Firework (FW) emission has strong impacts on air quality
and public health. However, little is known about the molecular composition
of FW-related airborne particulate matter (PM), especially the organic
fraction. Here we describe the detailed molecular composition of Beijing PM
collected before, during, and after a FW event in the evening of New Year's Eve in
2012. Subgroups of CHO, CHON, and CHOS were characterized using ultrahigh-resolution Fourier transform ion cyclotron resonance (FT-ICR) mass
spectrometry. These subgroups comprise a substantial fraction of aromatic-like
compounds with low O/C ratio and high degrees of unsaturation, some of which
plausibly contributed to the formation of brown carbon in Beijing PM.
Moreover, we found that the number concentration of sulfur-containing
compounds, especially the organosulfates, increased dramatically during the FW
event, whereas the number concentration of CHO and CHON doubled after the
event, which was associated with multiple atmospheric aging processes
including the multiphase redox chemistry driven by NOx, O3, and
•OH. These findings highlight that FW emissions can lead to a
sharp increase in high-molecular-weight compounds, particularly aromatic-like
substances in urban particulate matter, which may affect the light
absorption properties and adverse health effects of atmospheric aerosols.
Introduction
The widespread haze pollution in China has aroused much attention due to
its strong impacts on air quality, human health, and climate change
(Ramanathan et al., 2001; Pöschl, 2005; Lelieveld et al., 2015; Thomason
et al., 2018; Kaufman et al., 2002). The levels of haze pollution are strongly
dependent on the source of haze particles, e.g., industry, coal combustion,
vehicle emissions, cooking, and biomass burning (Sun et al., 2013; Zheng et
al., 2005). Among different haze particle sources the firework (FW) emission can be
expected to play an important role in urban air quality during festivals
(Feng et al., 2012; Jing et al., 2014; Jiang et al., 2015; Tian et al.,
2014). However, the chemical composition of FW-related aerosols, especially
the organic fraction, is not well characterized.
There are a high number of pollutants released by FW burning, such as sulfur
dioxide, nitrogen oxide, volatile organic compounds, and particles
comprising inorganic materials (e.g., potassium and sulfate), and organic
compounds (e.g., n-alkanes and
polycyclic aromatic hydrocarbons, PAHs) (Feng et al., 2012). They impose
threats on human health (Sarkar et al., 2010) and can reduce visibility
(Vecchi et al., 2008). Moreover, real-time chemical composition
measurements illustrated that FW-related organics were mainly secondary
organic material (Jiang et al., 2015). Nonetheless, all those studies
primarily focused on the inorganic chemical species and relatively-low-molecular-weight (LMW) organic compounds, while little is known about the
molecular-level characterization of high-molecular-weight (HMW) organic
compounds in urban aerosols during FW events, which contains important
chemical composition information of aerosols.
Water-soluble organic carbon (WSOC) is a ubiquitous component of atmospheric
aerosols. A large proportion of water-soluble organic matter is composed of
HMW organic compounds that contain a substantial fraction of heteroatoms (N,
S, O) (Lin et al., 2012a; Mazzoleni et al., 2012; Wozniak et al., 2008; Wang
et al., 2016). Highly oxygenated molecules contain a wide range of chemical
functional groups such as peroxides, hydroperoxides, carbonyls, and percarboxylic acids (Lee et al., 2019). Organic acids in oxygen-containing
species contribute significantly to aerosol acidity. Lots of nitro-aromatic
compounds in relatively-high-molecular-weight compounds, often observed in
biomass burning aerosols, are potential contributors to light absorption
(Laskin et al., 2015; Lin et al., 2015). Moreover,
organosulfates substantially contribute to the secondary organic aerosol
(SOA) mass (Tolocka and Turpin, 2012), which plays an important role
in exploring the formation pathway of SOA (Shang et al., 2016; Riva et
al., 2015, 2016; Passananti et al., 2016). Meanwhile, because of
their polar and hydrophilic nature, organosulfates can influence the
hygroscopic properties of aerosols (Estillore et al., 2016).
Hence, characterizing both the compound class and individual compound level
of organic aerosols (OAs) is important for exploring the formation
mechanisms, physicochemical properties, and environmental effects of
firework-related aerosols. Moreover, the large amount of firework emission
is an ideal event to understand the contribution of anthropogenic precursors
to the formation of organic aerosols.
A molecular-level characterization of chemical constituents in firework-related aerosols is challenging because of their highly chemical
complexity with vast numbers of compounds. Less than 10 %–20 % of
water-soluble organics, limited to LMW, can be characterized at a molecular
level by a combination of gas chromatography–mass spectrometry (GC-MS)
(Wang et al., 2006), ion chromatography, and high-performance liquid
chromatography (HPLC) (Hong et al., 2004). Recently, Fourier transform
ion cyclotron resonance mass spectrometry (FT-ICRMS), one of the
ultrahigh-resolution mass spectrometers (UHRMSs) with extremely high mass
accuracy, has been successfully used to characterize complex organic
mixtures of water-soluble organic matter in urban aerosols (e.g., Ohno et
al., 2016; Qi and O'Connor, 2014; Lin et al., 2012a; Wozniak et al., 2008; Jiang
et al., 2016; Mazzoleni et al., 2012; Kundu et al., 2012; Xie et al., 2020).
However up to date, little is known about the detailed molecular information
in firework-related aerosols. FT-ICRMS can characterize compounds with molecular weight from 100 to 1000 Da, especially for HMW compounds.
Moreover, compounds containing nitrogen, sulfur, and phosphorus atoms in the
organic mixture can be identified by FT-ICRMS with high resolution
(Hawkes et al., 2016).
In this study, the molecular-level composition of HMW organic compounds in
urban aerosols collected in Beijing during the firework events of
the traditional Chinese New Year's Eve and Spring Festival was investigated using
a 15 T ultrahigh-resolution FT-ICRMS. The chemical composition and
number concentrations of CHO, CHON, and CHOS subgroups in FW- and
non-FW-related aerosols were mainly discussed. In addition, the detailed
molecular characteristics of CHNOS species and their volatility using a
molecular corridor method will be present in another study.
The concentrations of chemical components in the Beijing
aerosol samples.
*ΣPAHs; the total concentration of 18 detected PAHs. NYE:
New Year's Eve (detailed days). LNY: Lunar New Year's Day (detailed days).
Materials and methodsAerosol sampling
Total suspended particle (TSP) sampling was conducted on the roof of a
building (8 m above ground level) in the campus of the Institute of
Atmospheric Physics, Chinese Academy of Sciences (39∘58′28′′ N, 116∘22′13′′ E), a representative urban site in the
central north part of Beijing. TSP samples were collected on a 12 h basis
from 21 to 23 January 2012 (i.e., sample ID: New Year's Eve
daytime, NYE D, before the FW event; New Year's Eve nighttime, NYE N, during
the FW event; Lunar New Year's Day daytime, LNY D, after the FW event; Lunar New Year's Day nighttime, LNY N), including episodes of short-term pollution
raised by FW emissions. Detailed sample information is shown in Table 1. The
48 h clustering air mass trajectories (Fig. S1 in the Supplement) show that all of them
mainly originated from the northwest. All aerosol and field blank samples
were collected using a high-volume air sampler (Kimoto, Japan) with
precombusted (6 h in 450 ∘C in a muffle furnace) quartz filters
(20cm×25cm, Pallflex). After the sampling, the filters were
stored in a refrigerator at -20∘C until analysis.
Chemical component analysis
One punch (diameter: 24 mm) of each filter sample was sonicated in 10 mL
ultrapure Millipore Q water for 20 min. The solution was then filtered with
0.22 µm hydrophilic PTFE filters (Anpel, China). The concentrations of
water-soluble SO42-, NO3-, Cl-, NH4+,
Na+, K+, Mg2+, and Ca2+ were measured using ion
chromatography equipped with IonPac AS11HC (Anion) and IonPac CS12 (Cation)
chromatographic column systems (Dionex Aquion, Thermo Scientific, America).
Concentrations of WSOC and total dissolved nitrogen (TDN) in the aerosol
extracts were measured by TOC-L and TNM-L (Shimadzu, Japan). Water-soluble
organic nitrogen (WSON) was calculated as the difference between TDN and the
sum of water-soluble inorganic nitrogen (WSIN, including NO3-,
NO2-, and NH4+) (Altieri et al., 2016). In
addition, the loadings of OC/EC (organic carbon / elemental carbon) and PAHs on filter
samples were measured using a Sunset OC/EC analyzer (Sunset Laboratory Inc., model 4) and gas chromatography–mass spectrometry (GC-MS), respectively.
There were 18 detected PAHs, including phenanthrene (PHE), anthracene
(AN), fluoranthene (FLU), pyrene (PYR), retene (RET), benz[a]anthracene
(BaA), chrysene/triphenylene (CHR), benzo[b]fluoranthene (BbF),
benzo[k]fluoranthene (BkF), benzo[e]pyrene (BeP), benzo[a]pyrene (BaP),
perylene (PER), anthanthrene (ANT), indeno[1,2,3-cd]pyrene (IcP),
dibenzo[ah]anthracene (DahA), (a,l)dibenopyrene, 1,3,5-triphenylbenzene,
benzo[ghi]perylene (BgP), and coronene (COR). More detailed information of
the water-soluble ions, WSOC, OC/EC, and PAH analysis was given elsewhere
(Yue et al., 2016; Ren et al., 2018; Fu et al., 2008).
FT-ICRMS measurement
Approximately 4.5 cm2 of each filter was extracted three times with
ultrapure Milli-Q water by sonicating for 10 min. The extract was combined
and loaded onto a solid-phase-extraction cartridge (Oasis HLB, Waters, US) for desalting,
which had been preconditioned with methanol and Milli-Q water. The majority
of inorganic ions, low-molecular-weight organic molecules, and sugars were
not retained by the cartridge (Lin et al., 2012a). Then, the
cartridge was washed with 5 mL Millipore Q water and dried under a nitrogen
flow for 1 h. Subsequently, the organic compounds retained on the
cartridge were eluted using 12 mL of methanol to avoid incomplete elution.
The eluate was immediately concentrated by a rotary evaporator and
redissolved in 4 mL of methanol. The pretreated extracts were finally
analyzed with a Bruker solariX Fourier transform ion cyclotron resonance
mass spectrometer (Bruker Daltonik, GmbH, Bremen, Germany) equipped with a
15.0 T superconducting magnet and an electrospray ionization (ESI) ion source. Because the target
species were water-soluble polar compounds, all the samples were analyzed in
the negative ionization mode and infused into the ESI unit by syringe
infusion at a flow rate of 120 µL h-1. Ions were accumulated for
0.1 s in a hexapole collision cell. The mass limit was from 180 to 1000 Da. To enhance the signal-to-noise ratio and dynamic range, two hundred
scans were averaged per spectrum. An average resolving power (m/Δm 50 %) of over 400 000 (at m/z=400 Da) was achieved. Field blank
filters were analyzed following the same procedure as the aerosol sample
analysis. Other details of the experiment setup can be found elsewhere
(Cao et al., 2016).
Molecular formula assignment
The mass spectra obtained by FT-ICRMS were internally recalibrated using an
abundant homologous series of sulfur-containing organic compounds in the
samples. Molecular formulae were assigned for peaks with a signal-to-noise
(S/N) ratio > 6 by allowing a mass error of 1.0 ppm between the
measured and theoretically calculated mass. A molecular formula calculator
was used to calculate formulae in the mass range between 185 and 800 Da with
elemental compositions up to 40 atoms of 12C, 100 of 1H, 40 of
16O, 2 of 14N, and 1 of 32S. The elemental ratio limits of
H/C < 2.5, O/C < 1.2, N/C < 0.5, and S/C < 0.2
and a nitrogen rule for even electron ions were used as further restrictions
for formula calculation (Koch et al., 2005, 2007; Wozniak et
al., 2008). Unambiguous molecular formula assignment was determined with the
help of the homologous series approach for multiple formula assignments
(Koch et al., 2007; Herzsprung et al., 2014). The isotopic peaks are
removed in the present study. Approximately 67 %–71 % of the
identified peaks were assigned in our samples. Their intensities
accounted for 70 %–75 % of the total signal. The molecular formulae in
blank filters with a signal-to-noise ratio greater than that of the aerosol
samples were subtracted from the real aerosol samples. In addition, because
of the instrument limitations, the absolute mass concentration of each
compound cannot be obtained. However, a semiquantitative method can make up the defect to some extent by using a normalized intensity, which has been applied in previous studies (Lin et al., 2012b; Jiang et al., 2016; Kourtchev et al.,
2016; Dzepina et al., 2015).
Parameter calculation
To explore the saturation and oxidation degree of organic constituents of
FW-related aerosols, we calculated the following useful parameters: double-bond equivalent (DBE) and aromaticity equivalent (Xc), and carbon
oxidation state (OSC), respectively.
The value of DBE is calculated along Eq. (1)
DBE=1+NC-NH2+NN2,
where the NC, NH, and NN represent the number of C, H, and N
atoms in a molecular formula, respectively. Molecular formulae with DBE
< 0 and formulae that disobey the nitrogen rule were discarded.
The value of Xc is used to characterize aromatic and poly-aromatic
compounds in highly complex compound mixtures. Xc normally ranges from 0 to 3.0 and is calculated as follows (Yassine et al., 2014):
Xc=3DBE-mNO-nNS-2DBE-mNO-nNS.
If DBE ≤mNO+nNS, then Xc=0,
where m and n correspond to a fraction of O and S atoms involved in π-bond structures of a compound and are various for different functional
groups. For instance, carboxylic acids, esters, and nitro functional groups
have m=n=0.5. When compounds contain functional groups such as
aldehydes, ketones, nitroso, cyanate, alcohol, or ethers, m and n are
adjusted to 1 or 0. Because ESI- mode is the most sensitive to
compounds containing carboxylic groups, we used m=n=0.5 for the
calculation of the Xc in this study. 2.5≤Xc < 2.71
indicates the presence of mono-aromatics and Xc≥2.71 indicates
the presence of poly-aromatics.
The OSC is used to describe the composition of a complex mixture of
organics undergoing oxidation processes. OSC is calculated for
assignable molecular formulae using Eq. (3) (Kroll et al.,
2011):
OSC=-∑iOSininC,
where OSi is the oxidation state associated with element i and
ni/nC is the molar ratio of element i to carbon within the molecule.
Results and discussionAbundances of typical aerosol constituents
Table 1 presents the mass concentrations of chemical components in the
urban aerosol samples. The atmospheric abundances of inorganic ions such as
SO42-, Cl-, and K+ increased dramatically in the
nighttime of the Chinese New Year's Eve (the FW event), which were 10 times
higher than the non-FW periods, and then decreased sharply afterwards.
K+, an indicator generally used for biomass burning (Cheng et al.,
2014), was the most abundant species among the measured ions and was about
50 times more than those during non-FW periods. Previous studies also
reported that the concentrations of K+ increased during FW events
(Cheng et al., 2014; Tian et al., 2014). This is reasonable because
K+ is a key component for the burst of FW. Similarly, Cl- also sharply increased during the FW event. But there was no influence of FW
burning on NO3-. Some studies showed that the concentrations of
NO3- increased in the NYE nighttime
(Zhang et al., 2017a), while others reported higher
concentrations after the FW event (Yang et al., 2014b; Zhang et al.,
2017a). These suggested that KClO4 and KClO3 are the main
components of the FW emission, though KNO3 is also the principal
oxidizer in black powder (Wang et al., 2007). Both SO42- and
NO3- are secondary inorganic ions; such diversity may be due to
the changes in emission sources. An increase in SO42- is
associated with orange flames from lots of FW burning (Moreno et al.,
2007). Fossil fuel combustion and vehicle emissions have been reported as
important sources of NO3- in Beijing (Ianniello et al.,
2010; Wang et al., 2014), while these sources minimized due to a sharp
decline in the population and vehicle; most of the people leave Beijing for
their hometowns during the Spring Festival (Yang et al., 2014a; Zhang et
al., 2017b). In addition, the concentrations of Mg2+ and Ca2+ were
slightly higher in the NYE nighttime than the non-FW periods. They were
mainly in the coarse-particle mode (Huang et al., 2013; Xu et
al., 2015).
Moreover, the mass concentrations of OC and EC during the FW event (sample
NYE N) doubled those during non-FW periods, particularly for EC.
Simultaneously, the WSOC concentration peaked sharply in the NYE nighttime.
Moreover, the WSOC / OC ratio was higher during the FW period than non-FW
periods, indicating more water-soluble OC was formed during the FW event.
Compared to the non-FW period, the total concentration of 18
detectable PAHs (Σ18 PAHs; PAH types are listed in Materials and
methods) significantly increased four times during the FW event,
agreeing with the urban aerosol study by Kong et al. (2015), which found
that FW burning was an important source for PAHs in Nanjing PM2.5
during the Spring Festival period in 2014. Furthermore, the detailed molecular composition of WSOC components was characterized by ESI FT-ICRMS and discussed below.
General molecular characterization of organic aerosols
The reconstructed mass spectra of all samples by ESI FT-ICRMS are exhibited
in Fig. 1. The peak intensity is mainly affected by the initial
concentration and ionization efficiency of the neutral compound (Lin
et al., 2012a). ESI is sensitive to polar compounds, and the compounds
reported in this study are easily ionized in the negative ion mode
(Qi et al., 2020). On this basis and considering the fact
that the spectra of all samples were obtained under the same ESI-MS
condition, the peak intensities of the same ions could be compared among
different samples by assuming that matrix effects were relatively constant
(Kourtchev et al., 2016; Lin et al., 2012a). To make a comparison among
different spectra, the most arbitrary abundant
C18H29O3S1- (m/z 325.18429) ion in the NYE N sample
(2.2×108 arbitrary units) was defined as 100 % (1 unit in
reconstructed mass spectra in Fig. 1); all peak intensities in the
measured samples were normalized to it.
Distribution of relative intensity, number, and intensity
fractions of CHO, CHNO, CHOS, and CHNOS compound in WSOC isolated from
aerosol samples detected in FT-ICRMS. The detailed molecular characteristics
of CHNOS are discussed in another study.
Thousands of formulae (∼6000–9500) were obtained in each
spectrum with the majority ranging from 150 to 700 Da. The molecular weights
of formulae with high intensity were primarily distributed between 300 and 400 Da, which was higher than previous studies with 200–300 Da (Jiang et al., 2016; Tao et al., 2014). On the one
hand, the compounds being explored in the present study have a larger mass
range; on the other hand, it was worth noting that some fractions of
compounds might be lost during our sampling preparation, particularly for
the low-molecular-weight ones. The formulae of different molecules are
classified into CHO, CHNO, CHNOS, and CHOS compounds. For example, CHNOS
compounds refer to formulae that contain carbon, hydrogen, oxygen, nitrogen,
and sulfur elements.
The relative number abundances of compounds in four classes are shown in
Fig. 1. The CHO and CHNO compounds accounted for 50 %–71 % in all four
categories, while sulfur-containing compounds account for less. The total
intensities of CHO and CHNO compounds were also dominant, accounting for
43 %–72 %, which demonstrate that these categories of compounds are
abundant in both the number and mass concentrations in the urban aerosols.
Nonetheless, the average number and intensity contributions of
sulfur-containing compounds were 32 % and 33 % during the non-FW
periods; they increased to 51 % and 57 % in the NYE nighttime,
respectively, suggesting that FW emissions contribute significantly to
sulfur-containing compounds.
The number of compounds in each subgroup and arithmetic
and weighted mean elemental ratio for each subgroup in NYE D and NYE N
samples.
ParametersNYE D NYE N All compoundsCHOCHONCHOSAll compoundsCHOCHONCHOSNumber58542045262311466836226025971979frequencyMolecular405±89407±100415±93385±76439±99424±107445±100433±97weight (Da)O/C0.38±0.140.33±0.110.36±0.120.44±0.150.37±0.130.30±0.120.33±0.120.37±0.14O/Cw0.380.330.360.410.370.300.330.37H/C1.23±0.361.14±0.371.13±0.321.49±0.421.23±0.371.18±0.371.11±0.281.35±0.42H/Cw1.251.101.111.631.241.161.081.40OM/OC1.70±0.221.53±0.151.65±0.161.87±0.231.70±0.231.50±0.161.61±0.171.75±0.22OM/OCw1.701.531.641.841.711.501.601.76DBE9.35±4.0110.7±5.0011.0±4.365.53±3.8510.1±4.8210.7±5.2912.1±4.438.03±5.19DBEw8.9410.511.04.219.5210.512.16.97DBE/C0.45±0.180.48±0.180.52±0.160.32±0.180.45±0.170.45±0.180.52±0.140.37±0.21DBE/Cw0.450.500.530.250.450.460.530.36
Tables 2, S1, and S2 show the number of compounds in each class and
the arithmetic and weighted mean elemental ratio for them in each sample. When
affected by the FW emissions, the number of compounds increased to 6836 in
the NYE nighttime and 9511 in the LNY daytime in comparison with 5854 in the
NYE daytime. Moreover, their average molecular weight increased from 405±89 Da in the NYE daytime to 439±99 Da in the NYE
nighttime and 448±97 Da in the LNY daytime. These results suggested
that FW emission contributes to the formation of relatively-high-molecular-weight compounds in urban aerosols. In addition, the average DBE values, an
indicative of degree of unsaturation, increased from 9.35±4.01 in
the NYE daytime to 10.1±4.82 in the NYE nighttime and 11.2±4.98 in the LNY daytime. Compounds with low O/C and H/C ratios and high DBE
values are likely to be aromatic-like species (Tong et al.,
2016; Kourtchev et al., 2016), indicating that FW burning plays a significant
role in the formation of these compounds. There was a similar trend for the
intensity-weighted mean elemental ratios of compounds with lower O/Cw
and H/Cw and higher DBEw. It is worth noting that the detected
compounds showed lower O/C and H/C ratios and higher DBE values than those
in previous studies (Table S3), suggesting more aromatic compounds in the FW-influenced aerosols in urban Beijing. More importantly, FW emission
dramatically increased the amounts of HMW (> 400 Da) organic
compounds from 3022 compounds in the NYE daytime to 4264 compounds in the
NYE nighttime and 5206 compounds in the LNY daytime, while the relative
abundance of three categories of compounds was different.
The number (a, b) and intensity (c, d)
of molecular formulae associated with three categories of compounds in NYE D
(before the FW event) and NYE N (during the FW event) samples, as well as common formulae present at both samples.
CHO compounds
CHO compounds detected in the ESI negative mode potentially include the carboxyl and/or hydroxyl functional group deprotonation effect (Cech and Enke,
2001). As shown in Table 2, there was no significant change for the number
of CHO species between NYE N (2260 compounds) and NYE D (2045 compounds).
Except for common compounds in two samples, the number and total intensities
of the unique compounds in the NYE N sample (591 compounds) were slightly
increased compared with those only in NYE D sample (376 compounds) (Fig. 2). However, the unique compounds increased considerably after the FW event,
that is, in the LNY daytime (Table S1 in the Supplement), with their number being up to 3120.
On the one hand, the precursors emitted by FW burning at the NYE night possibly
produced a large number of CHO compounds in the LNY daytime under the
photochemical reaction; on the other hand, they were affected by the spread
of the regional FW emission of pollutants in the surrounding regions of
Beijing during the Chinese New Year's Eve. In addition, the production
efficiency of oxygen-containing compounds during the day through
photochemistry should be more significant than that at night.
Classification of CHO species into subgroups according to
the number of O atoms in their molecules.
As shown in Figs. 3 and S2, CHO compounds had O1 to O15
subgroups, which were classified by the number of O atoms in their
molecules. As for O1–O8 subgroups, both their number and their intensity increased as the oxygen content increased, while they
decreased from O9 to O15 subgroups. Among them, O4–O10 subgroups dominated the total CHO compounds, and their number and
intensity accounted for 65 %–79 % and 64 %–85 % of the total
compounds, respectively. After the FW event, the abundance of each On
subgroup considerably increased in the LNY daytime, particularly for the
O>7 subgroups, highlighting the importance of photochemical formation.
Double-bond equivalent (DBE) vs. number of C atoms for
CHO species. The color bar denotes the number of O atoms. The size of the
symbols reflects the relative peak intensities of molecular formulae on a
logarithmic scale.
As shown in Fig. 4, the high-intensity CHO compounds in the fraction of
water-soluble organic matter in urban aerosols are primarily with C numbers
of 15–27 and DBE values of 6–15, indicating that they potentially have one
or more benzene rings in their molecules. The DBE values and C numbers of
CHO compounds in NYE N and LNY D samples vary in the ranges of 0–29 and
6–40, respectively, higher than those of 0–22 (DBE) and 7–35 (C number)
for other samples. There were many HMW CHO compounds with a high degree of
unsaturation in the FW-related aerosols. Moreover, they are high-oxygen-containing compounds with more than eight O atoms, which are
potentially the highly oxidized and condensed aromatic compounds. In
addition, a high intensity of compounds with low DBE values and O atoms
was present in the FW-related aerosols, such as C16H32O2,
C18H36O2,C20H40O2,C22H44O2,
and C24H48O2. They have an even carbon advantage, which could
be fatty acids (Li et al., 2018; Kang et al., 2017; Fan et al., 2020).
Van Krevelen diagrams (the H/C via O/C ratios) for the
CHO compounds with various aromatic index (AI) value ranges. The dashed
lines separate the different AI regions. The size of the symbols reflects
the relative peak intensities of compounds on a logarithmic scale.
CHO compounds with aromatic index (AI) > 0.5, a characteristic of
condensed aromatic ring structures, were also more abundant in the LNY
daytime (777 compounds) than in the NYE nighttime (484 compounds) (Table 3).
The H/C and O/C ratios of CHO compounds in different samples with various AI
regions were shown in Fig. 5. Obviously, compounds with AI > 0.5 had low H/C ratios (< 1). The majority of them have DBE values
above 7, indicating that they correspond to oxidized aromatic compounds,
which are primarily of anthropogenic origin (Tong et al., 2016).
Moreover, more of their species fall into the area of AI > 0.5 in
the LNY daytime. This suggests that the pollutants emitted by FW burning may
be oxidized into aromatic CHO compounds under the oxidation by nighttime
chemistry, while the photochemical reaction during the day is more
efficient.
The number of compounds with various AI values.
Sample IDParameterCHOCHONCHOSNYE DAI =0861755060 < AI < 0.5146020846080.5≤ AI < 0.67457361320.67≤ AI4230NYE NAI =088785830 < AI < 0.51686217512710.5≤ AI < 0.674263401200.67≤ AI5815LNY DAI =0124914010 < AI < 0.5221929547720.5≤ AI< 0.67692545750.67≤ AI85140LNY NAI =01271604890 < AI < 0.51926193110500.5≤ AI < 0.67512418850.67≤ AI5362Normal DAI =0541055580 < AI < 0.5162919707600.5≤ AI < 0.67440296800.67≤ AI4571Normal NAI =079964420 < AI < 0.5154516427330.5≤ AI < 0.67413402680.67≤ AI3400
Different families of compounds with heteroatoms (e.g., O, N, S) overlap in
terms of DBE, which may be inadequate to explain the level of unsaturation
of organic compounds and to identify whether a molecular formula potentially
has a (poly-)aromatic structure or not (Kourtchev et al., 2016; Yassine
et al., 2014; Tong et al., 2016; Reemtsma, 2009). For instance, divalent atoms
such as oxygen and sulfur do not influence the value of DBE, but they may
contribute to the potential double bonds of that molecule. Unlike the
parameter AI, the use of parameter Xc can avoid this problem and help
to more precisely identify and characterize aromatic and condensed aromatic
compounds in highly complex WSOC mixtures (Yassine et al., 2014). The
H/C and O/C ratios versus the MW and Xc under different samples are
shown in Fig. S3. There are much more formulae in samples with an Xc > 2.5 (indicative of aromatic compounds) when using the Xc
classification than AI due to a large fraction of alkylated aromatics in the
present study, which would be wrongly assigned as nonaromatics by AI
(Kourtchev et al., 2016). The highest number of the aromatic compounds in
the samples was observed for formulae with a pyrene core structure (Xc=2.83). The number of compounds with an ovalene core structure (Xc=2.92) and highly condensed aromatic structures or highly unsaturated structures (Xc > 2.93) significantly increased by the FW burning event
until the LNY daytime, suggesting the importance of photochemical oxidation
(Fig. S3).
OSC is an ideal parameter to describe the oxidation processes of a
complex mixture of organics. Figure 6 shows overlaid OSC symbols for CHO
compounds in NYE D, NYE N, and LNY D samples. Because of the direct and
indirect influence by FW emissions, OSC shifted towards a less oxidized state with more than 15 carbon atoms in NYE N and LNY D
samples. The difference in OSC becomes even more significant with the increased number of C in the detected CHO compounds. As shown in Fig. 6,
a different OSC value and C number indicate different types of compounds
as previously characterized by Kroll et al. (2011). The
semivolatile and low-volatility oxidized organic aerosol (SV-OOA and
LV-OOA) have the values of OSC between -1 and +1 and less than 13 carbon atoms, which are associated with those that are produced by multistep oxidation reactions. The biomass burning organic aerosol (BBOA) has lower
OSC, with OSC between -0.5 and -1.5 and carbon atoms more than
7. The molecules with OSC less than -1 and carbon atoms more than 20
might be associated with hydrocarbon-like organic aerosol (HOA).
Overlaid carbon oxidation state (OSC) symbols for
CHO compounds in NYE D (a), NYE N (b), and LNY D (c) samples. The size and
color bar of the markers reflect the relative peak intensities of compounds
on a logarithmic scale. The gray areas were marked as SV-OOA (semivolatile
oxidized organic aerosol), LV-OOA (low-volatility oxidized organic aerosol),
BBOA (biomass burning organic aerosol), and HOA (hydrocarbon-like organic
aerosol) (Kourtchev et al., 2016; Kroll et al., 2011).
More compounds with long carbon chains are found in aerosols affected by the
FW emission. A large number of compounds with high peak intensities have
similar OSC to the SV-OOA, while they have longer carbon chains, from
C15 to C30 in the NYE N sample. Another important part of the
FW-affected ions in NYE N and LNY D samples falls into the category of the
BBOA, which is associated with primary particulate matter directly emitted
into the atmosphere. Moreover, unlike compounds before the FW event, there
were plenty of molecular formulae with low OSC in the area of HOA in
both NYE N and the LNY D samples, which were possibly aromatic-like
compounds.
CHON compounds
The trend of CHON species was similar to that of CHO (Fig. 2). Although
massive FW emissions occurred in the NYE nighttime, the number and total
intensities of CHON and CHO showed minor changes between daytime and
nighttime, while both of them clearly increased in the following LNY
daytime. Their average molecular weights were 445±100 Da in the NYE
nighttime and 472±112 Da in the LNY daytime, respectively, compared
to 415±93 Da in the NYE daytime. It indicates that FW emissions
contribute to the formation of HMW CHON compounds, particularly under
daytime photooxidation. These newly formed compounds with low H/C and O/C
ratios and high DBE values are likely oxidized aromatic compounds. As shown
in Fig. S4 and Table 3, nitrogen-containing compounds with AI > 0.5 were also more abundant in the LNY daytime (559 compounds) than the
previous NYE nighttime (341 compounds).
Classification of CHON species into subgroups according
to the number of N and O atoms in their molecules.
CHON compounds were classified into N1O3–N1O14 and
N2O3–N2O13 subgroups in all samples by the number of N
and O atoms in their molecules (Figs. 7 and S5). The total abundance of
N1On subgroups was twice as much as that of N2On
subgroups in each sample. After the FW period, with the reaction of
photooxidation, each subgroup considerably increased in the LNY daytime,
particularly for the N1,2O>7 subgroups, highlighting the
importance of photooxidation to the formation of CHON compounds. As shown in
Fig. 8, the high-intensity CHON compounds in samples were primarily with C
numbers of 15–25, O numbers of 2–8, and DBE values of 5–15, indicating
that they potentially had one or more benzene rings in their molecules.
However, there were numbers of CHON compounds with high carbon and oxygen
content and high unsaturation in LNY D sample. These unique compounds were
likely HMW nitro-aromatic compounds.
Double-bond equivalent (DBE) vs. number of C atoms for
CHON species. The color bar denotes the number of O atoms. The size of the
symbols reflects the relative peak intensities of molecular formulae on a
logarithmic scale.
Ion intensity distributions of selected tentatively
identified compounds in individual samples. They may be nitro-aromatics and
their potential structures have been reported by Lin et al. (2015).
Nitro-aromatic compounds are often observed in biomass burning aerosols
(Iinuma et al., 2010; Kitanovski et al., 2012) and are potential
contributors to light absorption as a component of brown carbon (Laskin
et al., 2015; Lin et al., 2015; Desyaterik et al., 2013). Although
nitrogen-containing compounds did not increase significantly at NYE night,
some biomass burning compounds might. Figure 9 displays the ion intensity
distributions of four nitro-aromatic compounds (i.e.,
C10H7O3N, C11H9O3N, C12H11O3N,
and C16H79O3N) detected in biomass burning aerosols by Lin et
al. (2015), which were just assigned by their molecular
composition but not the chemical structure. Their intensities increased in
the NYE N sample, particularly for C11H9O3N with its
intensity being doubled. Nitro-aromatic compounds are produced in the
atmosphere via the oxidation of aromatic precursors in the presence of
NO2 (Laskin et al., 2015), and their relative yields increase
with NO2/NO3 concentrations (Sato et al., 2007; Jang and Kamens,
2001), which can be released in large quantities during FW combustion
processes. Moreover, highly abundant PAHs from FW emission in the NYE can
react more efficiently with NO2 than their single-ring aromatic
counterparts (Nishino et al., 2009).
CHOS compounds
More than 1000 CHOS compounds were assigned in the samples,
accounting for 13 %–21 % of all identified formulae. As shown in Fig. 1,
the relative intensities of CHOS compounds were the highest among four
elemental compositional categories. FW emissions do influence the number and
intensity of the CHO and CHON. However, CHOS compounds increased dramatically at NYE
night, while they did not increase too much under sunlight in the following
LNY daytime. As shown in Table 2, the number of CHOS compounds was 1146 in
the NYE D sample, while it increased to 1979 during the FW event (NYE N).
Moreover, during the FW event, not only the number concentrations but also
their intensities sharply increased to approximately twice as much as those
before the FW event. Previous studies reported that a great deal of air
pollutants released via FW burning in the NYE nighttime lead to a short-term
pollution (Jiang et al., 2015; Tian et al., 2014). For example, higher
concentrations of sulfate, n-alkanes (C16–36), PAHs, and n-fatty acids
(C8–32) were observed in the FW burning night than the normal nights
(Kong et al., 2015). These compounds might be the precursors of CHOS
species (Riva et al., 2015, 2016; Passananti et al., 2016; Tao
et al., 2014; Shang et al., 2016). The amount of sulfur in the firework was
released into the air with the form of sulfur oxides during the combustion
process and further produced acidified sulfate seed aerosol, which
considerably contributed to the formation of a large number of CHOS
compounds via acid-catalyzed reaction with biogenic and anthropogenic
volatile organic compounds (VOCs) (Surratt et al., 2008; Riva et al.,
2015). For instance, the CHOS compounds derived from monoterpenes and
sesquiterpenes, such as limonene, α/γ-terpinene and β-caryophyllene, were detected only under acidic or strongly acidic sulfate
seed aerosol conditions (Surratt et al., 2008; Iinuma et al., 2007a, b; Chan et al., 2011). Meanwhile, the high levels of nitrogen
oxides emitted by FW burning can promote the formation of some CHOS
compounds (Surratt et al., 2008).
Unlike the normal daytime and nighttime, there was a noticeable change in
CHOS species affected by the FW event in the NYE nighttime (Fig. 1).
Figure 2 shows that CHOS species doubled in the NYE nighttime relative to
the NYE daytime. On the contrary, both the number concentration and
intensity were lower at night than in the daytime in the normal day, which
implies that FW plays an important role in the formation of CHOS compounds
at night.
Classification of CHOS species into subgroups according
to the number of O and S atoms in their molecules.
CHOS compounds were classified into O4S1–O13S1 subgroups
by the number of O and S atoms in their molecules (Figs. 10 and S6). Most
of them had more than or equal to four O atoms of each S atom in their
molecules, which supports the assignment of a sulfate group in the
molecules. They are likely organosulfates (OSs), which is an important
component of SOA formed by both daytime photooxidation and nighttime
chemistry such as NO3 oxidation. Unlike the CHO and CHON compounds,
during the FW event, both the number and the intensity of each subgroup
considerably increased in the NYE nighttime, not the LNY daytime,
highlighting the importance of nighttime chemical oxidation to form CHOS
compounds.
Table 2 demonstrates the arithmetic and weighted mean elemental ratios for
each species of samples. The average molecular weights of CHOS compounds
increased from 385±76 to 433±97 Da. Under the influence of
firework emission, both the O/C and H/C ratios decreased in the NYE N
aerosol, while the DBE and the DBE/C ratio increased. Formulae with 0 < H/C≤1.0 and O/C≤0.5 dominantly have high DBE values
(≥7.0), which is consistent with oxidized PAHs; e.g., the smallest PAH,
naphthalene (C10H8), has an H/C of 0.8 and a DBE of 7
(Tong et al., 2016; Feng et al., 2012). Moreover, as shown in Figs. 11 and S7, in contrast to CHO and CHON species, numbers of CHOS
compounds with high DBE (≥7.0) were only detected in the NYE N aerosol;
most of them fall into the area of AI > 0.5. The highest number
of these PAH-like CHOS compounds was found in the NYE N sample
with 125 ions, compared to only 68 ions in the normal N sample (Table 3). These
reflect that the FW emissions have an important influence on particle
composition, especially for the aromatic-like compounds.
The carbon chain length (a, e), H/C(b, f), and O/C(c, g) ratios; different group (d, h)
distributions via molecular weights of OSs in NYE D (before the FW event)
and NYE N (during the FW event) aerosols. Group A includes the aliphatic OSs
with DBE ≤2; group B includes the aromatic-like OSs with
Xc > 2.5; group C includes the biogenic OSs. The color bars on the right side of the figure represent DBE for panels (a) and (e) as well as Xc for panels (b), (c), (f), and (g).
To further evaluate the characteristics of CHOS species produced during the
FW burning, more than 92 % of them were found to contain only one sulfur
atom in each sample. Compounds that present a number of oxygen atoms greater
than or equal to 4S (O ≥ 4S), potentially with a –OSO3H group,
were tentatively regarded as OSs (Wang et al., 2016; Lin et al., 2012b).
They considerably contribute to the yield of SOA (Tolocka and Turpin,
2012). However, tandem MS experiments were not conducted on the ions
detected in samples. Hence, other sulfur-containing compounds, such as
sulfonates, may also be involved due to the lack of using tandem MS
experiments to provide insights into the exact structures (Riva et al.,
2015; El Haddad et al., 2013).
The detailed composition coupled with molecular weights of OSs in the NYE D
(1125 OSs) and NYE N (1945 OSs) samples were displayed in Fig. 11. Compared
to the OSs in the NYE D sample, a dense distribution of 1000
compounds with high DBE (> 7) was found during the FW event,
particularly for those within the high-molecular-weight (HMW; > 400 Da) region. Obviously, most of them had high Xc (> 2.5,
indicative of aromatics) and relatively low H/C (< 1.5) and O/C
(< 0.5) ratios. Moreover, these highly unsaturated compounds had
higher intensity, which indicated a larger abundance of them in urban
aerosols. These OSs with distinctive characteristics of high unsaturation
were aromatic OSs, which were probably derived from aromatic VOCs or PAHs.
Furthermore, to illustrate the differences among the measured OSs, they are
divided into three main classes. Group A includes aliphatic OSs with DBE ≤2, characterized by long alkyl carbon chains, which are highly
saturated. Group B includes aromatic-like OSs, detected by the Xc with
the value of Xc > 2.5, which has a high degree of
unsaturation. Group C includes the rest fraction except for groups A and B,
which has a moderate degree of saturation and is similar to biogenic OSs. As
shown in Fig. 11, there were 322 aliphatic OSs and 125 biogenic OSs in the NYE N sample, and there were 292 aliphatic OSs and 103 biogenic OSs in NYE D sample. Moreover, the aliphatic OSs of C12H24O5S, C18H36O6S, and
C10H16O9S, and the biogenic OSs of C10H18O5S
and C10H16O7S, which were separately derived from alkanes and
fatty acids (Riva et al., 2016; Passananti et al., 2016; Shang et al.,
2016) and α/β-pinene (Surratt et al., 2008) as well as their
corresponding family series (CnH2nO5,
CnH2nO6S, CnH2n-4O9S,
C10H2n-2O5S, and CnH2n-4O7S) were all detected
in the aerosols. Nonetheless, the number of aromatic-like compounds (1498
OSs) increased dramatically in the NYE nighttime, particularly for the HMW
compounds, compared to those (730 OSs) in the daytime. In addition,
aromatic-like OSs form not only in the daytime with photochemical reaction,
but also in the nighttime via unknown formation pathways such as
N2O5 oxidation. Plenty of aromatic-like ones rapidly formed in the
NYE nighttime due to FW emissions, releasing plentiful aromatic VOCs as
precursors of OSs. Riva et al. (2015) demonstrated the enhanced
formation of OSs and sulfonates from both naphthalene and 2-methylnaphthalene in the presence of
acidified sulfate seed aerosol via comparison of side-by-side experiments.
For instance, C9H10O5S, C10H10O6S, and
C10H10O7S, derived from 2-methylnaphthalene (Riva et al., 2015),
and their corresponding family series (CnH2n-8O5S,
CnH2n-10O6S, and CnH2n-10O7S) were detected in
FW-related aerosols. These potentially explain that high abundance of
aromatic-like OSs in aerosols could be formed through the sulfate ion and
PAHs emitted from FW burning at night without the presence of photoreaction.
Conclusions
We investigated HMW organic compounds in urban aerosols collected during the
Chinese New Year in Beijing, including the periods of before FW (in the NYE
daytime), during FW (in the NYE nighttime), and after FW (in the LNY
daytime), by the usage of ESI FT-ICRMS. Three dominant categories of organic
compounds, including CHO, CHON, and CHOS species, were measured and
discussed. About 6000 organic compounds were detected in the NYE daytime,
while up to almost 7000 were detected in the NYE nighttime and 9500 were detected in the LNY daytime.
Moreover, their DBE values, the indicator of unsaturation, also
clearly increased. Although they were increased by the effects of FW
emissions, the three species of compounds showed different behaviors. For
the CHO species, there was no significant change for both their number and
their total intensities detected during the FW event, while they doubled
with photooxidation in the LNY daytime, compared to the compounds before the
FW event. Similarly, there was a similar trend for the CHON species as well
as CHO groups. These phenomena indicated that photochemical reactions have a
great influence on the formation of CHO and CHON compounds.
Sulfur-containing compounds increased dramatically at the NYE night. The number of CHOS species was nearly twice as high in the NYE nighttime than that in the NYE daytime. About 92 % of them were OSs, which rapidly increased in
the NYE nighttime when large amount of pollutants were emitted. High abundances
of CHOS species with low H/C and O/C ratios and high DBE affected by FW
emission are dominated by aromatic-like compounds such as aromatic
carboxylic acids, nitro-aromatics, and poly-aromatic compounds; they are
potentially contributors to atmospheric brown carbon that affects the
physicochemical properties (e.g., light absorption and volatility) of
atmospheric aerosols. Our results highlight that FW emission is a
significant contributor to HMW organic aerosols, which needs to be
considered in atmospheric chemical models for regional air quality.
Data availability
The dataset for this paper is available upon request from the
corresponding author (fupingqing@tju.edu.cn).
The supplement related to this article is available online at: https://doi.org/10.5194/acp-20-6803-2020-supplement.
Author contributions
QX participated in the investigation, methodology, software development, formal analysis, and writing of the original draft. SS, SC, YX, and DC participated in the methodology and formal analysis. JC and PF collected the samples. LR, SY, WZ, YS, HT, YC, HS, ZW, KK, GJ, CL, and PF participated in validation as well as in reviewing and editing of the manuscript. GJ and DC provided the analytical resources. PF participated in the conceptualization, project administration, and funding acquisition.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
The authors thank Lianfang Wei and Linjie Li (Institute of Atmospheric Physics, Chinese Academy of Sciences, China) and Hong Ren (Tianjin University, China) for their helpful discussions. The authors thank the editor and three anonymous referees for their comments and suggestions to improve the quality of this paper.
Financial support
This research has been supported by the National Natural Science Foundation of China (grant nos. 41625014 and 41571130024).
Review statement
This paper was edited by Jason Surratt and reviewed by three anonymous referees.
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