Oxidation of sulfur dioxide
(
Hydroxymethanesulfonate (HMS;
In cloud and fog water,
Model simulations under low-light conditions in regions with slow
photochemistry, such as polluted cities in China and India, underestimate
sulfate (
The measurement of sulfate in ambient PM is common, whereas measurements of HMS have mainly been conducted for fog and cloud water. Studies reporting the presence of HMS in ambient PM using single-particle mass spectrometry have also been conducted (Neubauer et al., 1996, 1997; Whiteaker and Prather, 2003; Lee et al., 2003; Dall'Osto et al., 2009). Two main methods have been used: ion chromatography (IC) and mass spectrometry (MS). For IC a characteristic elution time is used for the identification of different ions, including sulfate. For MS the detailed mass spectrum, especially differences in fragmentation patterns, can provide a means to differentiate, in this case, different sulfur-containing species. Moreover, for MS, cations can be observed simultaneously in addition to sulfur-containing ions, whereas for IC a specified IC column with high sensitivity for sulfur-containing ions has to be used to identify them. In order to distinguish HMS from sulfate using IC or MS, the elution times or the mass spectra and fragmentation patterns, respectively, have to be distinct (Munger et al., 1986; Chapman et al., 1990; Neubauer et al., 1996, 1997; Dixon and Aasen, 1999; Zuo and Chen, 2003; Lee et al., 2003; Whiteaker and Prather, 2003; Dall'Osto et al., 2009).
Sulfate is traditionally measured using IC, but for measurements of PM
little attention has been given to the effect of HMS in PM on sulfate
measurements. An IC system with an alkanol quaternary ammonium analytical
column is widely used to separate the main inorganic ions, i.e.,
A variety of technical methods have been used to detect HMS, mainly IC using specific columns (Munger et al., 1986; Dixon and Aasen, 1999), reverse-phase ion pair high-performance liquid chromatography (HPLC) (Zuo and Chen, 2003), electrospray ionization–tandem mass spectrometry (ESI-MS) (Chapman et al., 1990) and single-particle mass spectrometry (single-particle analysis by laser mass spectrometry – PALMS; rapid single-particle mass spectrometer – RSMS; aerosol time-of-flight mass spectrometer – ATOFMS) (Neubauer et al., 1996, 1997; Lee et al., 2003; Whiteaker and Prather, 2003; Dall'Osto et al., 2009). In this work we present an IC method specifically developed to identify and quantify HMS, and we discuss the ability of the AMS to identify and quantify HMS in the presence of sulfate and different cations to evaluate the matrix effects under laboratory conditions. In addition, we compare these methods with the technical methods used in previous work.
Mass spectrometry has been used in the past to identify HMS. Chapman et al. (1990) reported its identification by using an electrospray ionization mass
spectrometer (ESI-MS). The characteristic
Neubauer et al. (1996, 1997) explored the possibility of separating
sulfur species, including HMS, by the use of rapid single-particle mass
spectrometer (RSMS) in aerosols. Particles are vaporized and ionized by a
pulsed laser (248 nm) and analysis is completed by a reflectron
time-of-flight mass analyzer. In contrast to ESI-MS, RSMS did not show an
Lee et al. (2003) conducted a field campaign measuring the chemical
composition of aerosols with 0.35–2.5
Overall, the quantification of HMS using single-particle MS methods is
challenging due to matrix effects in ambient samples
(Neubauer et al., 1996, 1997; Whiteaker and Prather, 2003).
The sensitivity challenges of these methods with respect to HMS
quantification yield the necessity of further study. Aerosol mass
spectroscopy (AMS) was used in this work to investigate the ability to
identify and quantify HMS and will be described in detail below. However,
all mass spectrometry techniques share the challenge that the majority of
the fragments, such as
Scheinhardt et al. (2014) provided evidence of the identification of HMS during
two field campaigns conducted in nine sites in Germany using capillary
electrophoresis (CE). CE achieved the efficient separation of HMS from other
compounds using a voltage of
Reverse-phase ion pair HPLC has successfully been used to separate
sulfur species (Zuo and Chen, 2003). A
cetylpyridinium-coated C
Sample analysis using the HR-ToF-AMS.
The sodium salt of HMS (Na-HMS) was purchased from Sigma-Aldrich (purity
95 %). Sodium sulfate
(
The Aerodyne high-resolution time-of-flight aerosol mass spectrometer
(HR-ToF-AMS) (DeCarlo et al.,
2006) was used to determine the mass spectral signatures of Na-HMS, sodium
sulfate and bisulfite. The mass spectra of sodium sulfate, sodium bisulfite
and sodium HMS were examined, and the concentration of each solution in the
atomizer was 10 mM. The pH of the sample solutions was 6. In addition,
solutions containing 20 % sulfate and 80 % Na-HMS, 40 % sulfate and
60 % HMS, 60 % sulfate and 40 % Na-HMS, and 80 % sulfate and 20 %
Na-HMS were analyzed to evaluate the ability of distinguishing the two
species at varying sulfate to Na-HMS ratios. A reference spectrum of
ammonium sulfate was also used to investigate the matrix effect. The
solutions were atomized by a particle generator (TSI 3076) and subsequently
dried before being sampled by the AMS. The AMS heater was set at the standard
operating temperature of 600
A Dionex ICS-5000
Samples of sodium bisulfite, sulfate and Na-HMS were analyzed individually
using the HR-ToF-AMS in order to determine the mass spectral signatures of
these compounds (Fig. 1). For Na-HMS the organic ions
Fractional contributions of
HR-ToF-AMS analysis of aqueous samples containing sodium sulfate
and sodium HMS.
The dominant sulfur-containing
The AG22–AS22 column pair was used to examine the ability to separate HMS
and sulfate as well as bisulfite/sulfite and sulfate ions. The AS22
analytical column has the same functional group, alkanol quaternary
ammonium, as columns used in previous work for the identification of HMS and for
ambient analysis during haze events
(Munger et al.,
1986; Dixon and Aasen,
1999; Wang et al.,
2005; Cao et al.,
2014; Cheng et al., 2016). The
AS22 analytical column provides a direct comparison to this class of
columns. The analytical columns can also be classified with respect to the
eluent. The types of columns used in previous studies were the Dionex Ionpac
AS11, AS11-HC and AS4A for which AS11 and AS11-HC are anion hydroxide
columns and AS4A is an anion carbonate column. The AS22 analytical
column (diameter
Detection and separation of sulfate and HMS using two ion
chromatography systems. The first system, corresponding to
Six samples containing only sulfate, bisulfite/sulfite, HMS,
a combination of HMS with bisulfite/sulfite, HMS with sulfate, and all three
sulfur-containing species were analyzed using the AG22–AS22 column pair in a
pH range of 3–12. At pH 3–6 the dissolved sulfur dioxide will be in the form
of bisulfite (
Detection and separation of HMS and sulfate using an ion
chromatography system with an AS12A analytical column and AG12A guard column.
In order to examine the possibility of separating sulfate and HMS we used
the AG12A–AS12A column pair. The AS12A analytical column has an alkyl
quaternary ammonium functional group. The AS12A analytical column
(diameter
Technical characteristics of the columns used for the ion chromatography analysis.
Calibration standards were prepared and analyzed to determine the retention
times (Fig. 5). Each sample was a single-component sample containing only
one of the sulfur species. The detection limit of sulfate and HMS was
experimentally determined as 0.2 and 0.8
Sample analysis of sulfate and HMS using two ion chromatography
systems. The first system, corresponding to
Comparing the results from the two column pairs, it was determined that for
the AS22 analytical column the HMS peak appears slightly after the sulfate
peak, whereas for the AS12A analytical column the HMS peak appears before the
sulfate peak. Using the AS12A analytical column, sulfate represents 55.2 %
of the total area signal and HMS 44.8 % when a sample of 2 mM of HMS and 2 mM of sulfate was analyzed. In contrast, for the AS22 analytical column the
area signal of sulfate was 63.6 % and HMS was 31.8 % for both pH
Considering the intensity of HMS and sulfate for AS12A in the mixed
sample, the intensity of the sulfate and HMS peaks was 26.2 and 21.3
The AS22 and AS12A columns have different technical characteristics (Table 2). The difference in the retention times is due to the functional groups (internal coating) of the columns and thus their ability to separate ions. Sulfate is more polar than bisulfite/sulfite, and therefore it is expected to have a stronger binding on the stationary phase (functional group), which results in a longer retention time. HMS and bisulfite/sulfite are not separated as they have very similar polarity. In addition, the AS22 analytical column is longer than the AS12A analytical column, which affects the retention time of the examined compounds. The eluent is also a technical aspect that differs between the two columns. The AS12A is an anion carbonate column, and thus the eluent is neutral with respect to the pH, whereas the AS22 column is an anion hydroxide column, and thus the eluent is basic with respect to pH. The stability of HMS has a strong pH dependence as it dissociates at high pH values. Therefore, the use of a neutral pH eluent allows us to avoid HMS decomposition during analysis. The majority of columns with an alkyl quaternary ammonium functional group require a neutral pH eluent, which results in the efficient separation of sulfur species.
Another factor that can affect the retention time of the compounds is the hydrophobicity of the stationary phase of the column. The AS22 analytical column has ultralow hydrophobicity, whereas the AS12A analytical column has medium hydrophobicity, resulting in a more efficient separation of species within a single family. An ultralow hydrophobicity results in faster retention for nonpolar compounds and will cause polar substances of the matrix to accumulate in the column, possibly leading to undesirable effects such as the misidentification of compounds and shifted retention times. Nonpolar compounds will be transferred down the column more readily, whereas polar compounds, such as sulfate and bisulfite/sulfite, might not be eluted efficiently by the eluent, resulting in unrealistic retention times and peak shapes in the chromatograph. This factor can possibly explain the longer retention time of HMS compared to sulfate when the AS22 column is used, as sulfate has a higher polarity than HMS.
This study investigates techniques used to identify and quantify HMS and
sulfate in PM that contains both species. Two main methods were examined: IC
and AMS. HMS and sulfate can be efficiently separated and quantified using
an IC system with an analytical column that has an alkyl quaternary ammonium
functional group (i.e., AS12A). However, using a column with alkanol
quaternary ammonium functional groups (i.e., AS22) the quantification of sulfate
and HMS is challenging as the peaks are not separated efficiently and they
may be identified as one species, typically sulfate. Hence, HMS could
possibly be mistaken as sulfate in field measurements. Using an IC system,
the detection limit of quantifying HMS and sulfate is 0.8
and 0.2
The results obtained in this study may help explain the case of the January 2013 haze event in northern China (Wang et al., 2014) for which models underpredicted sulfate levels compared to observations. During the study of the 2013 haze events, field measurements, analyzed using an alkanol quaternary ammonium column, showed 70 %–90 % increased sulfate concentrations compared to the model simulations (Wang et al., 2014), and one explanation that has been proposed is that HMS was quantified as sulfate. Similarly, AMS measurements may have identified HMS as sulfate as explained above. This is also consistent with the explanation provided by Moch et al. (2018) and Song et al. (2019).
Applications of both the IC and AMS methods to ambient samples from similar
conditions as the January 2013 haze event in the future will provide an
opportunity to characterize the efficiency of the identification and
quantification of HMS and sulfate in complex mixtures and the degree to
which nonoxidative reactions of
The data used within this work are available upon request. Please email Eleni Dovrou (edovrou@g.harvard.edu).
FNK initially conceived of the work. ED developed the specific ion chromatography method described in this work, performed the experiments and analyzed the data. CYL and ED conducted the aerosol mass spectrometry experiments, and CYL, ED and MRC analyzed the data. ED prepared the paper with contributions from CYL, MRC, JHK, DRW and FNK.
The authors declare they have no conflict of interest.
The authors thank J. William Munger for helpful statistical and HMS-related discussions, as well as Loretta J. Mickley and Jonathan M. Moch for helpful preliminary discussions.
This research has been supported by the Harvard Global Institute.
This paper was edited by Pierre Herckes and reviewed by three anonymous referees.