Determination of the Sulfur Isotope Ratio in Carbonyl Sulfide Using

Dec 2, 2014 - Precambrian Ecosystem Laboratory, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho,. Yokosuka ...
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Determination of the Sulfur Isotope Ratio in Carbonyl Sulfide Using Gas Chromatography/Isotope Ratio Mass Spectrometry on Fragment Ions 32S+, 33S+, and 34S+ Shohei Hattori,*,† Akari Toyoda,‡ Sakae Toyoda,‡ Sakiko Ishino,† Yuichiro Ueno,¶,§,∥ and Naohiro Yoshida†,‡,§ †

Department of Environmental Chemistry and Engineering and ‡Department of Environmental Science and Technology, Tokyo Institute of Technology, Yokohama 226-8502, Japan ¶ Department of Earth and Planetary Sciences and §Earth-Life Science Institute (WPI-ELSI), Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan ∥ Precambrian Ecosystem Laboratory, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan S Supporting Information *

ABSTRACT: Little is known about the sulfur isotopic composition of carbonyl sulfide (OCS), the most abundant atmospheric sulfur species. We present a promising new analytical method for measuring the stable sulfur isotopic compositions (δ33S, δ34S, and Δ33S) of OCS using nanomole level samples. The direct isotopic analytical technique consists of two parts: a concentration line and online gas chromatography-isotope ratio mass spectrometry (GC-IRMS) using fragmentation ions 32S+, 33S+, and 34S+. The current levels of measurement precision for OCS samples greater than 8 nmol are 0.42‰, 0.62‰, and 0.23‰ for δ33S, δ34S, and Δ33S, respectively. These δ and Δ values show a slight dependence on the amount of injected OCS for volumes smaller than 8 nmol. The isotope values obtained from the GC-IRMS method were calibrated against those measured by a conventional SF6 method. We report the first measurement of the sulfur isotopic composition of OCS in air collected at Kawasaki, Kanagawa, Japan. The δ34S value obtained for OCS (4.9 ± 0.3‰) was lower than the previous estimate of 11‰. When the δ34S value for OCS from the atmospheric sample is postulated as the global signal, this finding, coupled with isotopic fractionation for OCS sink reactions in the stratosphere, explains the reported δ34S for background stratospheric sulfate. This suggests that OCS is a potentially important source for background (nonepisodic or nonvolcanic) stratospheric sulfate aerosols. corresponding to an OCS sink of 34−64 Gg S years−1. This matches the mass of sulfur needed to sustain an SSA level of 30−170 Gg S years−1 according to the model described in Chin and Davis6 and references therein. The sulfur source that produces and maintains the background SSA has therefore not been quantitatively estimated. Isotopic analysis is used to trace the sources and transformations of atmospheric trace gases.12,13 The estimated isotopic composition of OCS is 11‰ in δ34S,14 but no measurement has confirmed this value. For background SSA, the δ34S value was reported to be 2.6‰,15 indicating that background SSA is slightly depleted in 34S compared with OCS. The isotopic fractionation constant for 34S is given by 34ε = 34 32 k/ k − 1, where k is the rate coefficient for OC32S or OC34S

C

arbonyl sulfide (OCS) is the most abundant sulfurcontaining gas in the atmosphere, with an average molar fraction of 5 × 10−10 (500 ppt) in the troposphere.1 OCS has an atmospheric lifetime longer than 2 years in the troposphere,2 which enables it to be transported into the stratosphere, where it is oxidized to form stratospheric sulfate aerosols (SSA) through atmospheric sink reactions. OCS has therefore been suggested as a source of background (nonvolcanic) SSA layers;3 these increase the Earth’ albedo and modulate the concentration of stratospheric ozone (O3) as a result of surface heterogeneous reactions.4,5 However, Chin and Davis6 used a one-dimensional model to show that atmospheric OCS levels are not sufficient to maintain the background SSA concentration; another significant SSA source may be deep convection of SO2 from the lower troposphere.7−10 In contrast, Barkley et al.11 found that the stratospheric lifetime of OCS was 64 ± 21 years from measurements via the Atmospheric Chemistry Experiment (ACE) satellite Fourier-transform spectrometer, © XXXX American Chemical Society

Received: July 21, 2014 Accepted: December 2, 2014

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loss in the atmosphere; and 34ε is given in units of per mil (‰). Apparent 34ε was reported as 73.8 ± 8‰ for stratospheric OCS loss in the lower stratosphere, based on OC32S and OC34S concentration profiles obtained using the NASA Jet Propulsion Laboratory MkIV Fourier-transform infrared spectrometer.16 On the basis of this finding, it was concluded that OCS is not the main source of background SSA. However, these large and positive 34ε values, and the conclusions drawn from these values, have been brought into question by recent studies. These studies, which investigated isotopic fractionation for OCS photolysis,17−19 the OCS + OH reaction,20,21 and the OCS + O(3P) reaction,22 obtained small and/or negative 34ε. To test whether or not OCS is a source of background SSA, the OCS isotopic composition needs to be determined precisely. In addition, sulfur isotope analysis of OCS provides information that enables us to understand the origins and sinks of atmospheric OCS. Consequently, it is important to develop a system for measuring the sulfur isotopic compositions of atmospheric OCS. The conventional method for measuring sulfur isotopic compositions is based on combustion of sulfur compounds to SO2 in an elemental analyzer, followed by measurement of SO2 (m/z: 64 and 66) using gas-source isotope ratio mass spectrometry (IRMS).23 Conversion to SF6, with analysis of SF5+ fragments (m/z: 127, 128, 129, and 131) by IRMS, has also been used to examine multiple sulfur isotope ratios using a dual-inlet (DI) system.24,25 Both types of analysis require several micromoles of bulk materials (i.e., powders), and the latter approach requires a hazardous chemical (F2) and lengthy sample preparation. There is, therefore, no simple and robust analytical method for the measurement of atmospheric sulfur species, which have concentrations of several hundred parts per trillion. Because a trace amount of atmospheric OCS (up to 500 ppt) exists in ambient air, it is necessary to collect nanomoles of OCS from ambient air and separate it from other species. Isotopic analysis by coupling gas chromatography (GC) and IRMS online, with preconcentration of OCS from ambient air, enables us to measure OCS sulfur isotopic compositions. S+ fragment ions produced from electron impact ionization of OCS in the IRMS ion source were used in this study. Although the fraction of S+ (m/z: 32) produced in the IRMS ion source is smaller than that of OCS+ (m/z: 60), the advantage of using S+ fragment ions from OCS is that this allows us to use typical triple faraday collector cups originally designed for monitoring oxygen isotopologues (m/z: 32, 33, and 34). This method is easier to carry out compared with using OCS+ molecular ions, which need special faraday collector cups for measuring OCS+ fragment ions (m/z: 60, 61, and 62). Furthermore, the relative abundance of these molecular ions depends not only on sulfur, but also on carbon (12C and 13C) and oxygen (16O, 17O, and 18O) isotopes, making it difficult to obtain sulfur isotopic composition. In the current study, we developed a rapid and simple method for sulfur isotopic analysis of OCS using GC-IRMS on fragmentation ions of S+, coupled with an online preconcentration system. The first measurement of ambient OCS in air is reported, and the atmospheric implications of the obtained OCS isotopic composition are discussed.

(sample B, Japan Fine Products) were used in this study. They were balanced with He gas. Laboratory-synthesized OCS samples (samples C and D) were prepared using the method described by ferm.26 This procedure involved the reaction of CO (99.99% purity; Japan Fine Products, Kawasaki, Japan) with 3.3−7.3 mg of elemental sulfur (sample C, Wako Pure Chemical Industries Ltd., Japan; sample D, Sigma-Aldrich, Japan). The elemental sulfur powders were placed in Pyrex glass tubes (i.d. 9 mm), and the air was evacuated. After evacuation, a stoichiometric excess of CO (2to 3-fold) was added, and the tubes were sealed. The reaction mixtures were heated at 573 K for 24 h, to convert them to OCS. The product was expected to include impurities such as CO, CO2, H2S, CS2, and other trace compounds. The OCS samples were purified using a GC equipped with a thermal conductivity detector (GC-14B; Shimadzu, Kyoto, Japan) and a packed column (Porapak Q, 2 mm i.d. 2.4 m; GL Science, Tokyo, Japan) maintained at 333 K. Ultrahigh-purity He (>99.99995% purity; Japan Air Gases Co., Tokyo, Japan) was used as the carrier gas at flow rates of 32 mL min−1. The GC oven temperature was kept at 333 K for 12 min, raised from 333 to 473 K at a rate of 25 K min−1, and then kept at 473 K for 2 min. OCS, which has a retention time of ca. 5 min was trapped in a U-shaped glass tube packed with quartz turnings at 77 K. Reanalysis of the purified OCS sample confirmed that the concentrations of impurities were below the detection limit (99.99995% purity; Taiyo Nissan, Japan) purified using a commercial purifier (VICI filters, Valco Instruments Co., TX, U.S.A.) and a stainless-steel column (with 7.53 mm i.d., and 0.7 m length) packed with 5A Molecular Sieve (Sigma-Aldrich, Japan) cooled at 77 K with liquid N2 to avoid any trace contamination by O2. He was used as the carrier gas with a flow rate of 2 mL min−1. In the IRMS ion source, electron impact ionization of OCS produced S+ fragment ions, as well as its molecular ion (OCS+). At 82 eV electron energy, the fraction of S+ (m/z: 32) relative to OCS+ (m/z: 60) was about 0.6. The sulfur isotope ratios in OCS were therefore determined by measuring the fragment ions 32S+, 33S+, and 34S+ using triple faraday collector cups that were originally designed for monitoring the three ions derived C

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Table 2. Averages and Standard Deviations (1σ) of OCS Sulfur Isotopic Compositions Measured by the New GC-IRMS Method and the Conventional DI-IRMS Methoda GC-IRMS (S+)

DI-IRMS (SF5+)

sample

n

δ S (‰)

δ S (‰)

Δ S (‰)

n

δ S (‰)

δ34S (‰)

Δ33S (‰)

A C-1 C-2 D-1 D-2

6 4 3 3 2

7.3 ± 0.4 −3.2 ± 0.6 −3.7 ± 0.7 −7.4 ± 0.0 −7.9 ± 0.1

14.3 ± 0.6 −6.2 ± 0.4 −7.1 ± 0.2 −15.2 ± 0.1 −15.5 ± 0.1

0.00 ± 0.23 0.05 ± 0.41 −0.02 ± 0.01 0.31 ± 0.04 0.01 ± 0.01

3 1 1 1 1

7.3 ± 0.1 −3.7 −3.4 −7.9 −7.7

14.3 ± 0.2 −6.2 −6.8 −15.4 −15.2

0.00 ± 0.002 0.057 0.049 0.076 0.072

33

34

33

33

δ and Δ values for the GC-IRMS method are based on measurements in which total peak areas of OCS (m/z: 32, 33, and 34) were above 5 Vs; sample A was used as the internal standard for conversion to VCDT notation.

a

from oxygen isotopologues 16O16O, 16O17O, and 16O18O. The natural abundances of the three sulfur isotopes are different from those of oxygen isotopes, so the signal amplifiers associated with the cups for m/z 33 and 34 were replaced with optimal ones. The resistors for the 32S, 33S, and 34S measurements were set at 3 × 108 Ω, 3 × 1010 Ω, and 1 × 1010 Ω, respectively. Sample A was purified with liquid N2 (77 K) and then introduced via a conventional DI system as the reference gas because purified OCS, which is toxic, is not commercially available in Japan. Definition. Sulfur isotopic compositions are reported as δ 33,34S =

R sample R standard

−1

related to MDF. Samples C-1, C-2, D-1, and D-2 were measured similarly, although only a single analysis was conducted because of their limited amounts (Table 2). Sample C-3 was lost during the experimental procedure. Sulfur Isotopic Measurements Using the GC-IRMS Method. The δ33S, δ34S, and Δ33S values as a function of the OCS peak areas (m/z: 32, 33, and 34) are shown in Figure 2.

(1) 33

32

34

32

where R are the isotope ratios ( S/ S and S/ S) of the samples and standards. The isotope ratios of sulfur are reported relative to the Vienna Canyon Diablo Troilite (VCDT) standard, and quoted as per mil values (‰). In addition to the δ34S values, the mass-independent fractionation (MIF; or non-mass-dependent fractionation) of sulfur, which causes a deviation from the mass-dependent fractionation (MDF) line, is an important quantity. The capital delta notation (Δ33S) is used to distinguish between mass-dependent and massindependent isotopic compositions, where Δ33S = δ 33S − [(δ 34S + 1)0.515 − 1]

(2) 33

This notation describes the excess or deficiency of S relative to a reference MDF line. In our GC-IMRS system, all the isotopic values were initially calculated relative to a reference sample measurement using GC-IRMS (i.e., a daily measurement of approximately 4−5 nmol of sample B). However, the data sets were corrected to values relative to the international standard (VCDT) notation using the isotopic composition of sample A, which was determined using the offline method (SF5+ measurements with DI-IRMS), as described below.

Figure 2. Sulfur isotopic compositions of samples A and B showing sample size dependence in measurements using the GC-IRMS method. All values are relative to an internal standard (approximately 4−5 nmol of sample B), measured first on all measurement days.

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RESULTS AND DISCUSSION Isotopic Compositions of Commercial and Synthesized OCS Samples Using the Offline System. Three replicated analyses of sample A using the offline method (SF5+ measurements with DI-IRMS) yielded 7.3 ± 0.1‰, 14.3 ± 0.2‰, and 0.00 ± 0.002‰ for δ33S, δ34S, and Δ33S, respectively, relative to VCDT. The errors are one standard deviation and represent errors associated with gas handling, wet chemistry (hydration and precipitation), fluorination, and MS analysis. Note that the error for Δ33S is smaller than those for δ33S and δ34S, indicating that most of the experiment error is

Note that these δ and Δ values are relative to daily measurements of sample B (ca. 4−5 nmol). The δ and Δ values of OCS determined using the online method (S+ measurements via GC-IRMS) were constant when approximately 8 to 15 nmol of OCS were injected into the analytical system. When the total peak area of OCS (m/z: 32, 33, and 34) was above 5 Vs (OCS sample amount >8 nmol), typical precisions (1σ) for sample A (n = 6) were 0.42‰, 0.62‰, and 0.23‰ for δ33S, δ34S, and Δ33S, respectively (Table 2). D

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However, these values increased when the amount of injected OCS decreased (Figure 2). O2, which has the isotopologues 16 16 O O, 16O17O, and 16O18O, is a potential background species and may interfere with the measurement of m/z 32, 33, and 34. However, it is worth noting that natural abundances of 17O (ca. 0.038%) and 18O (ca. 0.205%) are much lower than those of 33 S (ca. 0.75%) and 34S (ca. 4.21%), respectively. If interference of O2 caused the sample size dependence, then δ33S and δ34S should decrease as the amount of injected OCS decreases. Thus, the sample size dependence does not appear to be related to O2 interference. The δ33S, δ34S, and Δ33S values of sample A were determined by offline measurements of the SF5+ fragment using the DIIRMS system. Therefore, all the experimental δ33S, δ34S, and Δ33S values obtained using the new GC-IRMS system can be converted to those relative to VCDT. However, elevated δ33S, δ34S, and Δ33S values were observed when the amount of injected OCS into the GC-IRMS system was less than 8 nmol (peak areas of OCS (m/z: 32, 33, and 34) were below 5 Vs). To avoid the effect of sample size on the normalization of the experimental data relative to VCDT, all the experimental δ and Δ values obtained using the GC-IRMS method relative to sample B were converted to those relative to VCDT using the average isotopic composition of sample A. For sample A used in this normalization, samples with the total peak areas of OCS (m/z: 32, 33, and 34) above 5 Vs (n = 6) were used. In Figures 3−6 and the following discussion, the δ and Δ values for samples A to E are relative to VCDT, normalized with sample A as the internal standard. The δ and Δ values on the VCDT scale are shown in Figure 4. Trends reflecting sample size dependence in samples B, C, D, and E were similar to that for sample A. Averages and precisions of the δ33S, δ34S, and Δ33S values of the peak areas of OCS above 5 Vs for samples A to E are shown in Table 2. Typical precisions (1σ) for samples C1, C2, D1, and D2 were less than 0.7‰, 0.4‰, and 0.4‰ for δ33S, δ34S, and Δ33S, respectively. Calibration of OCS Isotopic Measurements Using S+ Fragments by Conventional DI-IRMS Method. The accuracy of the online GC-IRMS method measuring S+ fragment ions was evaluated by comparing the results for the three isotopically different OCS samples (A, C, and D) with those obtained using the offline DI-IRMS method measuring SF5+ fragment ions (Figure 3). In Figure 3, the δ and Δ values obtained using the online GC-IRMS method are shown on the VCDT scale, normalized with sample A as the internal standard. For both δ33S and δ34S, no significant difference was observed between methods, and all values plot on a straight line with slope 1, within an error of 1σ. Thus, the results obtained using our new measurement system show no systematic deviation and are comparable to those obtained by conventional measurement of sulfur isotopic compositions on the VCDT scale. Sulfur Isotopic Composition of the Atmospheric Sample. The determination of OCS isotopic composition in atmospheric samples requires several nanomoles of OCS to be concentrated in trap 2 of the system (Figure 1). In our study, trap 2 on the preconcentration line was cooled to dry ice/ ethanol temperature (195 K), allowing OCS to be separated from other atmospheric compounds such as N2, O2, and Ar. Sample E (42−317 L) was introduced directly into the custombuilt preconcentration line. In some analytical runs, the total OCS peak area for m/z 32, 33, and 34 indicated that recovery

Figure 3. Comparison of sulfur isotopic compositions of commercial and laboratory-synthesized OCS using both the online S+ method and offline SF6 method.

of the OCS introduced into the preconcentration system was as low as 50%. In particular, for large volume samples, no more than 3.5 nmol of OCS were collected. In addition, the OCS peak shapes for some of the sample E analytical runs were broader than those for samples A to D. Of 12 analyses conducted (Figure 4), only seven gave isotopic results that were considered to be unaffected by partial loss of OCS or interference by other species (see the Supporting Information). Sample size dependence was observed for δ and Δ values in sample E, as for samples A−D, and this is thought to be partly caused by the presence of background species with different abundances for m/z 32, 33, and 34. Furthermore, for samples A−D, the δ and Δ values for large samples were constant and consistent with conventional DI-IRMS measurements (Figure 3 and Table 2). To estimate δ33S and δ34S of sample E, correlation between δ values and the inverse peak area were determined with slopes of 2.1 ± 0.5 for δ33S and 1.8 ± 0.3 for δ34S (Figure 5). The intercepts (i.e., the expected sulfur isotopic compositions of sample E) were 1.6 ± 0.5‰ and 4.9 ± 0.3‰ for δ33S and δ34S, respectively (Figure 5). It is worth noting that the δ33S and δ34S values for 2−3 nmol of sample E for spec 104, 161, and 190 (Table S1 in the Supporting Information), normalized from similar sample sizes of sample A, were 1.8 ± 1‰ and 5.2 ± 1‰ for δ33S and δ34S, respectively. These results indicate that our analysis using the correlation between δ values and the inverse peak area is consistent with normalE

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Our estimated δ34S for OCS in the atmospheric sample E collected in Kawasaki, Japan, was lower than the previously estimated value of 11‰ for δ34S based on a simple mass balance for terrestrial and marine emissions of OCS.14 The Δ33S value for sample E, calculated from estimated δ33S and δ34S, was −0.9 ± 0.9‰. This value indicates slightly depleted 33 S on average, although no significant deviation from the MDF line was observed within the error and uncertainty of the measurements. Three-Isotope Plot. The relationship between δ33S and 34 δ S (the so-called three-isotope plot) is shown in Figure 6. A

Figure 6. Three-isotope plots for OCS samples. Dotted line is MDF line with slope 0.515. Star indicates isotopic composition of sample E, estimated using correlation between δ values of OCS and the inverse peak area.

correlation between δ33S and δ34S was seen for all samples. The slopes ranged from 0.8 to 1.5, but were different from that of the MDF line (0.515) due to their sample size dependence, as discussed above. The reason for this MIF trend related to sample size is not clear. However, we speculate that isotopic fractionation occurs in the ionization chamber, as revealed by the pressure dependence of O2 isotopic measurements.29 Although there are no reports of Δ33S values for background SSA, polar ice core records of sulfate from SSA show initially positive Δ33S values,30,31 suggesting that MIF chemistry is involved in the stratospheric sulfur chemistry. In addition, some positive Δ33S values were also observed for tropospheric sulfate.32,33 One of these studies33 attributed these positive sulfur isotope anomalies (Δ33S) in aerosol sulfates in Xianghe County (China) to contributions from other atmospheric sources (upper troposphere or lower stratosphere). Thus, MIF in stratospheric sources appears to give positive Δ33S values. It has been suggested that sulfur MIF in the atmosphere is derived from CS2 oxidation,34 SO2 photolysis,35 photoexcitation,36,37 and intersystem crossing.38 In contrast, OCS is not expected to produce MIF during oxidation processes in atmospheric sink reactions.17−19,21,22 Consequently, the −0.9 ± 0.9 ‰ value of Δ33S for sample E (atmospheric OCS sample), and OCS sink reactions do not explain the positive Δ33S values observed for tropospheric sulfate aerosols. Further studies are required to determine whether sample E Δ33S values are related to MIF in OCS source and sink reactions. Atmospheric Implications. In this study, we measured the δ34S value of OCS for an atmospheric sample for the first time. If this value is representative of the global OCS signal, then we can evaluate whether or not OCS is a source of background SSA using a similar analysis to that described in Leung et al.,16

Figure 4. OCS sulfur isotopic compositions of samples A−E, relative to VCDT, as a function of OCS peak area.

Figure 5. Correlation between δ values of OCS for commercial compressed air (sample E) and inverse peak area from seven analyses of commercial compressed air (sample E).

ization using analytical runs of small sample size from sample A. Furthermore, analyses for samples A−D were used to estimate differences between the intercept values and average values for sample amounts greater than 8 nmol, as discussed above. The differences for samples A−D were smaller than 0.6‰ and 0.6‰ for δ33S and δ34S, respectively. Thus, the error estimated for using this analysis for sample E is expected to be in the same range as these values for a precision of 1σ. F

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updated with δ 34 S values for tropospheric OCS and fractionation constants from recent literature. Previous studies of isotopic fractionation of OCS photolysis,17−19 the OCS + OH reaction,20,21 and the OCS + O(3P) reaction,22 indicate that the combined value of the 34S fractionation constants (34εtotal) for the three OCS sink reactions is small and negative, ranging from −8.7 to −4.4‰ (based on theoretical calculations of OCS photolysis19), or from −8.0 to −2.3‰ (based on cross-section measurements of OCS photolysis17) for altitudes of 16, 20, and 24 km (see Schmidt et al.19 for more detailed calculations). Chin and Davis6 estimated that 9% of the total OCS transported into the stratosphere is oxidized to sulfate and remainder re-enters the troposphere, indicating that the fraction of remaining OCS ( f) is 0.91. The δ34S of this remaining OCS is estimated to range from 5.1 to 5.7‰, based on the Rayleigh equation (from δ34S-OCS = δ34S0-OCS + 34εtotal ln(0.91), where δ34S0-OCS is 4.9 ± 0.3‰). Here, δ34S of the products from OCS sink reactions can be calculated using the isotope mass balance equation below. δ 34S0‐OCS = δ 34S‐product (1 − f ) + δ 34S‐OCS f

than 8 nmol) to mitigate sample size dependence of the measurements using the newly developed GC-IRMS method; hence, the trapping system for collecting OCS needs to be improved by using a larger collection loop and/or an in situ preconcentration method. In addition, separation from CO2, which is 106 times more abundant than OCS and has a similar vapor pressure, might be required to avoid interfering with the collection of OCS in trap 2; this should be improved through future study. Finally, on the basis of isotope mass balance using the OCS isotopic composition derived in this study and isotopic fractionation in OCS sink reactions, we conclude that OCS is a potentially important source for background SSA. An atmospheric modeling study of the sulfur isotope budget is also needed to better understand stratospheric OCS loss and sulfate production. Our isotopic analysis for OCS can be used to investigate its sources and sinks in the troposphere to better understand its cycle and its contribution to the background SSA.

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ASSOCIATED CONTENT

* Supporting Information

(3)

S

This material is available free of charge via the Internet at http://pubs.acs.org.

The δ S value of the product is estimated to be between −3.4 and +2.7‰. The δ34S value of the products from OCS sink reactions for the stratosphere are eventually oxidized to sulfate. Although SO2 oxidation processes involve isotopic fractionation,39 it is assumed that SO2 oxidation processes do not contribute to the isotopic composition of the background SSA because all SO2 is oxidized to the sulfate. This is supported by field observations on the levels of depletion of SO2 at altitudes between 10 and 20 km.40 Thus, the δ34S of background SSA is estimated to be −3.4 to +2.7‰. This value agrees well with a previously reported value for background SSA (δ34S = 2.6 ± 0.3‰15). Consequently, we conclude that OCS is a potentially important source for SSA from isotope mass balance considerations. 34

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We wish to thank T. Watanabe, S. O. Danielache, Y. Endo, and M. Nakagawa for their technical assistance with the experiments. We also wish to thank Y. Sone and M. Ohori of Thermo Fisher Scientific for her technical support for the IRMS analytical method. We are grateful to M. S. Johnson, J. A. Schmidt, K. Yamada, and other members of Yoshida’s laboratory for their advice and assistance. Comments from two anonymous reviewers improved the manuscript. This work is supported by the Global Environmental Research Fund (A0904) of the Ministry of the Environment, Japan, and a Grantin-Aid for Scientific Research (S) (23224013) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. S.H. is partially supported by a Grant-inAid for Young Scientists (Start-up) (25887025) from MEXT, Japan. S.T. was supported by a Grant-in-Aid for Young Scientists (B) (17740356) and Scientific Research on Priority Areas (19030007) from MEXT, Japan. Y.U. is partially supported by the NEXT program, MEXT, Japan.

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CONCLUSIONS In this study, a new GC-IMRS measurement system using S+ fragment ions was developed. The precisions of measurements for peak areas above 5 Vs (sample OCS amount >8 nmol) were 0.42‰, 0.62‰, and 0.23‰ for δ33S, δ34S, and Δ33S, respectively. Both the δ and Δ values increased when samples were smaller than 8 nmol. The δ values measured using the GC-IRMS system were compared to those obtained with a conventional DI-IRMS measurement system using SF 5 + fragments, and good agreement was observed. This suggests the new measurement system is accurate and that the isotopic compositions of OCS measured by this system are comparable to those relative to VCDT. For the first time, we succeeded in measuring sulfur isotopic compositions of compressed air collected at Kawasaki, Kanagawa, Japan. We determined the δ34S value of OCS of an atmospheric sample to be 4.9 ± 0.3‰. Our result is lower than the previous estimate of 11‰ based on mass balance equations for emissions. Although we successfully estimated the sulfur isotopic compositions of atmospheric OCS, it was based on analyses from only one atmospheric sample. Future studies are clearly needed to investigate global OCS isotope distributions and effects of seasonal changes. To do this, the preconcentration system for atmospheric samples needs to be improved. Foremost, we need to use large volume samples (i.e., more

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REFERENCES

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Analytical Chemistry

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DOI: 10.1021/ac502704d Anal. Chem. XXXX, XXX, XXX−XXX