Neutralization of Calcite in Mineral Aerosols by Acidic Sulfur Species

Aug 5, 2009 - Neutralization of Calcite in Mineral Aerosols by Acidic Sulfur Species Collected in China and Japan Studied by Ca K-edge X-ray Absorptio...
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Environ. Sci. Technol. 2009, 43, 6535–6540

Introduction

coal burning in China (1-3). Among the various effects caused by sulfate aerosols (1-4), acidification can be mitigated by the neutralization of the acidic SOx species (SO2, sulfuric acid, and acidic sulfate salts in this study) by alkaline salts such as calcium carbonate in mineral aerosols (e.g.. refs 5-8) that can be produced by dust events in Mongolia and western China. Although various studies have indicated the neutralization effect of such mineral aerosols (2, 5-8), these results have been derived mainly from indirect chemical analyses of aerosol samples, from which it may not be easy to determine the chemical processes occurring in aerosols. It is necessary, therefore, to obtain a clearer understanding of the neutralization effect by speciation of the Ca and S in mineral aerosols. X-ray absorption near-edge structure (XANES) is effective in the speciation of Ca. Takahashi et al. (9) reported that Ca K-edge XANES can determine the ratio of gypsum to total calcium [Gyp]/[Ca2+]t in aerosols collected in Aksu, near the Taklimakan Desert, where [Gyp] and [Ca2+]t represent the abundances of gypsum and total Ca species. In their study, it was found that the [Gyp]/[Ca2+]t ratio was around 0.1 for mineral aerosols during the dust period (April 2002), whereas the ratio during the nondust period in winter (January 2002) was more than 0.8 for samples with a particle size from 0.65 to 3.3 µm. Considering the large amount of SO2 emissions during winter in China, the much larger [Gyp]/[Ca2+]t ratio in winter may be related to the neutralization of SOx species by calcite and the formation of gypsum. It was also shown that information on the Ca species in the bulk and at the surface of mineral aerosol particles can be provided by the fluorescence (FL) and the conversion electron yield (CEY) modes, respectively, in the XANES measurement. Although the neutralization effect was indicated in the previous study based on the seasonal variation at Aksu (9), it was not confirmed whether or not the neutralization effect occurs during the long-range transport of mineral aerosols to East Asia from western China and other source areas. In this study, similar XANES methods were applied to aerosol samples simultaneously collected at Aksu, Qingdao, and Tsukuba during a large dust event on 20 March, 2002, which is well documented in other studies (10-12). Aksu is close to the Taklimakan Desert, an important source area of mineral dust during that event, although the Gobi Desert was the main dust source (10-12). Qingdao is a typical large city in east China, and Tsukuba is on the main island of Japan. The samples were collected as part of the Japan-China joint project, “Asian Dust Experiment on Climate Impact (ADEC)” (13). During the ADEC project, the dust event of March 2002 was one of the largest events simultaneously observed in Aksu, Qingdao, and Tsukuba (10-12). It can be regarded as a series of events because of the long-range transport of mineral aerosols from the Taklimakan Desert to Qingdao and subsequently to Tsukuba by the East Asia westerlies. The pH values of water in contact with the aerosol samples and the sulfur speciation data related to the abundance of calcium in the aerosols were also considered as part of this study.

Large amounts of sulfate aerosol are produced in East Asia as a result of large emissions of SO2, mainly derived from

Materials and Methods

* Corresponding author e-mail: [email protected]. † Department of Earth and Planetary Systems Science, Hiroshima University. ‡ Laboratory for Multiple Isotope Research for Astro- and Geochemical Evolution (MIRAGE), Hiroshima University. § Geological Survey of Japan.

Samples. Aerosol samples were collected at Aksu, Qingdao, and Tsukuba (Supporting Information (SI) Figure S1) in the ADEC project from 2000 to 2005 (13-15). Samples at the three sites were also collected during a large dust event recorded from 20 to 22 March, 2002 (SI Table S1). Samples collected at the same sites in winter (January 2002) without

Neutralization of Calcite in Mineral Aerosols by Acidic Sulfur Species Collected in China and Japan Studied by Ca K-edge X-ray Absorption Near-Edge Structure Y O S H I O T A K A H A S H I , * ,†,‡ TAKURO MIYOSHI,† MASAYUKI HIGASHI,† HIKARI KAMIOKA,§ AND YUTAKA KANAI§ Department of Earth and Planetary Systems Science, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan, Laboratory for Multiple Isotope Research for Astro- and Geochemical Evolution (MIRAGE), Hiroshima University, Hiroshima 739-8526, Japan, and Research Center for Deep Geological Environments, Geological Survey of Japan, AIST, Central 7, Higashi, Tsukuba, Ibaraki 305-8567, Japan

Received April 5, 2009. Revised manuscript received July 15, 2009. Accepted July 20, 2009.

Calcium species in mineral aerosols collected simultaneously in Aksu (near the Taklimakan Desert), Qingdao (eastern China), and Tsukuba (Japan) during dust and nondust periods were determined using Ca K-edge X-ray absorption near-edge structure (XANES). From the fitting of XANES spectra, it was found that (i) calcite and gypsum were the main Ca species in the aerosol samples, and (ii) the gypsum fraction versus total Ca minerals [Gyp]/[Ca2+]t increased progressively in the order Aksu < Qingdao < Tsukuba. Surface-sensitive XANES in the conversion electron yield mode (CEY) showed that the gypsum is formed selectively at the surface of mineral aerosols for all the samples except for that taken in Aksu during the dust period. The decrease of the [Gyp]/[Ca2+]t ratio with an increase in particle size showed that the neutralization effect proceeds from the particle surface. For the Aksu sample in the dust period, however, (i) the [Gyp]/[Ca2+]t ratios obtained by XANES measured in the fluorescence (FL; regarded as bulk analysis) and CEY modes were similar and (ii) size dependence was not found, showing that neutralization is not important for the sample because of the large supply of mineral aerosol with little neutralization effect in Aksu. It was also found that the pH of the aerosol and the ratio of (NH4)2SO4 to gypsum were positively and negatively correlated with the Ca (or calcite) content, respectively. The speciation of Ca by XANES revealed the neutralization processes of acidic sulfur species by calcite during the long-range transport of mineral aerosols.

10.1021/es9010256 CCC: $40.75

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associated intense dust events were also studied for comparison (SI Table S1). All the samples were collected using a low-volume Andersen-type air sampler (AN-200, Sibata, Tokyo) to obtain size-fractionated aerosol samples. The sampler had eight stages and a backup filter. The particle size classification by aerodynamic diameter is as follows: > 11 µm (sampling stage 0; 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; ref 3) in the atmosphere during the dust period were much higher than those during the nondust period by factors of 16, 7.0, and 3.6 in Aksu, Qingdao, and Tsukuba, respectively. Air mass backward trajectories during the sampling period in March 2002 in Qingdao and Tsukuba were computed using a NOAA hybrid single-particle lagrangian integrated trajectory (HYSPLIT) model at 1-3 km altitude (http://www.arl.noaa.gov/ ready/). As a result (SI Figure S1), it was revealed that the air mass in the vicinity of Aksu on 20 March arrived in Qingdao and Tsukuba on 21 and 22 March, respectively. These results are consistent with other studies (10-12), showing that the Taklimakan Desert was an important source area for the 20 March western China dust event transported to east China and Japan. Consistent with these facts, the mass concentrations of mineral aerosol decreased successively from Aksu > Qingdao > Tsukuba (SI Figure S2). Based on these results, it is suggested that the samples collected in this study at the three sites can trace the chemical transformation of elements in the mineral aerosols during their long-range transport from west to east China (Qingdao) and Japan (Tsukuba). It must be noted that source areas other than the Taklimakan Desert also contributed to the supply of mineral aerosol to east China and Japan during the 20 March dust event (10-12). However, the calcite contribution from all the source areas is likely much greater than that of other Ca minerals (21-24). Thus, the discussion on the neutralization effect of calcite given later in this paper would similarly apply to mineral aerosols derived from alternative source areas. Among the various species measured using ion chromatography (SI Figure S2), the ratio of SO42- to Ca2+ is of interest with respect to the neutralization effect of SOx species by mineral aerosols or calcite (5-9, 12). It was evident that the [SO42-]t/[Ca2+]t ratio in the dust period increased in the order of Aksu (0.41) < Qingdao (0.58) < Tsukuba (2.0). This result may indicate that calcite in Aksu was abundantly supplied from the Taklimakan Desert, whereas the [SO42-]t/[Ca2+]t ratio was larger in Qingdao and especially in Tsukuba because of the neutralization of calcite by SOx species and the formation of gypsum and other sulfates. NH4+ is mainly available to the finer fractions of mineral aerosols, suggesting that (NH4)2SO4 is mainly formed as sulfates in these fractions as demonstrated by sulfur K-edge XANES, discussed later in this paper. A similar discussion based on the bulk chemical analyses has been included in various studies (5-8). However, as that

FIGURE 1. (a) Normalized Ca K-edge XANES spectra of Ca reference materials (gypsum, calcite, Ca(NO3)2, and anorthite) and some aerosol samples collected in Qingdao (Jan-1, Jan-4, March-1, and March-4). The sample name Jan-1, for example, denotes the sample at Stage-1 collected in January. An example of the fitting of the sample spectrum by the linear combination of calcite and gypsum (dotted lines) was also indicated for March-1. A spectrum was obtained for the desert sand in the Taklimakan Desert also used in ref 21. (b) Normalized Ca K-edge XANES spectra for some samples in both the FL and the CEY modes. Ak: Aksu; Qd: Qingdao; Tk: Tsukuba. Broken lines indicate the fitted spectra obtained by the combination. information is derived indirectly, we cannot prove that gypsum is actually present as a product of the neutralization effect in the mineral aerosols. Direct measurement of such components is necessary to depict the neutralization processes more precisely. Calcium Species in the Mineral Aerosols. The neutralization process indirectly suggested in the bulk chemical compositions is clearer from the results of Ca speciation by XANES. The spectra of the samples at Stage-1 and Stage-4 collected in Qingdao in January and March are shown with some reference spectra (Figure 2a; spectra for other reference materials are shown in SI Figure S3). The variation in the XANES region at the Ca K-edge has been known to be sensitive to the local structural environment of Ca because of the multiple scattering of photoelectrons, by which we can identify the Ca species (9, 25, 26). For all the mineral aerosols studied here (particle size >1.1 µm), the spectra can be perfectly fitted by a combination of those of calcite and gypsum by least-squares fitting in the energy range between 4031 and 4049 eV using a routine supplied in REX2000 (Rigaku Co., Tokyo, Japan). The inclusion of silicate minerals such as anorthite and other Ca species such as apatite and Ca(NO3)2 did not improve the fit, showing that the contributions in the samples of other Ca species are minimal. Hence, the fit of the spectra of mineral aerosols allowed us to estimate the ratio of calcite to gypsum, or the gypsum fraction among total Ca ([Gyp]/[Ca2+]t; mole ratio), as [Ca2+]t is approximately equal to the sum of calcite and gypsum for the samples studied here. The precision of the [Gyp]/[Ca2+]t value determined by the fit was estimated to be better than 6.5% obtained from the analyses of reference samples with known [Gyp]/[Ca2+]t ratios (9). The results obtained by the fit were consistent with those expected from the bulk chemical analyses. In the Aksu

samples it was estimated that more than 85% of the Ca in the mineral aerosols during the dust period was calcite (Figure 2; all the spectra are shown in SI Figure S3). This result is consistent with the main Ca mineral in the Taklimakan Desert being calcite, as estimated from the XANES spectrum in this study (Figure 1a) and as suggested by other studies (8, 21, 22). The consistency shows that the mineral aerosols collected at Aksu were directly transported from the source area without significant chemical transformation during the transport in the dust period. When comparing the three stations, the [Gyp]/[Ca2+]t ratio for the mineral aerosols in the dust period (Figure 2) increased progressively as Aksu < Qingdao < Tsukuba, or in the direction of the dust movement, suggesting that the conversion of calcite into gypsum occurred during the long-range transport of mineral aerosols containing calcite. As described previously, the source area of the dust event was not only the Taklimakan Desert; the Gobi Desert was also an important source area (10-12). However, taking account of the dominance of calcite as a Ca mineral in these source areas (22-24), we may suggest that the [Gyp]/[Ca2+]t ratio during the dust period should increase progressively from “Aksu and the source areas of mineral aerosols” < Qingdao < Tsukuba, because of the long-range transport neutralization effect. In the nondust period (January), the [Gyp]/[Ca2+]t ratios were greater than those during the dust period in the three stations because of the neutralization of SOx species by calcite in the particles. This result is reasonable considering that (i) SO2 emissions tend to be larger in the winter season especially in China (27-29), (ii) the amount of mineral aerosol calcite is lower during the nondust period than during the dust period, and (iii) gypsum is formed by the neutralization of SOx species by calcite. VOL. 43, NO. 17, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Gypsum fraction among total calcium [Gyp]/[Ca2+]t at various particle sizes in Aksu, Qingdao, and Tsukuba during the dust (March 2002) and nondust (January 2002) periods. The results obtained in both the FL and the CEY modes are shown. If the neutralization reaches a certain depth below the surface of the calcite particles, it is suggested that the [Gyp]/ [Ca2+]t ratio will be larger for smaller particles because of a more thorough reaction within it. As indicated in Maria et al. (30), the size dependence suggests that the neutralization reaction is relatively faster than diffusion of SOx species into the interior of the calcite particle. The size effect was observed for the samples taken during the nondust period in Aksu and during both dust and nondust periods in Qingdao and Tsukuba. The absence of the effect during the dust period in Aksu also shows that the neutralization effect is negligible for the samples collected during the dust event because of the large supply of mineral aerosols. Comparing the results in the three sites (Figure 2), the size-dependence trace for Tsukuba is relatively flatter, suggesting that the neutralization reaches the core of calcite particles, possibly because the Tsukuba samples were transported longer distances than those in Aksu and Qingdao. Ca Species at the Surface of Particles Revealed by CEYXANES. Although the transformation of calcite to gypsum is suggested for the mineral aerosols, it is possible that gypsum comes directly and locally from terrestrial and marine sources near the sampling site (31). If we can show that the gypsum is selectively found at the surface of mineral particles, however, the formation of gypsum by reaction with SOx components in the atmosphere may be strongly suggested. For this purpose CEY-XANES can be a powerful tool because the method is sensitive to Ca minerals at a particle surface to depths of less than 0.25 µm (9, 32). On the other hand, the FL mode, with a probing depth of 5.4 µm is considered to be a bulk analysis, because most of the aerosol samples are smaller than 10 µm. The spectra obtained in the CEY mode can again be fitted successfully by a combination of calcite and gypsum. The gypsum fractions determined in the CEY mode were compared with those measured in the FL mode (Figure 1b). For a synthetic mixture of gypsum and calcite with similar particle sizes prepared from reagents, the [Gyp]/ [Ca2+]t ratio determined using CEY-XANES was identical to that determined using FL-XANES. On the other hand, if gypsum is selectively formed at the surface, the [Gyp]/[Ca2+]t ratio determined using CEY-XANES should be larger than that using FL-XANES because of the difference in their probing depths. XANES spectra in the CEY mode for desert sand in Taklimakan Desert and Chinese loess particles in the Loess Plateau are similar to those in the FL mode (SI Figure S4), 6538

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suggesting that Ca minerals at the surface and in the bulk are identical and that the contribution of such local particles to the dust can give different results from those transported the long distance. Such effects may be reflected in the Aksu dust period samples in which the [Gyp]/[Ca2+]t ratio in the CEY mode was similar to that in the FL mode (Figure 2). The results show that gypsum was not formed by the reaction at the particle surface, but was directly transported from the source area without chemical transformation during the transport. This result is consistent with low [Gyp]/[Ca2+]t ratios in the FL mode and also with the flat size dependence as suggested in the previous section. On the other hand, it was evident that the [Gyp]/[Ca2+]t ratios in the CEY mode were systematically larger than those in the FL mode in Qingdao and Tsukuba by 20-30% during the dust period. Considering the possible error in the fit (6.5%), the difference is significant, which shows that gypsum is distributed selectively on the surface of the mineral aerosol particles collected in Qingdao and Tsukuba. These results strongly suggest that gypsum in the mineral aerosols in Qingdao and Tsukuba was formed by the reaction in the atmosphere at the surface of calcite during long-range particle transport from external source areas. The [Gyp]/[Ca2+]t ratios in the CEY mode were also larger than those in the FL mode for samples collected during the nondust period at the three sites. Because the difference suggests the conversion of calcite into gypsum in the atmosphere, the results during the nondust period imply that calcite, which can neutralize SOx species, was also present in background mineral aerosols during the nondust period (33), even though its concentrations should be less than those during the dust period. This suggestion is consistent with the [Gyp]/[Ca2+]t ratio determined using FL-XANES showing a progressive increase from Aksu < Qingdao < Tsukuba even during the nondust period (Figure 2). As shown here, a combination of the FL- and CEY-XANES approaches can be an important way to consider the sources of mineral aerosols based on the selective detection of reaction products in the aerosol particle surfaces. The Effect of Neutralization on pH and Sulfur Compounds. The neutralization effect must be related to the pH of water equilibrated with the aerosol samples. For such a pH analysis, a piece of filter (1 × 1 cm2) used for the highvolume air sampler was soaked in 10 mL of Milli-Q water. The pH of the solution measured by a pH meter (D-21, Horiba, Kyoto) was compared with the total Ca abundance [Ca2+]t

FIGURE 3. (a) Relationship of [Ca2+]t with pH and [(NH4)2SO4]t/[Gyp]t. The sample in Qingdao during the dust period was not available for the pH measurement. (b) Sulfur K-edge XANES spectra at various particle sizes (Stg: Stage) for the sample in Qingdao during the nondust period (January 2002) with those of gypsum and (NH4)2SO4. Broken lines indicate the fitted spectra obtained by the combination. The absolute amounts of gypsum and (NH4)2SO4 determined by the fitting of S- and Ca- K-edge XANES spectra and ion chromatography data are given in the inset figure. initially supplied as calcite at the three sites (Figure 3a). It was clear that the pH value increased as [Ca2+]t in the aerosols increased because (i) the pH value progressively increased in the order Tsukuba < Qingdao < Aksu in relation to the calcite supplied to each site and (ii) a higher pH was observed in the dust period because of the larger supply of calcite and a lesser degree of the neutralization effect during the dust period. These results show that the calcite neutralization effect alters the acidity of water in contact with the aerosols. The effect of the degree of neutralization on sulfur species is also discussed here, because it produces a large amount of gypsum in the mineral aerosols. In a manner similar to the calcium case, the sulfur species were determined based on sulfur K-edge XANES. For example, the spectra for the aerosols collected in Qingdao during the dust period were shown with the spectra of gypsum and ammonium sulfate, (NH4)2SO4 (Figure 3b). Takahashi et al. (20) demonstrated that the postedge structure is sensitive to the sulfate formed in the sample. Consequently, it was found that the spectra can be well fitted by a combination of gypsum and (NH4)2SO4 (20). The linear combination of the two end members thus enabled us to obtain the main sulfur compounds in the aerosols. Coupled with sulfate content data from ion chromatography, the concentrations of gypsum and (NH4)2SO4 in the atmosphere were obtained (an example is shown in Figure 3b and SI Figure S5). The ratio of total (NH4)2SO4 and gypsum [(NH4)2SO4]t/[Gyp]t including all the size fractions was less for the sample having a larger Ca abundance (Figure 3a). The (NH4)2SO4 mainly found in smaller particles (Figure 3b) is formed primarily as a result of the reaction of gaseous NH3 and SOx compounds in the atmosphere (3, 20). Conversely, the amount of gypsum distributed mainly within larger particles increases when the supply of calcite increases especially during the dust period. It is thus suggested that the [Gyp]t generally increases with an increase in [Ca2+]t, whereas [(NH4)2SO4]t is relatively independent of [Ca2+]t. The results therefore indicate that the neutralizing agent of SOx

derivatives in the atmosphere can vary from NH3 to calcite, depending on the supply of Ca, or calcite, to each site. Finally, the masses of calcite and SO2 emitted from China are compared (34-36). It is estimated that about 800 Tg/y of mineral dust is released from the arid area in China (35). The calcium content in the dust and the carbonate fraction of [Ca2+]t were roughly about 12 wt.% and 15 mol.% (Figure 2), respectively, indicated by the Aksu data in the present study and Yabuki et al. (14). Accordingly, 0.36 Tmol/y of calcite is supplied into the atmosphere, which can neutralize 0.36 Tmol/y of SOx species assuming a 1:1 reaction of calcite and SOx compounds. Considering the total annual emission of SO2 in China in 2000 (20.4 Tg/y ) 0.32 Tmol/y; ref 36), about 89% of SOx compounds can be neutralized by calcite in China. The Tsukuba site is located on the eastern edge of East Asia. Thus, the calcite fraction within the [Ca2+]t () [Cal]/ [Ca2+]total) in Tsukuba can be considered as the final value of calcite remaining in the mineral dusts from China. According to the FL data (Figure 2), the total [Gyp]/[Ca2+]t value of all the size fractions was about 70-80% and 60-70% in the dust and nondust periods, respectively, which is roughly consistent with the estimated value of 89%. As shown here, Ca and S speciation by XANES is a powerful tool to quantify the neutralization of calcite by SOx species in the atmosphere.

Acknowledgments This study was partly supported by the Special Coordination Funds of the Ministry of Education, Culture, Sports, Science and Technology of Japan and the W-PASS project. This work has been performed with the approval of the Photon Factory (Proposal No. 2006G116, 2007G669, 2007G670, and 2008G683).

Supporting Information Available One table, six figures, and discussion on the sampling of aerosols. This material is available free of charge via the Internet at http://pubs.acs.org. VOL. 43, NO. 17, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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