Introduction
Atmospheric aerosol particles represent a highly complex blend of inorganic
and organic matter originating from a wide variety of both natural and
anthropogenic sources. The organic fraction typically constitutes 20–90 %
of the total submicron aerosol mass and is much less constrained in terms of
chemical composition than the inorganic fraction (Jimenez et al., 2009;
Hallquist et al., 2009). Only ∼ 10–30 % of the particulate
organic matter has been identified as specific compounds despite years of
effort and the use of the most sophisticated techniques available (Hoffmann
et al., 2011). The insufficient knowledge of the composition of organic
aerosol particles at the molecular level hinders a better understanding of
the sources, formation and atmospheric processes of organic aerosol as well
as their physicochemical properties and effects on climate and human health
(Noziere et al., 2015).
Organosulfates are ubiquitous in atmospheric aerosol and have been detected
in ambient aerosol particles from America, Europe, Asia and the Arctic
during the last decade (e.g., Surratt et al., 2008; Iinuma et al., 2007;
Stone et al., 2012; Hansen et al., 2014; Kourtchev et al., 2016; Surratt et
al., 2007). Due to the presence of the deprotonated functional group
R–O–SO3-, organosulfates are acidic and highly water soluble and
thus can enhance the aerosol hygroscopicity. These characteristics,
together with the light-absorbing property of organosulfates, lead to
potential impacts on climate (Lin et al., 2014).
Organosulfates are tracers of secondary organic aerosol (SOA) formation and
have been demonstrated to be produced from heterogeneous and multiphase
reactions (e.g., Surratt et al., 2008; Iinuma et al., 2007; Chan et al.,
2011; Zhang et al., 2012). Chamber studies have found that the oxidation of
biogenic volatile organic compounds (BVOCs) including isoprene,
monoterpenes and sesquiterpenes can form organosulfates on acidified
sulfate particles (e.g., Surratt et al., 2008; Iinuma et al., 2007; Chan et
al., 2011; Zhang et al., 2012). A very recent study revealed a previously
unrecognized pathway for organosulfate formation through the heterogeneous
reaction of SO2 with the unsaturated bond in oleic acid (Shang et al.,
2016). A number of biogenic organosulfates have been observed in ambient
aerosol, in particular, isoprene-derived organosulfates (e.g., Kristensen et
al., 2011; He et al., 2014; Liao et al., 2015; Budisulistiorini et al.,
2015). A recent study reported the formation of aromatic organosulfates by
photochemical oxidation of polycyclic aromatic hydrocarbons (PAHs) in the
presence of sulfate seed particles (Riva et al., 2016). Aromatic
organosulfates have also recently been observed in urban aerosol from
different locations in Asia. The presence of aromatic organosulfates was
first suggested by Stone et al. (2012) based on analysis of
aerosol samples collected at four sites in Asia. Kundu et al. (2013) quantified benzyl sulfate (C7H7SO4-) and
identified its homologous series with increasing number of methylene groups
(C8H9SO4- and C9H11SO4-) in Lahore,
Pakistan. Furthermore, Staudt et al. (2014) synthesized
phenyl sulfate, benzyl sulfate, 3- and 4-methylphenyl sulfate and 2-, 3-,
and 4-methylbenzyl sulfate and quantified them in aerosols collected in
urban samples from Lahore and Pasadena, USA as well as Nepal. Ma et al. (2014) reported the contribution up to 64 % from aromatic
organosulfates to the sum of identified organosulfates during winter in Shanghai,
while Wang et al. (2016) found aromatic organosulfates to
constitute less than 22 % of the detected number of organosulfates in
Shanghai, Nanjing and Wuhan.
Organosulfates have been estimated to contribute 5–10 % of the organic
mass in fine particles in the USA (Tolocka and Turpin, 2012). However,
quantification of organosulfates is a challenging task due to the lack of
authentic standards and incomplete understanding of the sources, precursors
and formation processes of organosulfates. To date, many studies of
organosulfates have remained at the qualitative level, although a limited
number of studies have provided quantitative or semi-quantitative analysis
of certain organosulfates (e.g., Kundu et al., 2013; Staudt et al., 2014; Ma
et al., 2014; Olson et al., 2011; Hettiyadura et al., 2017). Moreover,
several studies show that organosulfates are present as a wide range of
species with individual species such as the organosulfate derived from
isoprene epoxydiols (IEPOXs) contributing 0.2–1.4 % of the total organic
aerosol mass (Liao et al., 2015). This further complicates the
quantification of organosulfates. A few organosulfate standards have been
synthesized for quantification purposes. For example, Olsen et al. (2011) measured 0.4–3.8 ng m-3 lactic acid sulfate and
1.9–11.3 ng m-3 glycolic acid sulfate in samples of PM2.5 (particulate matter
with an aerodynamic diameter < 2.5 µm) from the US, Mexico
City, and Pakistan. Kundu et al. (2013) measured
monthly average concentrations of benzyl sulfate ranging from 0.05 to 0.5 ng m-3 in PM2.5 samples from Lahore, Pakistan. Staudt et al. (2014)
quantified benzyl sulfate ranging from 4 to 90 pg m-3 in
PM2.5 samples from Lahore (Pakistan), Godavari (Nepal), and Pasadena
(California), while methylbenzyl sulfates, phenyl sulfate, and methylphenyl
sulfates were observed intermittently in these three locations. Furthermore,
Hettiyadura et al. (2015) developed a hydrophilic
interaction liquid chromatography method using an amide stationary phase
providing excellent retention of carboxy-organosulfates and isoprene-derived
organosulfates, which was validated using six model organosulfates including
aliphatic and aromatic organosulfates.
Previous field studies focusing on organosulfates were conducted mainly in
Europe (e.g., Iinuma et al., 2007; Kristensen et al., 2011;
Gómez-González et al., 2008, 2012;
Nguyen et al., 2014; Martinsson et al., 2017), North America (e.g.,
Surratt et al., 2007; Nguyen et al., 2012; Worton et al., 2011) and a
few in China (He et al., 2014; Ma et al., 2014). The particulate air
pollution has been a serious environmental problem during recent winters in
China, characterized by high secondary aerosol concentrations including
sulfate and SOA (e.g., Huang et al., 2014; Elser et al., 2016; Wang et al.,
2017). As organosulfates are tracers for SOA, more studies on organosulfates
will help to better understand and constrain the SOA formation mechanisms in
highly polluted regions (e.g., China) and to reconcile the underestimation
of particle-phase organic carbon in atmospheric models.
In this study, nine organosulfate standards (phenyl sulfate, 3-methylphenyl
sulfate, benzyl sulfate, 2-methyl benzyl sulfate, 3-methyl benzyl sulfate,
2, 4-dimethyl benzyl sulfate, 3, 5-dimethyl benzyl sulfate, hydroxyacetone
sulfate and glycolic acid sulfate) were synthesized using an approach
modified from Staudt et al. (2014) and Hettiyadura et al. (2015). These authentic standards were used to optimize
an ultra performance liquid chromatography electrospray ionization-tandem
mass spectrometric method (UPLC–ESI–MS/MS) for the quantification of
organosulfates. The presence and concentration of four of these
organosulfates, namely, benzyl sulfate, phenyl sulfate, glycolic acid
sulfate and hydroxyacetone sulfate, were determined in ambient PM2,5
collected in urban air in Xi'an, China. The rest five organosulfates were
not quantified in ambient PM2.5 because the standards were synthesized
at a later stage of the study.
Material and methods
Chemicals and synthesis
The chemicals used for the synthesis of organosulfates included
hydroxyacetone (99 %, Sigma Aldrich), glycolic acid (99 %, Sigma
Aldrich), phenol (99.5 %, Tic), benzyl alcohol (99.8 %, Aladdin,
Shanghai, China), m-cresol (99 %, Sigma Aldrich), sulfur trioxide pyridine
complex (98 %, Sigma Aldrich), pyridine (99.9 %, Sigma Aldrich) and
Dowex® 50WX8 (hydrogen form, 100–200 mesh, Sigma Aldrich).
MilliQ water (18.2 MΩ) was used, and all other reagents were
analytical grade and used without further purification.
The organosulfate standards were synthesized using a general approach
modified from Staudt et al. (2014) and Hettiyadura et al. (2015). Figure 1 shows the reaction scheme. In general,
alcohol (7.0 mmol) and sulfur trioxide pyridine complex (1.2 equiv.) was
dissolved in dry pyridine (10 mL) in an oven-dried, three-necked flask
provided with magnetic stirring under nitrogen. The reaction mixture was
stirred at 30 ∘C for 24 h, and then the solvent was removed via
distillation under vacuum at 50 ∘C. The residue was redissolved
in distilled water (10 mL) and titrated with 0.9 M KOH until pH was above
12. Neat ethanol (40 mL, 65 ∘C) was added to the aqueous
solution. The resulting solution was heated to reflux followed by a quick
vacuum filtration to remove the stark white precipitate. The mother liquor
was then placed in a freezer (-25 ∘C) overnight. The potassium
salts of organosulfate formed in the mother liquor were collected by vacuum
filtration, rinsed with cold ethanol three times and dried to obtain the
target product. The synthesized organosulfate standards were stored in
refrigerator (∼ 4 ∘C) and no decomposition was observed
after 2 years as confirmed by nuclear magnetic resonance (NMR) analysis.
The optimized ESI-MS/MS parameters and UPLC retention time of
measured organosulfates.
Organosulfate
Deprotonate molecule
Product ion
Cone voltage
Collision energy
Retention time
(m/z)
(m/z)
(V)
(eV)
(min)
Phenyl sulfate
C6H5SO4-
SO3- (80)
41
20
0.86 ± 0.02
(173)
C6H5O- (93)
21
Benzyl sulfate
C7H7SO4- (187)
HSO3- (81)
42
19
0.96 ± 0.02
(187)
SO4- (96)
22
Hydroxyacetone
C3H5SO5-
SO3- (80)
32
18
1.10 ± 0.02
sulfate
(153)
HSO4- (97)
20
Glycolic acid
C2H3SO6-
C2H3O3- (75)
26
18
5.78 ± 0.03
sulfate
(155)
HSO4- (97)
14
The fragmentation (left) and mass spectra (right) of benzyl
sulfate (a), phenyl sulfate (b), hydroxyacetone sulfate (c) and glycolic
acid sulfate (d).
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Characterization
The synthesized products were characterized with NMR and ESI–MS. 1H
NMR and 13C NMR spectra were recorded on a Bruker Advance-III 40 MHz
spectrometer at 400 and 100 MHz, respectively using trimethylsilane (TMS) as
an internal standard. Chemical shifts are reported in ppm downfield from the
internal reference. The NMR spectra are shown in Supplement.
The following abbreviations are used for the multiplicities: s = singlet,
m = multiplet. The yield for phenyl sulfate was 45 %, 1H NMR (400 MHz, D2O): δ/ppm 7.29–7.43 (m, 5H), 13C NMR (100 MHz,
D2O): δ/ppm 121.6, 126.4, 129.8 and 151.2. The yield for benzyl
sulfate was 70 %, 1H NMR (400 MHz, DMSO-d6): δ/ppm 7.25–7.40
(m, 5 H), 4.76 (s, 2 H), 13C NMR (100 MHz, DMSO-d6): δ/ppm 67.9,
127.8, 128.0, 128.6 and 138.4. The yield for hydroxyacetone sulfate was 45 %,
1H NMR (400 MHz, DMSO-d6): δ/ppm 4.22 (s, 2 H), 2.11 (s, 3 H),
13C NMR (100 MHz, DMSO-d6): δ/ppm 26.9, 71.4 and 207.0. The yield
for glycolic acid sulfate was 35 %, 1H NMR (400 MHz, DMSO-d6): δ/ppm 4.07 (s, 2 H) and 13C NMR (100 MHz, DMSO-d6): δ/ppm 65.0,
173.1. The organosulfate standards were recrystallized in ethanol for
purification and purity of these synthesized standards is > 95 %, confirmed
by NMR analysis. Exact mass spectra were recorded on a high-resolution mass
spectrometer (HR–MS, Q Exactive Plus, Thermo Scientific, USA) equipped with
an ESI source in the negative ion mode (ESI-). The ESI conditions were as
follows: spray voltage -3.2 kV, collision energy (CE) 40 V for benzyl
sulfate and 45 V for hydroxyacetone sulfate, 3-methylphenyl sulfate,
glycolic acid sulfate and phenyl sulfate, capillary temperature 350 ∘C, aux gas heater temperature 320 ∘C, sheath gas flow
rate 35 and aux gas flow rate 10. The mass resolving power was 70 000. Data
acquisition was performed with m/z ranging from 50 to 200.
PM<inline-formula><mml:math id="M93" display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">2.5</mml:mn></mml:msub></mml:math></inline-formula> samples
The 24 h integrated PM2.5 samples were collected on pre-baked (780 ∘C, 3 h) quartz-fiber filters (8 × 10 inch, Whatman, QM-A, USA)
using a high-volume sampler (Tisch, Cleveland, OH, USA) at a flow rate of
1.05 m3 min-1 from 18 December 2013 to 17 February 2014. After
collection, the filter samples were immediately wrapped in pre-baked
aluminum foil and stored in a freezer (below -20 ∘C) until
analysis. The sampling site was located on the rooftop of the Institute of
Earth and Environment (∼ 10 m above the ground), Chinese
Academy of Sciences (IEECAS, 34.23∘ N, 108.88∘ E), which
is surrounded by residential, commercial and trafficked areas.
Sample analysis
A portion of the filter (6 × 0.526 cm2 punch) taken from each
sample was sonicated for 25 min in 9 mL of acetonitrile (ACN)/water mixture
(95:5, V/V). The extracts were filtered through a 0.22 µm polypropylene
membrane syringe filter to remove insoluble material. The eluate was
concentrated almost to dryness with a gentle stream of purified nitrogen
(99.999 %) at 45 ∘C using an evaporation system
(TurboVap® LV, biotage), then redissolved in 500 µL of
acetonitrile/water mixture (V/V, 95:5). The prepared samples were stored at
4 ∘C in the refrigerator and analyzed within 24 h. The separation
and quantification were realized using a ACQUITY UPLC system (equipped with
a quaternary pump, autosampler, and thermostated column compartment) coupled
to a tandem mass spectrometer (Xevo TQ MS, Waters, USA). The separation was
carried out using a BEH amide column (2.1 mm × 100 mm, 1.7 µm
particle size, Waters, USA) equipped with a pre-column. The column was
maintained at 35 ∘C and the flow rate of mobile phase was 0.25 mL min-1. A 5 µL injection volume was used for quantitative analysis
of samples and standards. The optimized mobile phase A (organic) consisted
of ammonium acetate buffer (5 mM, pH 8.5) in ACN and ultra-pure water (95:5,
V/V) and mobile phase B (aqueous) consisted of ammonium acetate buffer (5 mM, pH 9) in ultra-pure water. A mobile phase gradient was used: mobile
phase A was maintained at 98 % for 2 min, then decreased to 60 % from 2
to 5 min and then held there for 2 min; from 7 to 12 min mobile phase A was
returned to 98 %. Organosulfates were detected by a TQ MS equipped with an
ESI source in the negative ion mode. The mass spectrometer was operated in
multiple reaction monitoring (MRM) mode. Optimized MS conditions for the
four organosulfates chosen for the field studies (e.g., cone voltages and
collision energies) are listed in Table 1. The capillary voltage was 2.7 kV,
source temperature was 150 ∘C, desolvation temperature was 350 ∘C, desolvation gas (N2) flow rate at 800 L h-1, cone
gas (N2) flow rate was 150 L h-1, and collision gas (Ar) flow rate
was 0.16 mL min-1. All data were acquired and processed using MassLynx
software (version 4.1). All samples and standard spectra were background
subtracted.
Typical chromatograms of organosulfates from the mixture of
authentic standard solution and ambient PM2.5 samples, measured with
the UPLC–ESI–MS/MS method. Note, the intensity of benzyl sulfate and
phenyl sulfate refers to the left y axis, and the intensity of hydroxyacetone
sulfate and glycolic acid sulfate refers to right y axis.
![]()
Analytical performance of the UPLC-ESI-MS/MS method for
organosulfate analysis.
Organosulfate
Linear range
Linearity
Recovery
LOD (pg),
LOQ (pg),
LOD*
LOQ*
(ng mL-1)
(R2)
%
injection volume
injection volume
(pg m-3)
(pg m-3)
(5 µL)
(5 µL)
Phenyl sulfate
0.1–40
0.998
80.4
0.13
0.43
1.1
3.5
Benzyl sulfate
0.1–40
0.998
89.6
0.13
0.43
1.1
3.4
Hydroxyacetone sulfate
0.3–120
0.997
93.2
2.1
6.9
16.7
55.6
Glycolic acid sulfate
2–800
0.995
92.0
0.27
0.88
2.1
7.1
* For analyzing 6 × 0.526 cm punches of filters collected with
high-volume samplers (sampling at 1.13 m3 min-1 for 24 h on
8” × 10” filters).
The quantification of organosulfates at Xi'an and comparison with
data reported in the literature.
Organosulfate ng m-3
Location
Date
PM2.5
OC
benzyl
phenyl
hydroxyacetone
glycolic acid
Ref.
µg m-3
µg m-3
sulfate
sulfate
sulfate
sulfate
Riverside, CA
27/07/05
16.5
7.6
–
–
–
3.3
Olson et al. (2011)
Mexico City (T0)
26/03/06
40
8.5
–
–
–
4.1
Mexico City (T1)
26/03/06
33
5.2
–
–
–
7.0
Cleveland, OH
15/07/07
12.7
3.9
–
–
–
1.9
Bakersfield, CA
16–18/06/10
11.1–12.0
4.0-4.8
–
–
–
4.5–5.4
Lahore, Pakistan
02/11/07
327.5
174.7
–
–
–
11.3
Lahore, Pakistan
12/01/2007–
–
–
0.05–0.50
–
–
–
Kundu et al. (2013)
13/01/2008
Lahore, Pakistan
Mar/07
177.1
44.6
0.09
0.004
–
–
Staudt et al. (2014)
Godavari, Nepal
Feb/07
42.0
4.7
0.004
ND
–
–
Pasadena, CA
5–6/06/10
41.8–44.1
7.3–7.6
0.006–0.007
ND
–
–
Centreville, AL
10–11/07/13
–
–
ND
ND
2.7–5.8
9–14
Hettiyadura et al. (2015)
Shanghai
5–7/04/12
–
–
0.3–0.8
–
–
–
Ma et al. (2014)
12–14/07/12
27–29/10/12
14–16/01/13
Xi'an (n= 10)
18/12/13–
94.7–314.5
14.9–68.5
0.03–0.06
0.04–0.31
0.9–2.6
18.1–155.5
This study
17/02/14
Quality control
For every 10 analyses, a procedural blank and a spiked sample, real
ambient samples spiked with known amounts of a standard solution of
organosulfates to be quantified, were measured to check for interference and
cross-contamination. The external standard method was used for quantitative
determination of the analytes. The limits of detection are defined as the
minimum detectable peaks of individual species with a signal-to-noise (S/N)
ratio of 3:1. The recoveries were determined by the analysis of the spiked
samples: we first measured a filter punch without spike and then measured
the second punch from the identical filter spiked with known amounts of a
standard solution of organosulfates. The differences between these two
measurements were divided by the amounts of organosulfates spiked to
calculate the recoveries of individual organosulfates. This recovery test
also provides an indication of potential matrix effect. The reproducibility
(relative standard deviation, RSD) was determined by measuring five
identical samples that were subjected to the same pretreatment procedure.
The field blank samples were collected and analyzed, and the data reported
here were corrected for the field blanks.
Time series of organosulfates (ng m-3), OC (µg m-3), SO42- (µg m-3) and the fraction of
individual organosulfates in total sulfur and OC (a). The average
concentrations of individual organosulfates and the fractional contribution
in total sulfur and OC during high and low pollution days are also shown (b).
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Results and discussion
Mass spectral fragmentation and UPLC separation
Each synthesized organosulfate was analyzed by high resolution tandem MS
(MS/MS). The molecular ion for each organosulfate was assigned to the
deprotonated molecule (R–O–SO3-). Major sulfur-containing
product ions included the sulfite ion radical (⚫ SO3-,
m/z 80) that is formed from the homolytic cleavage of the O–S bond, the
sulfate ion radical (⚫ SO4-, m/z 96) that is formed from
the homolytic cleavage of the C–O bond, the bisulfite anion
(HSO3-, m/z 81) that is formed from hydrogen abstraction followed by
the heterolytic cleavage of the O–S bond, and the bisulfate anion
(HSO4-, m/z 97). Phenyl sulfate, 3-methylphenyl sulfate, and glycolic
acid sulfate produce phenoxide (C6H5O-, m/z 93),
3-methylphenoxide (C7H7O-, m/z 107) and glycolate
(C2H3O3-, m/z 75) anions, respectively, formed from neutral
loss of SO3. The mass spectra of these compounds are shown in Fig. 2.
The mass spectrum of phenyl sulfate is similar to that reported by Staudt et al. (2014), the mass spectra of hydroxy acetone sulfate and
glycolic acid sulfate are similar to those reported by Hettiyadura et al. (2015) and the spectrum of benzyl sulfate is similar to
that reported by Kundu et al. (2013), confirming the identity
of the compounds.
The ESI-MS/MS in MRM mode is applied for the quantification of individual
organosulfates. This can greatly enhance the selectivity and sensitivity by
monitoring a transition pair of precursor and product ions and thus
eliminating potential interferences from the complex aerosol matrix. Table 1
shows the optimized ESI- conditions and the transition pairs for each
organosulfate studied. The organosulfate standards were separated by UPLC
using a BEH amide column that retains extremely polar compounds through
ionic, hydrogen bonding and dipole interactions. A gradient elution
procedure was applied and the aqueous portion of the mobile phase increased
from 7–43 %, leading to the baseline separation of four organosulfates
within 6 min (Fig. 3a). The retention time was 0.86 min for phenyl sulfate,
0.96 min for benzyl sulfate, 1.10 min for hydroxyacetone sulfate and 5.78 min for glycolic acid sulfate, respectively. The mobile phase was buffered
to slightly basic pH to maintain the deprotonated state of the
organosulfates, which favors the separation. The amide functionalization of
the BEH stationary phase introduces hydrogen bonding and strengthens
interaction with organosulfates particularly for those containing carboxyl
and hydroxyl functional groups. It should be noted that the chromatographic
peak-broadening occurred particularly for phenyl sulfate and hydroxyacetone
sulfate when analyzing the ambient samples (Fig. 3b). This might be
explained by matrix effects due to the complex samples, which can influence
the partitioning of analyte between the stationary phase and mobile phase,
particularly for those analytes with weak retention on the column. However,
the quantification of organosulfates is not affected by the peak broadening
because the transition pair of precursor and product ions used in the MRM
mode of the mass spectrometer guarantees selectivity and accuracy.
Method validation
Table 2 shows the analytical performance of the method under optimized UPLC
and MS/MS conditions. The calibration curves of each organosulfate are
highly linear (R2≥ 0.995), ranging from 0.1–40 ng mL-1 for
phenyl sulfate and benzyl sulfate, from 0.3–120 ng mL-1 for
hydroxyacetone sulfate, and 2.0–800 ng mL-1 for glycolic acid sulfate.
The recoveries, determined by analyzing ambient samples spiked with known
amounts of organosulfate standards, ranging from 80.4–93.2 %. The good
recoveries indicate high extraction efficiency, low sample matrix effect and
low error from sample pretreatment and the UPLC-MS measurement. The limit of
detection (LOD, S/N = 3) and limit of quantification (LOQ, S/N = 10) ranged
from 0.03 to 0.42 ng mL-1 and 0.09 to 1.4 ng mL-1 of the extracts,
respectively. This corresponds to LODs of 1.1 to 16.7 pg m-3 and LOQs
of 3.4 to 55.6 pg m-3, respectively, using the current set-up (see
experimental section).
Quantification of organosulfates in ambient aerosol
Ambient PM2.5 samples were extracted and analyzed by UPLC-MS/MS
following the same procedure as the OS standards. The four selected
organosulfates were identified according to the transition pairs of
precursor and product ions of individual compounds on the MS/MS as well as
the UPLC retention time. Table 3 shows the concentrations of phenyl sulfate,
benzyl sulfate, hydroxyacetone sulfate, and glycolic acid sulfate in
PM2.5 samples collected at Xi'an (this work), together with
concentrations reported in the literature from other locations worldwide for
comparison. In our samples from Xi'an glycolic acid sulfate (average 77.3 ± 49.2 ng m-3, range 18.1–155.5 ng m-3) was the most
abundant species of the identified organosulfate followed by hydroxyacetone
sulfate (average 1.3 ± 0.5 ng m-3, range 0.9–2.6 ng m-3),
phenyl sulfate (average 0.14 ± 0.09 ng m-3, range 0.04–0.31 ng m-3) and benzyl sulfate (average 0.04 ± 0.01 ng m-3, range
0.03–0.06 ng m-3).
The concentration of glycolic acid sulfate quantified in this study is about
one order of magnitude higher than those reported in the literature (see
Table 3), indicating the substantial formation of this secondary organic
compound in polluted urban Xi'an. Glycolic acid sulfate can form efficiently
from glycolic acid relative to glyoxal in the presence of acidic sulfate
particles (Olson et al., 2011). While both organic precursors (glycolic acid
and glyoxal) have biogenic and anthropogenic origins, they form mainly from
the oxidation of anthropogenic emissions during winter in Xi'an. The
concentrations of particle-phase glyoxal and glycolic acid measured at Xi'an
during winter have been reported to be significantly higher compared to
other studied regions (e.g., Kawamura and Yasui, 2005; Miyazaki et al., 2009;
Cheng et al., 2013), which thus may explain the elevated glycolic acid
sulfate. The concentration of the other three organosulfates quantified in
this study was much lower, but falling into the ranges measured in other
regions.
It is noted that the time series of glycolic acid sulfate, phenyl sulfate
and benzyl sulfate is similar to that of organic carbon (OC) and
SO42-, while the concentration of hydroxyacetone sulfate did not
show an increasing trend when the concentrations of OC increased (Fig. 4a).
Hydroxyacetone sulfate can form from photochemical oxidation of isoprene
and/or isoprene ozonolysis in the presence of acidic sulfate aerosols
(Surratt et al., 2008; Riva et al., 2015), although hydroxyacetone was also
suggested to originate from anthropogenic emissions (e.g., biomass burning
and fossil fuel combustion) (Hansen et al., 2014). Also, the formation rate
of biogenic hydroxyacetone sulfate and anthropogenic hydroxyacetone sulfate
may different. This may explain the lack of correlation between
hydroxyacetone sulfate and OC during winter in Xi'an. The average
concentrations of glycolic acid sulfate, phenyl sulfate and benzyl sulfate
were 1.3–3.2× higher during high pollution days (PM2.5 range of
293.7–314.5 µg m-3 with an average of 300.6 µg m-3) than
during low pollution days (PM2.5 range of 94.7–121.2 µg m-3
with an average of 106.4 µg m-3), while the average concentrations
of hydroxyacetone sulfate were rather similar between high pollution days
and low pollution days (Fig. 4b). These four organosulfates together account
for 0.25 % of total sulfur and 0.05 % of OC, respectively.