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

Abstract

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.

Depth-dependent Calcium Speciation in Individual Aerosol Particles by Combination of Fluorescence Yield and Conversion Electron Yield XAFS Using X-ray Microbeam

“…In this paper, we report analysis of aerosol samples with particle diameter 3.34.7¯m. Bulk Ca speciation of the same samples was already reported in Takahashi et al 9 Details of the sampling locations etc. were given in Supporting Information.…”

“…It is expected that the degree of neutralization of calcite is much lower than that in eastern China, such as in Qingdao. Bulk FYand CEY-XANES spectra using a millimeter-size beam for the Aksu sample 9 showed that the Ca species in the bulk samples are mainly calcite and gypsum. The gypsum fractions of the bulk analysis determined by the FY-and CEY-modes were almost identical i.e.…”

“…8 Takahashi et al observed the neutralization between CaCO 3 and H 2 SO 4 in dust during its long-range transport from a desert area in western China to Japan. 9 They analyzed samples collected in Aksu (near the Taklimakan Desert), Qingdao (urban area in eastern China), and Tsukuba (eastern Japan) during a large dust event in March, 2002, using sulfur (S) and Ca speciation by S and Ca K-edge X-ray absorption near-edge structure (XANES) spectroscopy, or the structure near absorption edge in X-ray absorption fine structure (XAFS). 10 The main Ca species determined by the spectra were calcite (CaCO 3 ) and gypsum (CaSO 4 ¢2H 2 O) and the mole ratio of gypsum to total Ca (= measured by ion chromatography) were estimated.…”

“…10 Hence, the combination of FY-and CEY-modes makes it possible to determine Ca species in the whole and at the surface of each particle. 9,10 XANES spectra obtained were fitted by a linear combination of spectra of reference materials as reported in Takahashi et al 10 to estimate mole ratios of Ca species present in individual particles. The least-squares fitting of the spectra was conducted using REX2000 (Rigaku Co., Tokyo, Japan).…”

“…The result of average gypsum fraction by XAFS using mm-size X-ray beam for particles on the aerosol filter in FY-and CEY-modes were 37 and 55 mol %, respectively, for the particles having diameter 3.3 4.7¯m. 9 This result suggested that gypsum can be formed selectively at the particle surfaces, but it is not clear whether the neutralization reaction occurs at the surface of calcite, because a possible interpretation is that gypsum is simply coated on any particles. This ambiguity can be excluded using the depthdependent μ-XAFS.…”