Detection of S(IV) species in aerosol particles using XANES spectroscopy.

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.