotopic composition
of C2 in the PM2.5 samples (Fig. 6). Previous studies have
demonstrated that Gly, mGly, glyoxylic acid (ωC2), and pyruvic
acid (Pyr) are the precursors of C2 (Carlton et al., 2006, 2007;
Ervens et al., 2004; Wang et al., 2012). Thus, higher mass ratios of
C2 to its precursors indicate that organic aerosols are more oxidized
(Wang et al., 2010). As shown in Table 3, δ13C of C2 in
this work positively correlated with the mass ratios of C2 / ωC2, C2 / mGly, and TDOC / WSOC, demonstrating an enrichment of
13C during the aerosol oxidation process. Because decomposition (or
breakdown) of larger molecular weight precursors in aerosol aqueous phase is
the dominant formation pathway for C2 in the aerosol ageing process
(Kawamura et al., 2016; Gensch et al., 2014; Kirillova et al., 2013), during
which organic compounds release CO2 / CO by reaction with OH radical and
other oxidants, resulting in the evolved species enriched with lighter
isotope (12C) and the remaining substrate enriched in 13C due to
kinetic isotope effects (KIE; Hoefs, 1997; Rudolph et al., 2002).
Meteorological parameters and chemical compositions (µg m-3) of two
maximum PM2.5 between two pollution episodes in Beijing.
T (∘C)
RH (%)
Va (km)
PM2.5
OC
EC
SIAb
TDOCc
Event I
16.7 ± 0.8
82 ± 4
1.5 ± 0.5
349 ± 57
45 ± 12
12 ± 2
106 ± 39
2749 ± 1357
(8/10–11/10, Before APEC)
Event II
4.5 ± 1.7
62 ± 13
3.5 ± 1.5
259 ± 102
60 ± 21
17 ± 6
60 ± 32
831 ± 400
(18/11–21/11, After APEC)
a V: visibility. b SIA: secondary inorganic aerosols (the sum of sulfate, nitrate, and
ammonium). c TDOC: total detected organic compounds.
A 72 h backward trajectory analysis showed that air masses that moved to Beijing
during the whole sampling period can roughly be categorized into three types
(Fig. 6a; all trajectories during the entire study period can be found in
the Supplement). (1) Polluted type, by which air masses
originated inland and east coastal China and moved slowly into Beijing
within 72 h from its southern regions, i.e., Henan, Shandong, and Jiangsu
provinces. This type of air mass mostly occurred before APEC with high
PM2.5 concentrations. Air pollution has been widely distributed in the three
provinces (Wei et al., 2016); thus aerosols transported by this type of air
mass are of regional characteristics. (2) Mixed type, by which air masses
originated from Mongolia and North China, and moved quickly into Hebei
province and then turned back to Beijing. The air in Mongolia and North China
was clean but polluted in Hebei province, which is adjacent to Beijing. This
type of air mass is a mixture of clean and polluted air and is thus named
mixed type. Since the resident time of the mixed type of air mass within
Hebei province is very short, thus aerosols transported by this type of air
mass are of local characteristics and relatively fresh. (3) Clean type, by
which air masses originated from Siberia and moved rapidly into Beijing
directly via long-range transport. Aerosols from the clean type of air
masses are much more aged, while those from the mixed type of air masses are
fresh. Since severe air pollution is widespread in the southern regions,
gas-to-aerosol phase partitioning of precursors and subsequent aerosol-phase
oxidation to produce SOA including C2 continuously proceed during the
air-mass movement. However, such a partition for producing SOA is not
significant when air masses move from Siberia, Mongolia, and northern China
because of the much less abundant VOCs. Instead, aerosols in the clean air
masses are continuously oxidized, during which C2 is produced by
photochemical decomposition of larger molecular weight precursors.
Therefore, C2 in PM2.5 transported by the mixed type air masses
is not only fresh and abundant but also enriched in 12C, whereas
C2 in PM2.5 transported by the clean type air masses is aged,
less abundant, and enriched in 13C due to KIE effects, as illustrated by
the pink and light blue columns in Fig. 6b, respectively. C2 in
PM2.5 transported by the polluted type of air mass is most
abundant compared with that in other two types of air mass, which is not
only due to the severe air pollution in the Henan, Shandong, and Jiangsu
provinces but also due to the enhanced photochemical oxidation under the
humid, higher temperature and stagnant conditions that occurred mostly
before APEC, as discussed previously. Therefore, C2 in the polluted
type of air masses is not only abundant but also enriched in 13C (see
black columns in Fig. 6b).
Comparison of chemical composition of PM2.5 during
two air pollution events. (a) Percentages of major species in
PM2.5; (b, c) mass ratios of major species and organic tracers
in PM2.5; (d) stable carbon isotope composition of oxalic acid
(C2) (data about levoglucosan (Lev), PAHs, and hopanes are cited from
Wang et al., 2016a).
![]()
Different chemical characteristics of
PM<inline-formula><mml:math id="M564" display="inline"><mml:msub><mml:mi/><mml:mn>2.5</mml:mn></mml:msub></mml:math></inline-formula> between two severe haze events
From Fig. 1 and Table 4, it can be found that PM2.5 showed two
equivalent maxima on 9 October and 20 November during the
whole study period. However, the chemical compositions of PM2.5 during
these two pollution events are significantly different. As shown in Fig. 7a,
relative abundances of SIA (sum of SO42-, NO3-, and
NH4+) to PM2.5 are 30 % during event I and
23 % during event II. The relative abundance of OM
(21 %, Fig. 7a) during event I is lower than that (37 %) during
event II (Fig. 7b). In contrast, the ratios of WSOC / OC and TDOC / OC are
higher in event I than in event II, which is consistent with lower
levels of O3 after APEC (Table 1), suggesting a weaker photochemical
oxidation capacity during event II. Organic biomarkers in the PM2.5
samples have been measured for the source apportionment (Wang et al., 2016a)
and cited here to further identify the difference in chemical composition
of PM2.5 between the two events. Levoglucosan is a key tracer for
biomass burning smoke. The mass ratio of levoglucosan to OC in PM2.5 (Lev / OC) is comparable between the two events, suggesting a similar level
of contributions of biomass burning emission to PM2.5 before and
after APEC. However, the mass ratios of polycyclic aromatic hydrocarbons (PAHs) and hopanes to OC are lower in
event I than those in event II (Fig. 7c), which again demonstrates the
enhanced emissions from coal burning for house heating, because these
compounds are key tracers of coal burning smokes (Wang et al., 2006). As
seen in Fig. 7d, C2 in event I was enriched in 13C. Such
relatively more abundant SIA, WSOC, and TDOC and heavier C2 in
PM2.5 clearly demonstrate that PM2.5 during event I is
enriched with secondary products while the fine particles during event
II are enriched with primary compounds. After-APEC house heating activities
including residential coal burning were activated, which emitted huge
amounts of SO2, NOx, and VOCs as well as primary particles, resulting
in both absolute concentrations and relative abundances of CO and EC
30–40 % higher after APEC than before APEC (see Table 1). Li et al. (2015) reported that VOCs in Beijing were 86 ppbv before APEC, 48 ppbv
during APEC, and 73 ppbv after APEC. As shown in Table 4, temperature
(16.7 ± 0.8 ∘C for event I and 4.5 ± 1.7 ∘C for event
II) and relative humidity (RH; 82 ± 4 for event I and 62 ± 13 % for event II) are lower during event II than during event I.
Moreover, air masses arriving in Beijing during event II are the mixed
type, of which the resident time in Hebei province is short. Compared with
those in event I, such colder and drier conditions and a short reaction
time during event II are unfavorable for photochemical oxidation,
resulting in SOA not only less abundant but also enriched with lighter
12C during event II, although VOC levels are comparable before
and after APEC.
Summary and conclusion
Temporal variations in molecular distribution of SIA, dicarboxylic acids,
ketoacids, α-dicarbonyl, and stable carbon isotopic composition
(δ13C) of C2 in PM2.5 collected in Beijing before,
during and after the 2014 APEC were investigated. Absolute concentrations
and relative abundances of SIA and C2 in PM2.5 are highest
before APEC, followed by those after and during APEC, suggesting that the
fine aerosols before APEC are enriched with secondary products, mainly due
to an enhanced photochemical oxidation under the warm, humid and stagnant
conditions. Concentrations of SIA, oxalic acid and related SOA in PM2.5
during APEC are 2–4 times lower than those before APEC, which can be
ascribed to the effective emission controls and the favorable meteorological
conditions that brought clean air from Siberia and Mongolia into Beijing.
Positive correlations of C2 with sulfate mass, RH, ALWC, and aerosol
acidity indicate that the C2 formation pathway is involved in
acid-catalyzed aerosol aqueous phase oxidation. SIA, C2, and related SOA
in the polluted types of air mass are abundant with C2 enriched in
13C. On the contrary, those in the clean types of air mass are much
less abundant, although C2 is also enriched in 13C. By comparing
the chemical composition of PM2.5 and δ13C values of
C2 in two events that are characterized by the highest loadings of
PM2.5 before and after APEC, we further found that compared with those
before APEC fine aerosols after APEC are enriched with primary species and
C2 is depleted in heavier 13C, although SO2, NOx, and VOCs are
abundant during the heating season, again demonstrating the important role
of meteorological conditions in the secondary aerosol formation process,
which are warmer, humid, and stagnant before APEC and result in secondary
species being much more abundant than those during and after APEC.