Detection of S (IV) Species in Aerosol Particles Using XANES

Aug 25, 2009 - X-ray absorption near-edge structure spectroscopy (XANES) has been applied to the determination and quantification of S(IV) species in ...
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Environ. Sci. Technol. 2009, 43, 7357–7363

Detection of S(IV) Species in Aerosol Particles Using XANES Spectroscopy MASAYUKI HIGASHI† AND Y O S H I O T A K A H A S H I * ,† Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan

Received January 17, 2009. Revised manuscript received July 17, 2009. Accepted August 6, 2009.

X-ray absorption near-edge structure spectroscopy (XANES) has been applied to the determination and quantification of S(IV) species in aerosol samples collected at Qingdao in northeastern China. The XANES spectra showed that sulfite was found only in particles with larger diameters (mineral aerosols) collected in August 2001. Two oxidation treatments suggested that calcium sulfite (hannebachite) was the main S(IV) species in aerosols. No S(IV) species, however, were found at the surface of the aerosols as shown by surface-sensitive conversion electron/He ion yield XANES. The presence of hannebachite in the interior of aerosols demonstrates the importance of heterogeneous oxidation of SO2 (adsorption of SO2 at the surface of mineral aerosols such as calcite with subsequent oxidation). The fact that this process is supported from XANES analysis for natural samples is important because sulfite formed by the adsorption of SO2 has only been detected in laboratory studies so far. The contribution of heterogeneous oxidation to the total rate of SO2 oxidation is not clear at present. However, this study suggests that the adsorption of SO2 on mineral aerosols without oxidation can reduce the oxidation of SO2 in the atmosphere, especially in the presence of calcite.

1. Introduction Recent atmospheric modeling studies have suggested that aerosol particles have a potentially important role in the chemistry of the troposphere by interacting with anthropogenic gaseous species such as SO2, NOx, and NH3 (1). SO2 can be oxidized to sulfuric acid and subsequently to sulfate in some processes. Sulfate aerosols can greatly influence the radiation balance and climate of Earth, directly by scattering solar radiation and indirectly by serving as cloud condensation nuclei, having a cooling effect (2-4). Most SO2 is converted to sulfate either in the gas phase by OH radicals (4) or in liquid droplets by ozone and hydrogen peroxide (2, 5). Model calculations suggest that aqueous-phase oxidation is dominant on a global basis (3). In addition, heterogeneous uptake of SO2 on mineral particles such as calcite with subsequent oxidation has been proposed in recent studies on the basis of laboratory experiments (6-9). Although heterogeneous SO2 oxidation has been suggested in observational studies (10-12), intermediate S(IV) species such as sulfite in the oxidation of SO2 to sulfate were not * Corresponding author phone: +81-824-24-7460; fax: +81-82424-0735; e-mail: [email protected]. † Hiroshima University. 10.1021/es900163y CCC: $40.75

Published on Web 08/25/2009

 2009 American Chemical Society

directly detected in previous studies. Thus, it is important to identify sulfite or other S(IV) species in natural aerosol samples to show that heterogeneous oxidation is really occurring in natural aerosols. A complete understanding of the SO2 oxidation processes, including heterogeneous oxidation, will eventually allow us to model the oxidation rate of SO2 as a function of the concentrations of oxidants and aerosols. However, it is not easy to detect S(IV) species in natural aerosols because of their instability during analysis by some indirect methods and interference from other species in the sample. Thus, a direct method with high selectivity is essential to identify S(IV) species in natural aerosols. Synchrotron-based X-ray absorption fine structure (XAFS) spectroscopy is a powerful technique used to investigate chemical speciation in environmental samples (13-15). XAFS near the absorption edge is called X-ray absorption nearedge structure (XANES) and provides information about the electronic structure of absorbing atoms reflecting the oxidation state and coordination environment of the element. Different elements have different absorption edge energies, which minimizes interference from other elements in the sample. According to our previous study (16), sulfur K-edge XANES is capable of distinguishing various types of sulfate, showing that the main sulfate was CaSO4 · 2H2O (gypsum) in coarse aerosol particles and (NH4)2SO4 in finer particles. Two XANES modes, fluorescence yield XANES (FY-XANES) and conversion electron yield XANES (CEY-XANES), can be applied effectively in the chemical speciation of aerosols. The former method can be regarded as a bulk analysis considering the size of aerosol particles (16, 17), while the latter is surface sensitive because of the shallow escape depth of Auger electrons from atoms irradiated by X-rays (18, 19). Coupling these two modes enabled us to distinguish sulfur species formed at the surface and in the bulk of aerosol particles. Furthermore, an attempt was made to quantify S(IV) species in aerosols by FY-XANES. The objective of this study was to understand the process of SO2 oxidation on aerosols by observing natural samples, which allows us to extend our knowledge of heterogeneous oxidation of SO2 in the atmosphere. In particular, the reaction of SO2 on calcium-rich mineral aerosols is likely to play an important role on the basis of previous laboratory studies (6-9). This reaction would be especially significant for regions in eastern Asia, where (i) SO2 is derived from fossil fuel combustion and (ii) mineral aerosols are supplied from western China. Therefore, Qingdao, a coastal city in northern China, was selected as a sampling site in this study. The major forms of sulfur in aerosols are sulfates because aerosol particles are suspended in an oxidizing environment, including OH, O3, and H2O2. Even if sulfites coexist in aerosols, the abundance of sulfates can be larger at the surface under oxic conditions. Thus, it is expected that S(IV) species are mainly in the interior of the particle, which can be assessed by the combination of FY- and CEY-XANES. This discussion may be related to various processes of incorporation of S(IV) species in aerosol particles such as (i) SO2 diffused to the particle interior prior to oxidation (6), (ii) deposition of some other constituents on the surface inhibits heterogeneous oxidation, and (iii) any minerals containing S(IV) originally present in the particle.

2. Materials and Methods 2.1. Sample Collection and Characterization. Aerosol samples used in this study were collected at Qingdao (36.07 N, 120.33 E) in China as part of the Japan-China joint project, “Asian Dust Experiment on Climate Impact” (20-22). Two VOL. 43, NO. 19, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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aerosol samples collected from August 11-17, 2001, and March 20-23, 2002, were compared. Reference aerosol samples were collected during the same seasons at Aksu (40°61′ N, 80°73′ E), close to the Taklimakan Desert, and Tsukuba (36°06′ N, 140°14′ E), 60 km northeast of Tokyo in Japan. Chinese loess particles, CJ-1 and CJ-2, as certified reference materials (23) were also compared. Isentropic backward trajectories were calculated using the HYSPLIT4 (HYbrid Single-Particle Lagrangian Integrated Trajectory) model (24). Model relative humidity (RH) along the trajectories of air masses was also evaluated with the help of the HYSPLIT4 model, based on the National Weather Service, National Centers for Environmental Prediction database. An Andersen-type low-volume air sampler (Shibata, AN200) was employed to obtain aerosol particles of various diameters. The sampler has eight stages and a backup filter. The particle size distribution based on the aerodynamic diameter was >11 µm (sampling stage 0), 11-7.0 µm (stage 1), 7.0-4.7 µm (stage 2), 4.7-3.3 µm (stage 3), 3.3-2.1 µm (stage 4), 2.1-1.1 µm (stage 5), 1.1-0.65 µm (stage 6), 0.65-0.43 µm (stage 7), and 1 µm). According to Kanai et al. (26), large-scale dust events were observed during the period of March 19-22, 2002 in China and Japan. It is suggested that the dust event originating in the arid and semiarid area in China increased concentrations of mineral aerosols. In contrast, the air masses in August had passed over the East China Sea and Pacific Ocean. The size distribution of aerosols collected in August showed a steady increase in mass fraction with smaller particles and then a significant jump in the finest fraction, implying a considerable decrease in the amount of mineral aerosols compared with March. However, the Asian dust contributing to the background component was observed even in summer in the middle free troposphere (>4 km) under the influence of remaining westerly winds originating from the western part of China (27). Average relative humidity (RH) was especially high in August (65% RH) compared with that of March (38% RH). Therefore, the humidity effect should be taken into account in Qingdao because the heterogeneous reaction of SO2 with Asian dust particles proceeds at high RH (28). 3.2. Sulfur Species in the Aerosol Samples. XANES spectra were compared with various reference materials (Table S1 of the Supporting Information). As observed in Figure 1a, the absorption edge usually shifts toward higher energy with an increase in the oxidation state. The electronegativity of atoms bound to sulfur influences the energy of the absorption edge (29, 30). XANES spectra indicate a high sensitivity to their chemical environment. As a result, sulfite as Na2SO3, K2SO3, and hannebachite has a peak at 2.4762 keV. It was also found that the XANES spectra of organic S compounds [e.g., dimethyl sulfide (DMS), hydroxymethanesulfonate (HMS), and methane sulfonic acid (MSA)] could be distinguished from other species. The spectrum for QMar-0 (sample at stage 0 collected in March) shown in Figure 1b exhibits characteristics that are almost identical to gypsum. It was clear that the main sulfur species was gypsum in mineral aerosols constituting coarse particles above 1 µm and (NH4)2SO4 in finer particles below 1 µm (e.g., QMar-6). The normalized spectra of aerosols could be deconvoluted by a linear combination (LC) of the spectra of gypsum and (NH4)2SO4 using a least-squares fitting (16). The LC fit was conducted in the energy range between 2.4820 and 2.5020 keV. The R value, the goodness-of-fit parameter, was calculated as follows R)

∑ [I

M(E )

- IF(E )]2 /

∑ [I

2 M(E )]

FIGURE 1. Normalized XANES spectra at the sulfur K-edge for (a) various sulfur species and (b) QMar and QAug samples at different particle sizes. The sample name QMar-0 denotes the sample at stage 0 collected at Qingdao in March. Dotted line curves show the spectra fitted by the linear combination (LC) of gypsum and (NH4)2SO4. (c) Gypsum fractions of QMar and QAug with different particle sizes determined by the LC fits. where IM and IF are the absorption of the measured and fitted spectra, respectively. The R values for the synthetic mixtures of gypsum and (NH4)2SO4 in different mixing ratios were less than 0.040. According to the LC fits (Figure 1c), gypsum fractions for QMar samples were 72-95% in the coarse particles (>1 µm). The mean R value of the coarse particles was 0.043, implying high-quality fitting. However, it must be noted that the identification of the compounds by this method is not definitive but merely shows the consistency with the proposed model assuming the end members. Thus, we also compared the XANES results with the bulk chemical analyses (Figure 2a), where both results are basically consistent. The amount of Ca2+ increased in particles with diameters greater than 1 µm, especially in March. Calcite (CaCO3) is known to be the most abundant calcium mineral of Taklimakan Desert sand (17, 31). It has been shown that gypsum is formed by the neutralization of CaCO3 by H2SO4 formed by the oxidation of SO2 emitted from coal combustion in urban areas of Qingdao (32, 33). In the fine particles originating mainly from anthropogenic sources, large concentrations of NH4+ were found in March, which correspond with the LC fits. The spectrum for QAug-0 (Figure 1b), however, showed that the postedge structure of gypsum was not clear compared with that of QMar-0. According to the LC fits for QAug samples, gypsum fractions were 37-80% in the coarse particles. A lower contribution from gypsum was consistent with the analyses of water-soluble components, where smaller amounts of Ca2+ and SO42- were detected in August (Figure 2b). Note that the spectra for QAug-0 and QAug-2 exhibited another peak at 2.4762 keV because of a different electron transition, and the peak corresponded to sulfite of various reference materials. These were not found in the finer particles (e.g., QAug-6) at all. The presence of sulfite only in the coarse particles can be evidence of heterogeneous uptake of SO2 on mineral aerosols. However, the S(IV) species (such as SO32- and HSO3-) were not detected using ion chromatography after extraction under N2 atmosphere. The results indicate two possibilities: (i) the concentration of sulfite in aerosols is quite low compared with that of sulfate and/or

(ii) S(IV) species are oxidized to SO42- during the analyses. S(IV) in the aqueous phase can easily be oxidized to S(VI), which makes it difficult to detect sulfite in aerosols using normal ion chromatography. Although S(IV) is unstable once dissolved in water, some sulfite salts remained to some degree in aerosols as stable solid phases in ambient air. In aerosols collected at Aksu and Tsukuba in August and March, however, such sulfite salts were not detected using XANES (Figure S3 of the Supporting Information). As discussed previously, with respect to the difference in probe depths, FY and CEY modes of XANES were employed. No sulfite aerosols were found in the CEY-XANES spectra (Figure 3a), showing that sulfite is not present at the surface of particles within 0.08 µm of the surface. This result is reasonable because sulfite on the surface could be oxidized easily to sulfate by oxidants such as O3 and H2O2 in the atmosphere. 3.3. Confirmation of Formation of Sulfite in Aerosol Samples. Because S(IV) species are not very stable compounds in ambient air, two treatments, heat treatment (Figure S4 of the Supporting Information) and a water addition treatment (Figure 3b), were conducted to confirm the presence of sulfite in QAug samples. In the first treatment, aerosols on filters were heated at 260 °C for 24 h. The sulfite peak in the spectrum of QAug-2 remained as in the original spectrum. This result suggests that sulfite in the interior of particles is quite stable, even in the atmosphere. To compare the difference in stabilities among various sulfite salts, we also measured XANES spectra under the same conditions for heated reference sulfite materials. The results showed that (NH4)2SO3 · H2O was completely altered to (NH4)2SO4 and other sulfite (or bisulfite) species were partially oxidized to sulfate. However, XANES of hannebachite did not show any change, showing that oxidation was not found for the hannebachite. We cannot completely rule out the presence of other sulfites because unstable sulfite may be able to remain beneath the surface. Still, these results indicate that hannebachite is the most probable sulfite in aerosols on the VOL. 43, NO. 19, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Chemical compositions of aerosols with different particle sizes. The water-soluble components were determined using ion chromatography for QMar samples (a-I, a-II) and QAug samples (b-I, b-II). Cations (left) and anions (right) are shown separately. The range of the ordinate is different for QMar and QAug. basis of the difference in stabilities of sulfites and lower solubility of hannebachite than other sulfites as discussed below. XANES measurements were repeated after dropping 10 µL Milli-Q water onto an aerosol spot on the filters of QAug-0 and QAug-2 and subsequent air drying. The sulfite peak was not observed after the treatment (Figure 3b-I), whereas the postedge features of gypsum, three peaks at 2.4835, 2.4900, and 2.4970 keV, became more noticeable in the XANES spectra (Figure 3b-II). Mole fractions of gypsum for QAug-0 and QAug-2 increased from 37% to 62% and from 54% to 84%, respectively. These results show that the sulfite is dissolved by Milli-Q water and that SO32-/HSO3- is oxidized to SO42-/ HSO4- in the aqueous phase, which caused reprecipitation of gypsum with dissolved Ca2+ during air drying. The solubility product (Ksp) of hannebachite is 2.29 × 10-7 (34), which is much smaller than that of gypsum, Ksp ) 2.53 × 10-5 (35), and other sulfites (36). This suggests that hannebachite is more readily subject to precipitation than gypsum if aqueous SO32- and SO42- are present at similar concentration ranges at the aerosol surface containing Ca2+. This fact, coupled with the two oxidation treatments, implied that sulfite, in particular hannebachite, can be present in mineral aerosols, especially on calcite. Assuming that hannebachite and gypsum are the dominant sulfite and sulfate, respectively, the mixing ratio of S(IV) and S(VI) can be provided by XANES spectra of mechanically mixed samples of the two reference materials with different mole ratios (Figure 4a). The peak for the hannebachite of the normalized XANES spectra was extracted using a spline function used to interpolate the background contribution. The S(IV) ratio could be estimated by simply measuring peak heights at 2.4762 keV for the normalized spectra after the extraction of the peak. A calibration curve for this method 7360

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is illustrated in Figure 4b, representing a linear correlation between the peak heights and mole ratio of hannebachite. Thus, it is possible to obtain the S(IV) ratio of each compound from XANES spectra. The limit of detection (LOD) and range of quantitation (ROQ) were calculated as follows (37) LOD ) K(σ/S) ROQ e K(σ/S) where K ) 3.3 for LOD and 10 for ROQ, σ is the standard deviation of the measurements, and S is the slope of a calibration curve. For the method, the LOD was 1.0% and the ROQ was 2.9%. According to our results, S(IV) ratios calculated from the FY mode were 4.7% and 9.6% in QAug-0 and QAug-2, respectively, which are above the LOD value. The results showed that 5-10% of sulfur species in coarse particles collected in August are present as sulfite. The S(IV) ratios of March samples and reference samples collected at Aksu and Tsukuba were below the LOD. Thus, the amount of unoxidized S(IV) is likely quite small on a global basis. 3.4. Atmospheric Implications. As shown in the Introduction, three processes can be assumed as the reasons for the presence of sulfite in the aerosols: (i) diffusion of SO2 into the particle interior, (ii) deposition of other materials on the surface to inhibit the oxidation of S(IV), and (iii) S(IV) in the particle initially as some minerals. Absence of S(IV) species in XANES spectra of loess particles, CJ-1 and CJ-2, collected in the loess plateau in China (Figure S5 of the Supporting Information) shows that process iii may not be important. Our XANES analyses suggested that hannebachite and gypsum can be found in natural aerosols, possibly as the products of SO2-calcite reactions. This suggestion is con-

FIGURE 4. (a) Normalized XANES spectra at the sulfur K-edge for the mixture of gypsum and hannebachite at different mole ratios. Relationship between the mole ratio of hannebachite and peak heights at 2.4762 keV is shown in panel b. A line obtained by a least-squares analysis is indicated with the correlation coefficient (r).

FIGURE 3. (a) Comparison of the sulfur K-edge XANES spectra measured in FY and CEY modes for the representative coarse particles collected at stage 0 (>11 µm) and stage 2 (4.7-7.0 µm). (b) Sulfur K-edge XANES spectra before and after water addition treatments for QAug-0 and QAug-2 are indicated by circles and a solid line, respectively. sistent with previous laboratory studies (6-9, 38) showing formation of hannebachite and its conversion into gypsum by successive gas-water-solid interactions: (I) dissolution of SO2 from gas into water present on the calcite surface, (II) dissolution of calcite in the water, (III) precipitation of hannebachite, (IV) partial dissolution of hannebachite in the water, and (V) formation of gypsum by oxidation of SO32-/HSO3- present in the water. Within this scenario, processes i and ii can be the reason for the presence of sulfite in aerosols.

It was also indicated that Asian dust particles over Japan were coated with a solution containing sulfur species (16, 39). The surfaces of mineral aerosols play an important role as a sink for gaseous species and reaction sites for heterogeneous oxidation of adsorbed species (40). Because a water layer around a particle can be more readily formed at higher RH, the uptake of SO2 would be enhanced in summer because of high RH (9, 28). Such seasonal variation was consistent with our results that sulfite was found in QAug but not in QMar. In addition, a high concentration of SO2 may be important to maintain sulfite to the extent that we could observe it. Anthropogenic emissions represent a much larger source of SO2; about 80% of the global source of SO2 is estimated to be anthropogenic in a normal year (41). Sulfite was observed for the samples in Qingdao but not for those in Aksu and Tsukuba, which might be due to the difference in their locations. Sulfite can be observed in the environment with a large emission of SO2, which is not the case in Aksu. A large supply of calcite is also needed for the formation of sulfite, which may not be the case in Tsukuba. The sample in Qingdao meets the two requirements with the influences of SO2 in the city and calcite transported from the western part of China. The fact that the heterogeneous oxidation is supported from the XANES data for natural aerosol samples is important because sulfite formed by the adsorption of SO2 has only been detected in laboratory studies so far. However, the contribution of the heterogeneous oxidation to the total rate of SO2 oxidation is not clear at present. This study suggests that the adsorption of SO2 on mineral aerosols without oxidation can reduce the oxidation of SO2 in the atmosphere, especially in the presence of calcite and anthropogenic SO2. More quantitative modeling of SO2 oxidation must be carried out in the future, including the heterogeneous oxidation of SO2 on aerosols. VOL. 43, NO. 19, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Acknowledgments We gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for provision of the HYSPLIT transport and dispersion model and the READY website (http:// www. arl.noaa.gov/ready.html). This research has been supported by a Grant-in-Aid for Scientific Research in Priority Areas “Western Pacific Air-Sea Interaction Study (W-PASS)” under Grant 19030010. This work has been performed with the approval of KEK-PF (Proposal 2004G119, 2004G334, 2006G116, 2007G669, and 2008G683). This research is a contribution to the Surface Ocean Lower Atmosphere Study (SOLAS) and a Core Project of the International Geosphere-Biosphere Program (IGBP).

Supporting Information Available Figures S1-S5 and Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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