Reactivity between PbSO_4 and CaCO_3 particles relevant to the modification of mineral particles and chemical forms of Pb in particles sampled at two remote sites during an Asian dust event

摘要

During the transboundary transport of anthropogenic heavy metals by mineral particles providing reaction sites, the divalent metal salt PbSO_4 can be converted to PbCO_3 in the presence of water. We carried out laboratory experiments to study the transformation process under various conditions by incorporating test particles comprising CaCO_3 of a particulate mineral component, PbSO_4, and NaCl. After the immersion of PbSO_4 particles in contact with CaCO_3 particles in a water droplet, the conversion of PbSO_4 into PbCO_3 was confirmed by the change in morphology of the original particles to stick or needle form; the percentages of the chemical forms relative to the total Pb were determined by X-ray absorption near edge structure (XANES) analysis. Approximately 60-80% of PbSO_4 was converted to PbCO_3 after 24 h. A small amount of stick particles was detected when NaCl particles attached to PbSO_4/CaCO_3 particles were exposed to air with a relative humidity (RH) of 80-90% for 24 h. XANES measurements of the samples revealed that the molar percentage of PbCO_3 relative to the total Pb content was 4%. Field experiments were also conducted to determine the chemical forms of the Pb particles during the Kosa (Asian dust storm) event. Samples were collected from two remote sites in Japan and Korea. The mass size distribution of Pb aerosols collected in Japan was bimodal with two peaks in the coarse mode; the enrichment factor of Pb suggested that its source was anthropogenic. Pb L3 edge XANES measurements of both samples indicated that they had similar shapes. These measurements also indicated that the major Pb components for the samples collected in Japan were PbO, PbSO_4 PbCl_2, and PbCO_3, with molar percentages of 44%, 30%, 21%, and 5%, respectively. No significant differences were found between the component ratios of the samples collected in Japan and Korea, suggesting that definite transformation did not occur during the transport of the Kosa particles from Korea to Japan. On the basis of these observations, we postulate that the transformation process either occurred mainly before the particles arrived at Korea or did not take place after the particles left continental Asia.