Solubility and Freezing Effects of Fe2+ and Mg2+ in H2SO4 Solutions at Upper Tropospheric and Lower Stratospheric Temperatures and Compositions

Chemical elements characteristic of earth minerals and meteorites are present within background tropospheric and stratospheric sulfate aerosol particles. However, it is unknown (a) if these elements are present predominately in a solid matrix, or rather as soluble aqueous metal ions or complexes and (b) how these impurities could affect particle freezing. To address these questions, we have determined the total equilibrium metal solubility (viz. [Fe²⁺] _T and [Mg²⁺]_T ) of MgSO₄, FeSO₄ 7H₂O, and a mixture of metal sulfates representative of meteorite samples in 20 - 90 wt % sulfuric acid solutions over the temperature range 200 - 300 K. We estimate that soluble Fe²⁺ and Mg²⁺ respectively comprise ca. 0.02 - 0.3 wt % of the solutions at temperatures and acid compositions representative of the atmosphere. Bulk freezing experiments were also carried out on sulfuric acid containing soluble metal. It was found that 57.6 wt % H₂SO₄ containing soluble meteoritic metal and 60 wt % H₂SO₄ containing soluble Fe and soluble meteoritic metal froze approximately 12 - 20 K higher than solutions containing no soluble metal.