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Oxalate metal complexes in aerosol particles: Implications for the hygroscopicity of oxalate-containing particles

by T. Furukawa, Y. Takahashi
Atmospheric Chemistry and Physics ()
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Atmospheric aerosols have both a direct and an indirect cooling effect\nthat influences the radiative balance at the Earth's surface. It\nhas been estimated that the degree of cooling is large enough to\nweaken the warming effect of carbon dioxide. Among the cooling factors,\nsecondary organic aerosols (SOA) play an important role in the solar\nradiation balance in the troposphere as SOA can act as cloud condensation\nnuclei (CCN) and extend the lifespan of clouds because of their high\nhygroscopic and water soluble nature. Oxalic acid is an important\ncomponent of SOA, and is produced via several formation pathways\nin the atmosphere. However, it is not certain whether oxalic acid\nexists as free oxalic acid or as metal oxalate complexes in aerosols,\nalthough there is a marked difference in their solubility in water\nand their hygroscopicity. We employed X-ray absorption fine structure\nspectroscopy to characterize the calcium (Ca) and zinc (Zn) in aerosols\ncollected at Tsukuba in Japan. Size-fractionated aerosol samples\nwere collected for this purpose using an impactor aerosol sampler.\nIt was shown that 10–60% and 20–100% of the total Ca and Zn in the\nfiner particles (<2.1 μm) were present as Ca and Zn oxalate complexes,\nrespectively. Oxalic acid is hygroscopic and can thus increase the\nCCN activity of aerosol particles, while complexes with various polyvalent\nmetal ions such as Ca and Zn are not hygroscopic, which cannot contribute\nto the increase of the CCN activity of aerosols. Based on the concentrations\nof noncomplexed and metal-complexed oxalate species, we found that\nmost of the oxalic acid is present as metal oxalate complexes in\nthe aerosols, suggesting that oxalic acid does not always increase\nthe hygroscopicity of aerosols in the atmosphere. Similar results\nare expected for other dicarboxylic acids, such as malonic and succinic\nacids. Thus, it is advisable that the cooling effect of organic aerosols\nshould be estimated by including the information on metal oxalate\ncomplexes and metal complexes with other dicarboxylic acids in aerosols.

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