Environ. Sci. Technol. 2006, 40, 5052-5057
Speciation of Sulfate in Size-Fractionated Aerosol Particles Using Sulfur K-Edge X-ray Absorption Near-Edge Structure Y O S H I O T A K A H A S H I , * ,†,‡ Y U T A K A K A N A I , § HIKARI KAMIOKA,§ ATSUYUKI OHTA,§ HIROSHI MARUYAMA,† ZHIGUANG SONG,| AND H I R O S H I S H I M I Z U †,‡ Graduate School of Science and Laboratory for Multiple Isotope Research for Astro- and Geochemical Evolution (MIRAGE), Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan, Geological Survey of Japan, AIST, Central 7, Higashi, Tsukuba, Ibaraki 305-8567, Japan, and State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 510640 Guangzhou, China
X-ray absorption near-edge structure (XANES) at sulfur K-edge was used to determine the sulfate species present in size-fractionated aerosol particles based on the postedge structure after the main absorption peak in the XANES region. A comparison of the XANES spectra of reference sulfate materials and aerosol samples collected in Tsukuba in Japan clearly showed that (NH4)2SO4 was the main sulfur species in particles with a smaller diameter and CaSO4‚2H2O (gypsum) was the main sulfur species in particles with a larger diameter. A simulation of the XANES spectra by reference materials allows us to obtain the quantitative mixing ratios of the different sulfate species present in the aerosol samples. The presence of minor sulfur species other than (NH4)2SO4 and gypsum at the surface of mineral aerosols is suggested in our simulations and by a surface-sensitive conversion electron/He-ion yield XANES. In the absence of a contribution from a large dust event, the mole concentration of gypsum in the mineral aerosol fraction (particle diameter > 1 µm) determined by XANES is similar to that of Ca which is determined independently using ion chromatography. This shows that the Ca and sulfate in the mineral aerosols are present only as gypsum. Considering that calcite is the main Ca mineral in the original material arising from an arid and semiarid area in China, it is strongly suggested that gypsum is formed in aerosol during its long-range transportation by a reaction between calcite and sulfate ions.
1. Introduction Sulfate ions are a main constituent of aerosols, which can cause various environmental problems. For example, it is * Corresponding author phone: +81-82-424-7460; fax: +81-82424-0735; e-mail:
[email protected]. † Graduate School of Science, Hiroshima University. ‡ Laboratory for Multiple Isotope Research for Astro- and Geochemical Evolution (MIRAGE), Hiroshima University. § Geological Survey of Japan, AIST. | Chinese Academy of Sciences. 5052
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suggested that sulfate ion is an important component affecting radiative forcing (1, 2), respiratory health (3), and the acidification of soils and natural waters (3). In the evaluation of the influence of sulfate aerosols, it is necessary to determine what sulfate ions are contained in these aerosols. For example, the hygroscopic behavior of each sulfate species, e.g. (NH4)2SO4, Na2SO4, and so on is different (4), and this could influence radiative forcing differently, since the optical properties of aerosol are a function of its water content (57). In terms of acid deposition, the acidic fraction of sulfate ions in aerosol needs to be determined to assess its contribution as dry deposition to the acidification of the environment. Hence, the speciation of sulfate ions in aerosol is of great interest from various environmental viewpoints. Electron microscopy and X-ray diffraction (XRD) are frequently employed physiochemical methods for the speciation of elements in aerosol (7). However, electron microscopy can have several disadvantages, such as its semiquantitative nature, its low sensitivity toward lighter elements, and the necessity of employing high vacuum techniques, which may alter the volatile species in aerosol. Because an individual aerosol particle normally consists of various components, a clear identification of each mineral is not straightforward. The latter method, XRD, has been used as a direct speciation method of sulfate in many studies (3). However, XRD cannot be applied to amorphous or poorly crystalline compounds. In addition, the necessity to employ a large amount of sample makes it difficult for XRD to be applied to the small amount of size-fractionated samples collected by an Andersen-type sampler. Therefore, the establishment of any other methods for the speciation will contribute to the better characterization of sulfate species and related chemical processes in aerosols. X-ray absorption near-edge structure (XANES) has been used for the speciation of many elements in environmental samples due to its ability to determine the oxidation state and symmetry of the element (8, 9). However, to the best of our knowledge, the application of XANES to sulfur in aerosols has not been conducted yet. This could be due to the dominance of sulfate ions in most aerosols. One may think that more information cannot be extracted by the application of XANES, which is normally regarded as a method for examining the oxidation state of the element (10, 11). However, as applied to sulfate species in carbonates by Pingitore et al. (12), sulfur K-edge XANES is capable of distinguishing various sulfate species, depending on the countercation in the sulfate salt. Therefore, the aim of this study was to clarify what information we could elucidate from sulfur K-edge XANES as a speciation method for sulfur, or sulfate, in aerosol samples, especially for the sizefractionated samples collected by an Andersen-type sampler. In addition, we applied two detection methods to measure the spectra: fluorescence XANES and conversion electron/ He-ion yield (CEY) XANES (13, 14). The former method is considered to be a bulk analysis, since the analyzed depth is in the order of 10 µm. On the other hand, the latter method is a surface sensitive method, with an analyzed depth of around 10 nm (13, 14). Therefore, coupling these two methods enables us to estimate the differences in the sulfate species at the surface and in the core of aerosol particles.
2. Sample Analysis and Experimental Procedures 2.1. Sample and Reference Materials for XANES Analysis. The aerosol samples used in this study were collected at Tsukuba City in Japan (36.06N, 140.14E, 60 km northeast of Tokyo, collection height ) 44 m) as part of the Japan-China 10.1021/es060497y CCC: $33.50
2006 American Chemical Society Published on Web 07/12/2006
joint project, “Asian Dust Experiment on Climate Impact”, conducted from 2000 to 2005 (15). Two samples, T1 and T2, were examined in this study. Sample T1 was collected during April 8-12, 2002, during which a large dust event in East Asia took place. According to Yamada et al. (16), through the application of a meteorological simulation developed by Uno (Chemical Weather Forecasting System (CFORS) (17)), Kosa particles (Kosa literally means “yellow sand” in Japanese) were transported from the Taklimakan Desert in China to Japan during the collection period of T1 (denoted as the “Kosa period” hereafter). In contrast, there was no large Kosa event during the collection periods of sample T2 (April 24May 8, 2002) (i.e., non-Kosa periods). In our study, the samples were collected using a lowvolume Andersen-type air sampler (AN-200, Shibata, Tokyo) to obtain sulfur species in the aerosol particles with various diameters. The sampler had eight stages and a backup filter. The particle diameter classification by aerodynamic diameter was as follows: >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), 0.650.43 µm (stage 7), and
