Observation of transformation of calcite to gypsum in mineral aerosols by Ca K-edge X-ray absorption near-edge structure (XANES)

Calcium K-edge XANES was employed for the speciation of Ca in mineral aerosols collected by an Andersen-type air sampler in Aksu, close to the Taklimakan Desert in China. The sampling was conducted in April 2002 when dust events occurred. Additionally, the sample collected in January 2002 was studied for comparison purposes as well. It was found that the XANES spectra of mineral aerosols can be well simulated by those of calcite and gypsum, which can provide the ratio of gypsum to calcite (or total Ca minerals) in the samples. Moreover, we measured the XANES in both the fluorescence (FL) and conversion electron/He-ion yield (CEY) modes, which are bulk and surface-sensitive analysis, respectively. Based on the XANES data, several conclusions were deduced in this study: (1) the similarity of spectra recorded in the two modes in April suggests that the gypsum fraction ( 10%) in the period reflects the ratio of gypsum to the total Ca minerals in the source area without a significant effect of the formation of the secondary gypsum produced at the surface of the mineral aerosols; (2) a larger gypsum fraction detected in the CEY mode than in the FL mode in January reveals that gypsum is mainly found at the particle surfaces, suggesting that the gypsum in this period was mainly produced secondarily by the reaction of calcite with the acidic SOx species (=SO2, sulfuric acid, and acidic sulfate salts) during the transport; (3) considering the neutralization of acidic SOx species by calcite in mineral aerosols, the Ca speciation data obtained here may be explained by (i) the large supply of mineral aerosols including calcite in April as well as (ii) the large anthropogenic emission of SO2 generally found in winter in China (which is in January).