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Chemical elements characteristic of earth minerals and meteorites are present within background tropospheric and stratospheric sulfate aerosol particles. However, it is unknown if these elements are present predominantly as solids, including possible sulfates, carbonates, and oxides, or rather as soluble aqueous metal ions or complexes. Further, it is unclear how these impurities could affect particle freezing. To address these questions, we have determined the total equilibrium metal solubility ([Fe2+]T and [Mg2+]T) in 20–90 wt % sulfuric acid solutions over the temperature range 200–300 K. We have measured solubilities using samples of MgSO4, FeSO4·7H2O, and a mixture of metal sulfates representative of some meteorite samples. Because Fe2+ oxidation by O2 was found to be negligible over a 28 day period when air was bubbled through the solutions, we conclude that aqueous Fe2+ may be present at high concentrations in atmospheric sulfuric acid particles. We estimate that soluble Fe2+ and Mg2+ compose ∼0.01–0.28 wt % of the H2SO4/H2O/{Fe2+, Mg2+} solutions at temperatures and acid compositions representative of the atmosphere. Compared to composition measurements done by mass spectrometry of atmospheric particles showing ∼0.75 wt % Fe and 0.20 wt % Mg, we conclude it is plausible that solid Fe2+ and Mg2+ minerals often occur inside atmospheric sulfuric acid particles. Bulk freezing experiments were also carried out on H2SO4 solutions containing amounts of dissolved metal determined by the low-temperature solubility studies. It was found that 57.6 wt % H2SO4 containing soluble meteoritic metal and 60 wt % H2SO4 containing either soluble Fe or soluble meteoritic metal froze ∼12–20 K higher than solutions containing no soluble metal. Soluble metal ions in sulfuric acid solution may therefore provide a mechanism for sulfuric acid tetrahydrate formation in the lower stratosphere.


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Wise, M. E., S. D. Brooks, R. M. Garland, D. J. Cziczo, S. T. Martin, and M. A. Tolbert, Solubility and freezing effects of Fe2+ and Mg2+ in H2SO4 solutions representative of upper tropospheric and lower stratospheric sulfate particles, J. Geophys. Res., 108(D14), 4434, doi:10.1029/2003JD003420, 2003.


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