2012 年 27 巻 1 号 p. 41-50
In this manuscript, we briefly introduce X-ray absorption fine structure (XAFS: X-ray Absorption Fine Structure) as a speciation method of metal ions in aerosols. As an application, research on the Ca and Zn speciation related to their metal-oxalate complexes is introduced. Oxalic acid, one of the most dominant components of organic aerosols, has cooling effect on the Earth’s radiation budget by acting as cloud condensation nuclei (CCN) activator. However, it is uncertain whether the oxalic acid can exist as protonated form, dissociated form, or metal complexes in aerosols, though there is a large difference in their water-solubility. In this sense, XAFS is a powerful tool to quantify the amount of metal-oxalate complexes. As a result, Ca and Zn oxalates were observed in finer particles from Ca and Zn K-edge XAFS. Based on the total concentrations of oxalate, Ca, and Zn with speciation of Ca and Zn determined by XAFS, we found that Ca and Zn oxalate fractions reached about 20-80% of total oxalate in finer particles. Since Ca and Zn oxalates are highly insoluble, the results showed that the complexes cannot act as CCN activator and that the ability of oxalic acid as CCN activator must be evaluated by considering the formation of insoluble metal oxalate complexes.