Water-soluble humic-like substances (HULIS) absorb light in near-UV and
visible wavelengths and exert significant influence on the atmospheric
environment and climate. However, knowledge on HULIS evolution during haze
bloom-decay process is limited. Herein, PM2.5 samples were
obtained during a winter haze event in Guangzhou, China, and the light
absorption and molecular composition of HULIS were investigated by UV–Vis
spectrophotometry and ultrahigh-resolution mass spectrometry. Compared with
HULIS on clean days, the absorption coefficients (Abs365) of HULIS on
haze days were significantly higher but the mass absorption efficiencies
(MAE365) were relatively low, suggesting diverse and dynamic
absorption properties of HULIS during haze episodes. The CHO and CHON
compounds were the most abundant components in HULIS, followed by CHOS,
CHONS, and CHN. Haze HULIS presented comparatively high molecular weight; a
lower aromaticity index (AImod); and higher O/Cw, O/Nw, and
O/Sw ratios, indicating that HULIS fractions undergo relatively high
oxidation during haze days compared to clean days. Moreover, CHON and CHO compounds
with high AImod were the major potential chromophores in HULIS and
significantly contributed to HULIS light absorption. It is worth noting that
the proportions of these chromophores decreased during haze events,
mainly owing to their higher oxidation during haze episodes. Besides,
accumulated contribution of organic compounds emitted from vehicles and
formed from reactions of biogenic volatile organic compounds (bio-VOCs) also diluted light-absorbing compounds in
haze HULIS. These findings help us to understand HULIS evolution during haze
bloom-decay processes in the subtropic region of China.
Introduction
Water-soluble humic-like substances (HULIS), belonging to a class of highly
complex organic compounds with physical and chemical properties similar to humic
substances in natural aquatic/soil environments, constitute
30 %–70 % of water-soluble organic compounds in ambient aerosols and
are responsible for >70 % of light absorption (at 365 nm) in
water-soluble brown carbon (BrC) (Graber and Rudich, 2006; Laskin et al.,
2015; Huang et al., 2018). They are thought to be comprised of aromatic
structures containing aliphatic side chains and oxygenated functional groups
such as hydroxyl, carboxyl, nitrate, and organosulfate groups (Lin et al.,
2012; Song et al., 2018; Zeng et al., 2020). HULIS are ubiquitously
identified in atmospheric aerosols, fog, cloud, and rainwater and have
been demonstrated to have significant effects on both the atmospheric
environment and climate (Bianco et al., 2018; Wu et al., 2018; Zhan et al.,
2022). In addition, HULIS exert adverse health effects because they can
enhance the oxidative potential of organic aerosols (Ma et al., 2019; Cao et
al., 2021).
In recent years, severe particulate pollution (i.e., haze events) has frequently
occurred in some developing countries such as China, which has drawn extensive
public and scientific concerns (An et al., 2019; Zhang et al., 2020).
According to An et al. (2019), contributions of organic aerosols, including
primary organic aerosols and secondary organic aerosols (SOA), are
significant for severe haze events; in particular, the contribution of SOA
in China is expected to continuously increase because of stronger chemical
reactions in the atmosphere. HULIS are an important component in organic
aerosols, which originate from a variety of primary emissions (e.g., biomass
burning (BB), coal combustion, off-road engine emissions) (Fan et al., 2016;
Cui et al., 2019; Tang et al., 2020) and secondary chemical oxidation of
biogenic and anthropogenic volatile organic compounds (VOCs) (Yu et al.,
2016; Tomaz et al., 2018) and soot (Fan et al., 2020). During haze
episodes, a number of chemical processes occur in the aqueous phase (Wong et al.,
2017; Wu et al., 2018) and gas phase (Sumlin et al., 2017), which lead to
significant changes in the chemical composition and light absorption properties
of HULIS. For instance, recent studies on oxidation of BB-derived BrC have
indicated that although both enhancement and bleaching of BrC occur during
aging, bleaching of BrC becomes dominant over a long period (Fan et al.,
2020; Wong et al., 2017; Ni et al., 2021). However, multiphase reaction
between carbonyl and amine has demonstrated rapid formation of
light-absorbing organic compounds (Kampf et al., 2016). Nevertheless, it
should be noted that these results were mainly obtained from laboratory
experiments and may not reflect the complex evolution behavior of BrC in
atmospheric environments.
High concentrations of HULIS have been determined during typical haze
episodes in northern, eastern, and southern China (Fan et al., 2016; Zhang
et al., 2020; Wang et al., 2020) and have been demonstrated to
significantly influence atmospheric visibility, the environment, and
photochemical processes. Guangzhou is the biggest city in the Pearl River
Delta (PRD), one of the most developed regions in China, and is located in
the subtropical zone with a population of over 18 million people (Yu et al.,
2017). Although a remarkable decline in atmospheric particulate matter
(PM2.5) pollution has been observed in recent years owing to strict
regulatory controls, O3 and VOCs still remain at higher levels and
severe haze pollution caused by fine particulate matter frequently occurs in
winter (An et al., 2019; K. Li et al., 2019; Yang et al., 2022). Several
studies have investigated the optical, chemical, and molecular properties of
HULIS in the PRD region (Lin et al., 2010, 2012; Fan et al., 2016; Liu et
al., 2018; Jiang et al., 2020, 2021). For example, studies on the
temporal variations in water-soluble HULIS in Guangzhou have indicated that HULIS
had higher concentrations and mass absorption efficiencies (MAE365) in
the winter, which were attributed to the increasing contribution of BB and
secondary nitrate formation in the winter monsoon period (Fan et al., 2016;
Jiang et al., 2020, 2021). In addition, the molecular composition of HULIS
(and BrC) in the PRD region was also investigated, and it was demonstrated that the
levels of unsaturated and aromatic structures comprise the important factor
influencing their light absorption properties (Jiang et al., 2020). However,
detailed information regarding the evolution of light absorption and
molecular composition of HULIS during haze events is still scarce.
Recently, ultrahigh-resolution Fourier transform ion cyclotron resonance
mass spectrometry (FT-ICR MS) coupled with electrospray ionization (ESI)
sources has been frequently employed to investigate the molecular
characteristics of HULIS in ambient aerosols (Song et al., 2018, 2022; Tang
et al., 2020; Zeng et al., 2020). Owing to its extremely high mass
resolution and accuracy, this technique allows further exploration of the
evolution of HULIS during haze events. The present study performed
comprehensive characterization of HULIS in PM2.5 collected during a
haze event in Guangzhou, China. The abundances and light absorption
properties of HULIS were first measured, and carbonaceous fractions,
water-soluble ions, and levoglucosan (Lev) were determined. Subsequently,
four HULIS samples collected during different haze stages were analyzed
using FT-ICR MS operated in both ESI- and ESI+ modes. To the best of our
knowledge, the present study is the first to apply a combination of optical
properties and molecular characterization by FT-ICR MS to investigate HULIS
in a haze event in the subtropical zone of China. The results obtained
provide novel insights into the evolution of HULIS during haze events and
are important for predicting the environmental and climatic effects of HULIS
in southern China.
Material and methodsAerosol sampling
The PM2.5 samples were collected on the campus of Guangzhou Institute
of Geochemistry, Chinese Academy of Sciences, Guangzhou, China (23.14∘ N,
113.35∘ E), which is an academic and residential region. Traffic emissions and
residential activities are the potential pollution sources in the sampling
area. The 24 h PM2.5 sampling was conducted using a high-volume sampler
(Tianhong Intelligent Instrument Plant, Wuhan, China, with a flow rate of
1.0 m3min-1) during 7 to 30 January 2018, and a total of 24
samples were collected on the prebaked quartz filters (20.3cm×25.4cm, Whatman, Maidstone, UK). Field blank samples were collected by
keeping a blank filter in the sampler without pumping air. Before sampling,
the filters were wrapped in aluminum foil and prebaked at 450 ∘C
for 6 h to remove carbonaceous impurities. Before and after sampling, the
filters were weighed at 25 ∘C and 50 % RH on a microbalance
(Sartorius Model BP210D). The PM2.5 concentrations were determined by
weighing the filters before and after collection. Finally, all filter
samples were stored in a refrigerator at -20 ∘C until analysis.
The mass accuracy achieved was <2 % based on triplicate analyses
of filter samples. Meteorological data
(http://www.wunderground.com/history/airport/ZGGG, last access: 10 March 2020), including wind speed;
temperature; relative humidity; and concentrations of SO2, O3, and
NO2, for the sampling days are presented in Fig. 1 and Table S1 in the Supplement.
Temporal variation in meteorological parameters, concentrations of
chemical composition, and optical properties (Abs365, MAE365, and
AAE) of water-soluble BrC in the PM2.5 samples.
Isolation of HULIS
HULIS were isolated using a water extraction and solid-phase extraction
(SPE) procedure as described previously (Zou et al., 2020). This method has
been used in most previous studies because of its easy operation and high
reliability and reproducibility and low limit of detection (Fan et al., 2012); therefore, it was also used in this study. Briefly, portions of the
PM2.5 samples (100 cm2) were ultrasonically extracted with 50 mL
of ultrapure water for 30 min. The extracts were filtered through a
0.22 µm PTFE syringe filter to remove the suspended insoluble
particles. About 50 mL of water extracts was obtained from each sample, of
which 20 mL was used for the isolation and analysis of HULIS, 20 mL for
analysis of water-soluble organic carbon (WSOC), and the remaining extracts
for the analysis of inorganic ions. Then, the 20 mL water
extracts were adjusted to a pH of 2 with HCl and loaded on a preconditioned
SPE cartridge (Oasis HLB, 200 mg, 6 mL, Waters, USA). The hydrophilic
fraction (i.e., inorganic ions, high-polarity organic acids, etc.) was removed
with ultrapure water, whereas the relatively hydrophobic HULIS fraction was
retained and eluted with 2 % (v/v) ammonia/methanol. Finally, HULIS
solution was evaporated to dryness with a gentle N2 stream and
redissolved with ultrapure water for the analysis.
It is noted that the HULIS here is the hydrophobic portion of water-soluble
organic matter, which can be isolated with different types of SPE columns
(e.g., HLB, C-18, DEAE, XAD-8, and PPL) (Fan et al., 2012, 2013; Lin et al.,
2012; Zou et al., 2020; Jiang et al., 2020). Although each resin type has
its special chemical properties, the hydrophobic HULIS isolated with
different sorbents were similar in chemical, molecular properties based on
previous studies (Fan et al., 2012, 2013; Zou et al., 2020). Therefore, for
better comparison with other studies, the hydrophobic fractions isolated by
SPE methods were all termed as HULIS in the present paper.
Light absorption analysis
The absorption spectra of the WSOC and HULIS fractions were measured by a
UV–Vis spectrophotometer (UV-2600, Shimadzu) between 200 and 700 nm. Each
spectrum was corrected for the filter blanks. The light absorption
coefficients, absorption Ångström exponent (AAE), and mass absorption
efficiency (MAEλ) were calculated, and the detailed methods for this are
presented in the Supplement.
Chemical analysis
For FT-ICR MS analysis, the HULIS samples were isolated from PM2.5
collected during four periods: before haze days (clean-I days, 7–12
January), haze bloom days (haze-I days, 13–18 January), haze decay days
(haze-II days, 19–24 January), and after haze days (clean-II days, 25–30
January). A filter punch (18 cm in diameter) was taken from every sample,
and all six samples in each period were combined for the isolation of
HULIS fractions. The obtained HULIS samples were measured with an ESI FT-ICR
MS instrument (Bruker Daltonics GmbH, Bremen, Germany) equipped with a 9.4 T
refrigerated actively shielded superconducting magnet. The system was
operated in both ESI- and ESI+ modes. The scan range was set to m/z from
100 to 1000, with a typical mass-resolving power >450000 at m/z 319 with <0.2 ppm absolute mass error. The mass spectra were
calibrated externally with arginine clusters and internally recalibrated
with typical O5-class species peaks in DataAnalysis 4.4 (Bruker
Daltonics). Due to the inherent differences in the ionization mechanisms
between ESI- and ESI+ modes, the data detected by the two ionization modes
can provide complementary information on the molecular composition of
atmospheric HULIS (Lin et al., 2012, 2018). The details of data
analysis are provided in the Supplement.
The amounts of organic carbon (OC) and elemental carbon (EC) were determined
by an OC–EC analyzer (Sunset Laboratory Inc., USA) (Mo et al., 2018). The
concentrations of WSOC and HULIS were determined by a total organic carbon (TOC) analyzer (Shimadzu
TOC-VCPH, Kyoto, Japan). The water-soluble inorganic species
(NO3-, SO42-, Cl-, NH4+, K+,
Na+, Ca2+, Mg2+) were measured with a Dionex ICS-900 ion
chromatography system (Thermo Fisher Scientific, USA) as described
previously (Huang et al., 2018). The concentrations of Lev were analyzed
with gas chromatography–MS after derivatization with N,O-bis(trimethylsilyl)trifluoroacetamide and pyridine
at 70 ∘C for 3 h (Huang et al., 2018). Detailed information
regarding these measurements is provided in the Supplement.
Results and discussionAbundance and chemical composition of PM2.5
Figure 1 shows the meteorological conditions, PM2.5 concentration, and
concentrations of major chemical constituents, including carbon fractions
and water-soluble inorganic ions in PM2.5 samples obtained during a
haze bloom-decay process. Based on the variation in PM2.5
concentration, these samples were categorized into four groups: clean-I days
(before haze, 14–24 µgm-3), haze-I days (haze bloom, 45–114 µgm-3), haze-II days (haze decay, 58–115 µgm-3),
and clean-II days (after haze, 9–35 µgm-3). As indicated in
Table S1 and Fig. 1, the PM2.5 concentrations increased
from 18 ± 3.3 µgm-3 on clean-I days to 82 ± 26 and
84 ± 22 µgm-3 on haze-I and haze-II days, respectively,
and then decreased to 21 ± 10 µgm-3 on clean-II days.
This finding obviously indicated that the average PM2.5
concentrations during the examined haze episode are higher than the
second-grade national ambient air quality standard in China
(75 µgm-3, 24 h), whereas those during clean days are lower than the
first-grade national ambient air quality standard in China
(35 µgm-3, 24 h). However, the average PM2.5 concentrations during the
haze event are lower than those in the cities in winter haze, including
Shenyang (108 µgm-3) (Zhang et al., 2020) and Nanjing
(123 ± 28.5 µgm-3) (Li et al., 2020), Beijing
(158 µgm-3), and Xi'an (345 µgm-3) (Zhang et al., 2018).
As shown in Table S1, the average concentrations of OC and EC were 2.2–15
and 0.36–2.7 µgCm-3 in the four stages, respectively, implying
that the distinct changes in OC and EC were higher during haze episodes than
on clear days. During the entire study period, WSOC concentration
ranged from 0.5 to 12.5 µgCm-3 (4.3 ± 1.2 µgCm-3), which contributed to 53 %–57 % of OC in PM2.5. The
HULIS concentration noted in the present study ranged from 0.15 to 6.1 µgCm-3 (2.2 ± 1.9 µgCm-3), which was comparable to
those concentrations observed in the PRD region, such as Hong Kong (2.38 ± 1.62 µgCm-3) (Ma et al., 2019), Guangzhou (2.4 ± 1.6 µgCm-3) (Fan et al., 2016), and Heshan (2.08 ± 1.16 µgCm-3) (Jiang et al., 2020), but lower than those in northern cities of
China, such as Xi'an (12.4 ± 6.5 µgCm-3) (Huang et al.,
2020), Beijing (3.79 ± 3.03 µgCm-3) (Mo et al., 2018), and
Lanzhou (4.7 µgCm-3) (Tan et al., 2016). As shown in Fig. 1,
HULIS also exhibited obvious variations during the entire sampling period.
The average HULIS concentration was 0.46 ± 0.22 µgCm-3 on
clean-I days, which sharply increased to 4.5 ± 1.2 µgCm-3 on haze-I days, then decreased to 3.1 ± 1.2 µgCm-3 on
haze-II days, and rapidly declined to 0.75 ± 0.52 µgCm-3 on
clean-II days. This result was consistent with the changing trend of WSOC,
OC, and EC. In addition, the HULIS / WSOC ratios were about 0.50 ± 0.13
in the PM2.5 samples, which are in broad agreement with other studies
showing that HULIS comprise the major fraction of WSOC (Fan et al., 2016; Ma et
al., 2019; Jiang et al., 2020).
As illustrated in Fig. 1, obvious variations in chemical compositions were
also observed in these PM2.5 samples. Secondary inorganic aerosols
(SIA) (i.e., SO42-, NO3-, and NH4+), OC, and
EC exhibited a similar variation during the entire study period, and their
contents sharply increased from 10 January on clean-I days to 13–18 January on haze-I days, then slowly decreased on haze-II days, and finally
reached lower levels on clean-II days. It must be noted that the increasing
rate of EC was similar to that of SIA on haze-I days, indicating that direct
emissions and atmospheric reactions may play similar roles in PM2.5 increase during this haze bloom period. As indicated in Fig. 1f, the
highest values of NO3-/SO42- were observed on haze-I
days, implying the important influence of traffic exhaust emissions in the haze bloom
period (Mo et al., 2018). In addition, the high NO2 and O3
concentrations and the stable meteorological conditions with high temperature
also led to the outburst of fine particulate pollution in this period.
During haze-II days, the SIA and organic matter (OM) contents in PM2.5 slowly
decreased, whereas the concentrations of Na+, Cl-, and
unidentified materials in PM2.5 increased (Fig. 1e and h), suggesting
that local contribution weakened and regional contribution via sea salt
became more important (Jiang et al., 2021). This phenomenon was also
observed to be consistent with the changes in the pollutant sources
transported by air masses. As indicated in Fig. S1, the PM2.5 samples
on haze-II days included some contributors transported from the coastal area of
eastern Guangdong Province and Fujian Province, and the PM2.5 is
likely to be enriched with sea salt materials and mineral dusts.
Light absorption
The light absorption properties of WSOC and HULIS (Fig. 1d, i, and j and Table S2) exhibited obvious temporal variations during the sampling period. The
AAE values of WSOC and HULIS ranged from 4.1 to 6.4 and 5.6 to 6.6,
respectively. The AAE values for HULIS were obviously higher than those for
WSOC in the same sample (Fig. 1i), indicating that light absorption of
HULIS is more wavelength-dependent than that of WSOC. Similar results have
also been observed in previous studies (Park et al., 2018; Jiang et al., 2020;
Cao et al., 2021). These differences could be attributed to the differences
in chemical composition of chromophores in WSOC and HULIS. As shown in Table S2, the E250/E365 values of HULIS (5.3–5.6) are all higher than those
of WSOC (4.4–5.1), suggesting that the light-absorbing species in HULIS may
have lower aromaticity and/or lower molecular weight than those in
WSOC (Chen et al., 2016; Li and Hur, 2017). Therefore, the HULIS fractions
exhibit relatively high absorption at UV and short visible wavelengths and
relatively low absorption at long visible wavelengths, which results in
relatively high AAE values. Moreover, the AAE values of HULIS did not
present significant variation during the entire haze process, which could be
related to the evolution of HULIS chromophores in different stages (Jiang
et al., 2020; Huang et al., 2018; Deng et al., 2022). At first, the enhanced
oxidation of aromatic species on haze days could lead to the bleaching or
degradation of chromophores (a detailed explanation is provided in
Sect. 3.3) and thus a lower wavelength dependence (Forrister et al., 2015;
Zhan et al., 2022). In contrast, the outburst of secondary organic aerosols and the
photolysis of organic aerosols on haze days both tended toward higher AAE
values (Saleh et al., 2013; Dasari et al., 2019). Consequently, the
different trends in AAE were counterbalanced during the haze days, which
resulted in no significant AAE variations being observed for HULIS in the entire
sampling process. This is also consistent with the trends of the
E250/E365 ratios of HULIS in the four stages (Table S2).
Light absorption at 365 nm (Abs365) for WSOC and HULIS was 2.5 ± 2.0 and 1.8 ± 1.6 Mm-1, respectively (Table S2). HULIS
contributed to about 72 % of light absorption coefficients by WSOC,
implying that they enriched the major light-absorbing components in WSOC. As
shown in Fig. 1d, the Abs365 values for HULIS presented obvious
temporal variations. The Abs365,HULIS value was 0.55 ± 0.06 Mm-1 on clean-I days, which first increased to 3.4 ± 1.5 Mm-1 on haze-I days, then slowly decreased to 2.6 ± 0.85 Mm-1 on haze-II days, and finally rapidly declined to 0.64 ± 0.32 Mm-1 on clean-II days. This result was similar to the variations in
the mass concentration of HULIS. Furthermore, the Abs365 values for
HULIS in Guangzhou were found to be higher than those observed in the
southeastern Tibetan Plateau (0.38–1.0 Mm-1) (Zhu et al., 2018) but
obviously lower than those in Xi'an (7.6–36 Mm-1) (Shen et al., 2017)
and Beijing (3.7–10.1 Mm-1) (Du et al., 2014).
In general, MAE365 values can be used to assess the light absorption
capacity of target organic compounds (M. Li et al., 2019). As shown in Fig. 1j and Table S2, the average MAE365 value for WSOC was 1.0 ± 0.21 m2g-1C (0.68–1.3 m2g-1C), nearly the same as 1.1 ± 0.27 m2g-1C (0.77–1.8 m2g-1C) for HULIS, during
the entire sampling period. Moreover, the MAE365 values for HULIS
measured in the present study were noted to have dropped to the ranges of
those determined in Beijing (1.43 ± 0.33 m2g-1C) (Mo et
al., 2018), Xi'an (0.91–1.85 m2g-1C) (Yuan et al., 2021), and
Hong Kong (1.84 ± 0.77 m2g-1C) (Ma et al., 2019). The
average MAE365 values for HULIS exhibited some temporal variations. The
MAE365 values for HULIS were 0.91 ± 0.03 and 0.95 ± 0.11 m2g-1C on haze-I and haze-II days, respectively, which were lower
than those (1.3 ± 0.22 and 1.3 ± 0.27 m2g-1C,
respectively) observed on clean-I and clean-II days, suggesting that HULIS
have a relatively weaker light absorption capability on haze days. This
finding is consistent with the results reported by Zhang et al. (2017), who
found that the MAE365 values in the heating or non-heating seasons
during hazy days were lower than those on clean days. These differences in
MAE365 values may potentially contribute to the enhanced oxidation
reaction that was derived by the increased O3 levels and high
temperature and relative humidity (RH) during haze days (Fig. 1). This
oxidation process would lead the chromophores containing C=C unsaturated
bonds to be severely degraded (J. Wang et al., 2017; Zhang et al., 2017).
Besides, an increase in additional sources for HULIS in the study area, such
as weaker or non-light-absorbing compounds formed by atmospheric oxidation,
could also result in weaker light absorption of HULIS during the haze
episode (Liu et al., 2018).
Molecular evolution of HULIS during the haze process
For an in-depth understanding of the variation in HULIS at the molecular level
during the haze process, the four HULIS samples collected in different
stages of the haze process were analyzed by ESI FT-ICR MS in both negative
and positive modes. As shown in Fig. 2, thousands of peaks were detected
in the mass range between m/z 100 and m/z 700, with the high-intensity ions
noted within m/z 150–400. It is obvious that some organic compounds with
stronger arbitrary abundance were labeled, and their formulas, double-bond
equivalent (DBE), modified aromaticity index (AImod), and potential
sources are listed in Table S3. Compounds a (C7H7NO3) and b
(C8H6O4) both have high DBE values, which might be assigned
to aromatics such as methylnitrophenol and phthalic acid, whereas compound d
(C8H18O4S) with a low DBE value and high O/S ratio was probably
aliphatic organosulfate. According to previous studies, these organic
molecules might be derived from BB and diesel fuel, and thereby these results
suggest that both BB and vehicular emissions are important sources of BrC
in ambient aerosols (Mohr et al., 2013; Riva et al., 2015; Blair et al., 2017). Furthermore, compound e (C10H17NO7S) and compound f
(C10H18N2O11S) in Table S3 were found to be identical to
the oxidation products of monoterpenes, suggesting that biogenic sources could
contribute to the formation of HULIS (Surratt et al., 2008; Wang et al.,
2019). Thus, HULIS could be affected by multiple sources during the haze
process, possibly including BB, biogenic sources, and anthropogenic
emissions.
Mass spectra of HULIS detected in ESI- and ESI+ modes during the
haze process. The pie charts represent the intensity percent of different
compound groups.
The identified formulas could be divided into seven compound categories,
namely, CHO-, CHON-, CHOS-, and CHONS- detected in ESI- mode and
CHO+, CHN+, and CHON+ detected in ESI+ mode. As illustrated in
Fig. 2, the CHO compounds were the most abundant group in all the HULIS,
accounting for 43 %–50 % and 51 %–57 % of the overall compounds
detected in the ESI- and ESI+ modes, respectively. It must be noted that
relatively low contents of CHO- were detected during the haze episode
(haze-I and haze-II days) and of CHO+ molecules in haze-I HULIS. The CHON
compounds were the second most abundant group in all the HULIS. As shown in
Fig. 2, the relative content of CHON- was 23 % on clean-I days, which
slightly increased to 24 %–25 % in the haze episode and then decreased to
23 % on clean-II days. In contrast, the relative content of CHON+
compounds was 41 % on clean-I days, which increased to 45 % on haze-I
days and then fell to 42 % on haze-II days and 41 % on clean-II days. Both
CHOS- and CHONS- compounds were identified in all the four HULIS,
accounting for 19 %–22 % and 8 %–11 % of the total identified
compounds, respectively. The CHN+ compounds were the least abundant
(1.3 %–3.6 %) in the four HULIS samples and were relatively high
during the haze episode, especially on haze-I days.
Tables S4 and S5 show the relative-abundance-weighted elemental ratios,
molecular weight (MW), DBE, AImod, and carbon oxidation state
(OSC) for the identified compounds in HULIS. The MWw values for
HULIS determined in the ESI- mode on haze-I and haze-II days were 302 and
283, respectively, which were higher than those on clean-I and clean-II days
(266 and 264, respectively). Similar variation was also observed for
MWw for HULIS detected in ESI+ mode (Table S5). These results clearly
indicated that more higher-MW compounds constituted HULIS obtained during
the haze episode. Furthermore, the molecular properties of HULIS in
different stages of the haze process also exhibited some observable differences.
As shown in Table S4, the HULIS samples in the haze episode detected by the ESI-
mode presented relatively low AImod,w values and relatively high
O/Cw, O/Nw, and O/Sw ratios compared to those on clean days,
indicating that haze HULIS exhibited relatively low aromaticity and a higher
oxidation degree than clean HULIS. These differences can be attributed to
the enhanced oxidation degradation of aromatic compounds (e.g., phenols,
nitroaromatic compounds, and polycyclic aromatic hydrocarbons (PAHs)) during
the haze process. In addition, an increased contribution from traffic emissions
and secondary reactions of biogenic VOCs (bio-VOCs) also decreased the aromaticity and
increased the oxidation degree of HULIS (Liu et al., 2016; Tang et al.,
2020). These changes in HULIS compounds led to the decrease in their
MAE365 values during the haze episode, as described above (Zhong and
Jang, 2014; Song et al., 2019).
CHO compounds
The CHO compounds bear O-containing functional groups and have been
frequently detected in ambient aerosols. As shown in Fig. 2, the CHO
compounds were the predominant component in the four HULIS samples, and the
MWw values for CHO- and CHO+ compounds were 247–288 and
236–272, respectively, with relatively high MWw values observed for
the CHO group (CHO- and CHO+) in haze HULIS, especially in haze-I
samples. This finding may be related to the stronger oxidation of HULIS
during haze days because the aqueous oxidation of biomass burning aerosols
was found to yield high MW of organic products (Tomaz et al., 2018; Yu et
al., 2016).
The OSC is often used to describe the degree of oxidation of organic
species in the atmosphere (Kroll et al., 2011; X. K. Wang et al., 2017). Figure 3
shows plots of OSC versus the carbon number for the CHO compounds. As
indicated in the figure, CHO compounds exhibited OSC from -2 to +1
with up to 40 carbon atoms. Kroll et al. (2011) proposed that compounds with
OSC between -0.5 and +1 and <18 carbon atoms can be
attributed to semi-volatile and low-volatility oxidized organic aerosols
(SV-OOA and LV-OOA), which are mainly formed by complex oxidation reactions
in atmosphere. Compounds with OSC between -0.5 and -1.5 and 6–23
carbon atoms are related to primary biomass burning organic aerosols (BBOA).
In addition, compounds with OSC between -1 and -2 and ≥18
carbon atoms have been suggested to be hydrocarbon-like organic aerosols
(HOA), which are regarded as a primary combustion surrogate (Zhang et al.,
2005; Kroll et al., 2011; X. K. Wang et al., 2017).
Carbon oxidation state (OSC) plots for CHO- and CHO+. Formulas
with black, green, blue, and red are assigned to aliphatic (AI=0),
olefinic (0<AI<0.5), aromatic (0.5≤AI<0.67), and condensed aromatic (AI≥0.67) species (Koch and Dittmar, 2006), respectively.
As illustrated in Fig. 3 and Table S6, most of the CHO- compounds
clustered in the BBOA region, accounting for 40 %–46 % of the total
CHO- compounds, thus suggesting that BB may be a major contributor to CHO
compounds in HULIS. Figure 3 clearly indicates that the majority of aromatic
and condensed aromatic compounds produced signals in the OSC region
between -0.5 and 1.0 and with a carbon number of 3–18 (Fig. 3), which
corresponded to SV-OOA and LV-OOA. The proportions of SV-OOA and LV-OOA
accounted for 23 %–28 % and 1.9 %–2.4 % of the total CHO-
compounds, respectively, and presented no significant variation. In
contrast, the HOA components on haze-I days showed the highest abundance
(18 %), which were much higher than those (3.5 %–4.5 %) on haze-II,
clean-I, and clean-II days. This finding indicated that the increase in the
primary source is associated with fossil fuel combustion such as vehicle
emissions during the haze bloom period (Zhang et al., 2005).
As shown in Fig. 3, CHO+ compounds presented lower OSC (from -2.0
to 1.0) than CHO- compounds. Most of the CHO+ compounds occurred in the
BBOA region in all four HULIS samples, forming up to 60 %–72 % of the
total CHO+ compounds, which again suggests that BB is an important
contributor to CHO compounds in HULIS. The HOA among CHO+ compounds showed
the same changing trends as those among CHO- compounds, and higher HOA
abundance was observed during haze-I days. In addition, some high AImod
values of aromatics were found in the regions A1+ and A2+ (Fig. 3),
which implied that the highest AImod values (AI≥0.67) with DBE≥22 were only detected during the haze days, possibly owing to
soot-derived materials or oxidized PAHs (Decesari et al., 2002; Kuang and
Shang, 2020). It must be noted that the sampling site in the present study
is influenced by traffic sources; the enhanced oxidation of
vehicle-exhausted soot also results in the accumulation of water-soluble
high aromatic organic species (Decesari et al., 2002).
CHON compounds
In the present study, 1379–2217 and 2008–2943 formulas were assigned to
CHON compounds identified in the ESI- and ESI+ spectra, respectively,
which accounted for 23 %–25 % (ESI-) and 41 %–45 % (ESI+) of the
total identified compounds, respectively. Relatively high contents of
CHON- compounds were obviously detected in HULIS samples obtained during
haze-I days, suggesting the occurrence of more N-containing components in
HULIS during haze bloom days. As shown in Tables S4 and S5, the average
MWw values for CHON- and CHON+ compounds were 328 and 317 on haze-I
days, respectively, which were slightly higher than those determined on
haze-II days, and all were higher than those observed on clean-I and clean-II
days. Meanwhile, the AImod,w values for CHON- on haze days were
0.31–0.34, which were slightly lower than those on clean days (0.37 and
0.40). These findings indicated that more high-MW CHON compounds with lower
aromatic structures were formed during the haze episode.
The O/Nw ratios for CHON- and CHON+ during haze-I and haze-II
days were 5.3–5.7 and 3.8, respectively, which were higher than those ratios
determined during the two clean periods, confirming that these compounds
were highly oxidized during the haze episode (Tables S4 and S5). In general,
compounds with O/N≥3 may indicate oxidized N groups such as nitro
(-NO2) or nitrooxy (-ONO2), whereas compounds with O/N<3 may denote the reduced N compounds (i.e., amines) (Lin et al., 2012; Song
et al., 2018). In the present study, most of the CHON compounds
(79 %–91 % of CHON- compounds and 61 %–64 % of CHON+ compounds)
exhibited O/N≥3, suggesting that high concentrations of nitro
compounds or organonitrates were contained in the CHON compounds. Moreover,
these compounds were more abundant in the CHON- group during the haze
episode (87 %–91 %), when compared with those during clean-I and
clean-II days (79 %–82 %), again implying that CHON- compounds undergo
relatively high oxidization during the haze episode. As indicated in
Fig. 1, the increase in NO2 was consistent with increased production
of highly oxidized N-containing organic compounds (NOCs) during the haze
episode, which suggested the significant contribution of NO3-related
multigenerational chemistry to organonitrate aerosol formation (Berkemeier
et al., 2016).
The majority of aromatics and condensed aromatics produced clear signals in
regions associated with SV-OOA and LV-OOA (Fig. 4). BBOA also constituted
a significant proportion (33 %–39 %) in the CHON- group, and a
relatively low BBOA content was observed on haze-I days. The abundance of
HOA was relatively low, accounting for 2.3 %–7.8 % of the total CHON
compounds, and the relative abundance of HOA on haze-I days was much higher
than that on haze-II, clean-I, and clean-II days, suggesting the accumulation
of primary fossil fuel combustion during haze-I days.
Carbon oxidation state (OSC) plots for CHON- and CHON+. Formulas
with black, green, blue, and red are assigned to aliphatic (AI=0),
olefinic (0<AI<0.5), aromatic (0.5≤AI<0.67), and condensed aromatic (AI≥0.67) species (Koch and Dittmar,
2006), respectively.
The CHON+ compounds mainly occurred at the range of -2.0<OSC<1.5, with average OSC values of around -1.0 for
each sample, clearly indicating that CHON+ compounds were lower
than CHON- compounds. Most of the CHON+ compounds were detected in the
BBOA region, accounting for 60 %–76 % of the total CHON+
compounds. The relative contribution of BBOA on haze-I days was lower than
that on haze-II and clean days. Moreover, a large number of aromatic species
were observed at the region B1+ (Fig. 4), demonstrating that higher
aromatic compounds were only detected on haze-I days, which may be related
to soot or black carbon (BC). A similar trend was also exhibited by CHO+ compounds,
indicating the contribution of local combustion sources (e.g., traffic
emission) during haze-I days.
CHOS and CHONS compounds
In this study, 478–696 CHOS compounds and 306–589 CHONS compounds were
identified in ESI- mode (Table S4). Among these S-containing compounds,
>86 % of the CHOS compounds had O/S ratios >4,
whereas >89 % of the CHONS compounds presented O/S ratios
>7, suggesting that these S-containing compounds were possibly
organosulfates and nitrooxyorganosulfates. As listed in Table S4, the
AImod,w values for CHOS and CHONS were about 0.02 and 0.01 in the HULIS
fraction, which were much lower than those for CHO and CHON. Almost 99 %
of the CHOS and CHONS compounds in the HULIS fraction had AImod values
<0.5, while >93 % of the CHONS compounds had
AImod=0, indicating that they were mainly comprised of aliphatic
and olefinic organosulfates. These results are consistent with the previous
findings that the major S-containing compounds among organic aerosols in
Guangzhou are organosulfates formed by secondary oxidation reaction of
long-chain alkenes/fatty acids with SO2 (Jiang et al., 2020), which
generally possessed long aliphatic carbon chains and a higher degree of
oxidation. However, these compounds are different from the S-containing
compounds detected during the hazy days in Beijing (Jiang et al., 2016; Mo et al., 2018), which were determined to be aliphatic organosulfates with a low
degree of oxidation and higher amounts of aromatics and PAH-derived
organosulfates, having a strong correlation with anthropogenic emissions.
As described earlier, CHOS- and CHONS- compounds might be related to
organosulfates or nitrooxyorganosulfates, which have been observed to be
derived from atmospheric reactions of bio-VOCs such as α-pinene,
limonene, and isoprene (Huang et al., 2018; Surratt et al., 2008) and fossil fuel
combustion including coal combustion and off-road engine emissions (Song et
al., 2018, 2019; Cui et al., 2019). In the present study, the relative
contents of S-containing compounds (CHOS+CHONS) in the HULIS fraction on
haze days were all higher than those on clean days (Fig. 2). Moreover, the
CHOS and CHONS compounds in haze HULIS always have relatively high O/S ratios compared to those in clean HULIS. These findings suggested
the relatively high contribution of SO2-related chemical oxidation
during the haze event.
CHN compounds
The N bases (CHN) are usually identified in ambient aerosols and smoke from
BB. In the present study, 110–165 CHN+ compounds were identified in
ESI+ mode, with most of them (>86 %) presenting DBE≥2, suggesting that they might be nitrile and amine species (Lin et al.,
2012). As shown in Fig. 2, the abundances of CHN+ compounds were
2.0 %–3.6 % on the haze days, which were much higher than those noted
on clean days (1.3 %–1.4 %), indicating a higher contribution of CHN+
compounds to the HULIS fraction during the haze episode. The MWw values
for CHN+ compounds were 204–223, which were lower than those for the
other groups (i.e., CHO+, CHON+) (Table S5). However, the average
AImod values for N bases (0.37–0.48) detected in the ESI+ mode
were much higher than those for CHO+ (0.11–0.12) and CHON+ (0.20–0.22)
compounds, implying that these reduced CHN+ compounds exhibited more
unsaturated or aromatic structures.
To further understand the molecular distribution of CHN+ compounds during
the haze process, van Krevelen (VK) diagrams were constructed by plotting
the H/C ratio versus the N/C ratio (Fig. S2). It was obvious that this plot
could separate the compound classes with different degrees of AI. As shown in
Fig. S2, compounds (denoted in black color) in the upper region of the VK
diagram had one N atom with DBE=0, indicating that they are aliphatic
amines. It can be noted from Table S7 that the aliphatic group presented the
lowest abundance in all the samples, suggesting that the CHN+ compounds
possessed comparatively low aliphatic structures. Olefinic compounds
showed the highest abundance in the four samples, which accounted for
37 %–51 % of the total CHN+ compounds. Importantly, a large
proportion of the compounds (>39 %) exhibited high degree of
AI (AI>0.5) (Fig. S2 and Table S7), suggesting a large
extent of aromatic structure and N-heterocyclic rings in HULIS. Moreover,
the CHN+ compounds on haze-I days presented obviously lower content of
aromatic structures than those on haze-II, clean-I, and clean-II days,
signifying the relatively high contribution of fossil fuel combustion (which
generally emits more low aromatic CHN compounds) during the haze bloom
episode (Song et al., 2022). In addition, the CHN+ group also constituted a
large proportion of BBOA (Table S6), which indicated the significant
contribution of BB. However, it must be noted that a relatively low
content of BBOA was detected during haze-I days, which was consistent with
the changing trends of CHON- or CHON+ compounds during the haze episode.
These results suggested the relatively low contribution of BB during
haze-I days because quiet and stable weather conditions can prevent
regional transport of BB sources during this stage (Wu et al., 2018).
Factors influencing light absorption and molecular characteristics of
HULIS during the haze bloom-decay process
As described earlier, the light absorption properties of HULIS exhibited
obvious variation during the haze bloom-decay process. The average
Abs365 value for HULIS was 0.55 ± 0.06 Mm-1 on clean-I
days, which first increased to 3.4 ± 1.5 Mm-1 on haze-I days,
then slowly decreased to 2.6 ± 0.85 Mm-1 on haze-II days, and
finally rapidly declined to 0.64 ± 0.32 Mm-1 on clean-II days.
In general, the light absorption of HULIS can be related to their chemical
and molecular properties that are influenced by factors such as sources,
secondary formation, and the aging process. The results of principal component
analysis (PCA) obviously showed a positive loading for principal component 1
(PC1), and the Abs365 values for HULIS were clustered with EC,
Kbb+, Lev, NH4+, and NO3- (Fig. 5). These
results suggested that BB and other sources such as new particle formation
could contribute to light absorption of HULIS (An et al., 2019; Song et al.,
2019). Similarly, the findings of Pearson correlation coefficient analysis
revealed that the Abs365 values for HULIS exhibited significant
positive correlations with Kbb+ (r=0.728, p<0.01)
and Lev (r=0.800, p<0.01) (Table S8). As Lev and
Kbb+ are generally considered tracers derived from BB, these
results suggested the significant contribution of BB to light absorption of
HULIS. This observation was also supported by the abundance of BBOA
compounds detected in all the four HULIS samples (Table S6). The significant
positive relationships between the Abs365 values for HULIS and
secondary ions (i.e., NO3- (r=0.702, p<0.01),
SO42- (r=0.554, p<0.05), and NH4+ (r=0.899, p<0.01)) indicated the important impact of secondary
formation on the light absorption of HULIS. Besides, the Abs365 values
for HULIS were also strongly correlated with NO2, O3, and
NO2, which confirmed the important impact of atmospheric oxidation
reactions on the light absorption of HULIS.
Principal component analysis results for the optical properties of
HULIS and chemical compositions of PM2.5.
Formula number of potential BrC chromophores and the
intensity ratios of each group of potential BrC in total potential BrC and
each group of total identified formulas, respectively.
SamplesElementalESI-ElementalESI+compositionsNumberIntC/IntBrCIntBrC,i/IntbulkcompositionsNumberIntC/IntBrCIntBrC,i/IntbulkClean-ICHO-4240.480.25CHO+2630.370.07CHON-7730.460.53CHON+4800.560.15CHOS-630.030.05CHN+790.070.56CHONS-430.030.08all in ESI+8220.11all in ESI-13030.26Haze-ICHO-3560.440.21CHO+2440.290.09CHON-7910.500.45CHON+6140.620.22CHOS-430.030.03CHN+940.090.39CHONS-390.030.07all in ESI+9520.16all in ESI-12290.22Haze-IICHO-4440.450.26CHO+3330.340.06CHON-9410.490.49CHON+5950.560.13CHOS-670.030.03CHN+890.10.48CHONS-780.030.07all in ESI+10170.10all in ESI-15300.25Clean-IICHO-3910.460.27CHO+2340.380.09CHON-7070.480.59CHON+4620.560.18CHOS-640.030.05CHN+750.060.57CHONS-490.030.10all in ESI+7710.13all in ESI-12110.29
IntC: the intensity of each group of identified potential BrC;
IntBrC: the sum intensity of identified potential BrC;
Intbulk: the sum intensity of each group of total identified formulas. The bold formatting represents the relative high values in each group.
It must be noted that MAE365 is a key parameter signifying the light
absorption ability of HULIS. As listed in Table S2, the MAE365 values
for HULIS varied in different stages and were lower on haze days owing to
the variation in the chemical and molecular composition of HULIS during the
haze bloom-decay process. Furthermore, the AImod values for HULIS
varied in different stages (Tables S4) and were relatively low on haze
days, indicating that haze HULIS have a comparatively low degree of
conjugation or aromaticity. This finding suggested that the HULIS compounds
may undergo higher oxidation during the haze episode, causing a decline in
chromophores and reduction in the light absorption capacity of HULIS (Lin et
al., 2017). Besides, the accumulated contribution of organic compounds from
vehicle emission and secondary chemical reactions of bio-VOCs may also
dilute light-absorbing compounds in haze HULIS (Tang et al., 2020; Liu et
al., 2016).
Lin et al. (2018) reported that potential light-absorbing chromophores can
be determined in the region between DBE=0.5×C (linear
conjugated polyenes CxHyC2) and DBE=0.9×C
(fullerene-like hydrocarbons). In the present study, most of the
high-intensity CHON, CHO, and CHN compounds with high AI values were
clustered in a potential BrC chromophore region (Figs. S3 and S4), which
mainly comprised CHON (46 %–50 % in ESI- mode and 56 %–62 % in
ESI+ mode) and CHO (44 %–48 % in ESI- mode and
29 %–38 % in ESI+ mode) compounds
(Table 1). Although
the contribution of CHN+ compounds to BrC was relatively low, the
content of potential chromophores among the total CHN+ compounds was
higher than those in CHON+ and CHO+ compounds. Therefore, these three
groups of light-absorbing compounds (i.e., CHON+, CHN+, and CHO+
compounds) were further examined. As shown in Table 1, the
IntC/IntBrC values of CHO- (i.e., content of CHO- chromophores
in the total chromophores) decreased from 48 % to 44 %, whereas the
IntC/IntBrC values of CHON- increased from 46 % to 50 %
during the haze bloom process. These findings indicated that more NOC
chromophores were formed during this stage in which higher NO2
concentration may be preferred for the formation of N-containing
chromophores such as nitrophenols. However, it must be noted that the
proportions of both CHO- and CHON- chromophores among the total
identified compounds decreased from clean-I to haze-I days, suggesting the
occurrence of a stronger photo-bleaching process during the haze bloom stage
(Zeng et al., 2020). Likewise, both CHO+ and CHON+ compounds presented
similar variation during the entire study period. In addition, the CHN+
compounds also exhibited higher IntC/IntBrC values during the haze
bloom process, suggesting the accumulated contribution from the local
combustion process. Furthermore, the proportion of CHON+ chromophores in
the total CHON+ compounds increased with the decreasing content of CHN+
chromophores, possibly implying that some aromatic CHN compounds were transformed
to CHON+ compounds during the aging process.
Conclusions
This study investigated the evolution of light absorption and molecular
properties of HULIS during a winter haze bloom-decay process and examined
the key factors affecting the light absorption of HULIS in Guangzhou, China.
The results showed that HULIS exhibited significant variation in light
absorption during the haze bloom-decay process. First, higher Abs365
values were observed on haze days, indicating the presence of significant
quantities of light-absorbing organic compounds during the haze episode.
However, the MAE365 values for HULIS on haze days were lower
than those on clean days, suggesting the light absorption capabilities of
HULIS were weakened during the haze event. Furthermore, CHON and CHO
compounds, exhibiting a relatively high degree of conjugated structure, were
the most abundant groups in all the HULIS samples and were also the major
contributors to the light absorption capacity of HULIS. Importantly, the
molecular properties of HULIS dynamically varied during the entire haze
episode. When compared with HULIS on clean days, those on haze days
presented relatively low AImod values and higher O/Cw,
O/Nw, and O/Sw ratios, suggesting the predominance of compounds
with low aromaticity and higher oxidation in HULIS during haze episodes.
These results indicated that HULIS compounds undergo relatively strong
oxidation during the haze days. Moreover, PCA and Pearson correlation
analysis revealed that BB and secondary chemical formation both contributed
to the variation in the light absorption properties of HULIS. Both primary
sources (such as the accumulated contribution of organic compounds formed from
local traffic emissions) and secondary sources (such as stronger chemical
reactions) led to the rapid increase in HULIS during the haze bloom days.
However, stronger oxidation of HULIS compounds were observed during the haze
episode, and some potential BrC chromophores were degraded. In addition, the
chemical reactions of bio-VOCs such as isoprene also diluted the
light-absorbing compounds in HULIS.
Thus, the present study provides novel insights into the light and molecular
evolution of HULIS during haze events, which are important for predicting the
environmental and climatic effects of HULIS. However, as this study examined
only one haze bloom-decay process in winter in Guangzhou, the results
obtained may be not adequate for understanding all the haze episodes in
southern China. Therefore, there is a need for a comprehensive investigation of
haze episodes in different seasons and regions in the future.
Data availability
The research data are available in the Harvard Dataverse (10.7910/DVN/DYGYQT, Song, 2022).
The supplement related to this article is available online at: https://doi.org/10.5194/acp-23-963-2023-supplement.
Author contributions
JS and PP designed the research together. CZ, TC, and ML carried out the PM2.5 sampling experiments. CZ and TC extracted and analyzed the WSOC and HULIS samples. BJ analyzed the HULIS samples by FT-ICR MS. CZ and JS wrote the paper. JL, XD, ZY, and GZ commented on and revised the paper.
Competing interests
The contact author has declared that none of the authors has any competing interests.
Disclaimer
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
This is contribution no. IS-3298 from GIGCAS.
Financial support
This research has been supported by the National Natural Science Foundation of China (grant nos. 42192514 and 41977188) and Guangdong Foundation for Program of Science and Technology Research (grant nos. 2020B1212060053 and 2019B121205006).
Review statement
This paper was edited by Roya Bahreini and reviewed by two anonymous referees.
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