Airborne Mineral Dust Adds Sulfate To Aerosols

Aerosol particles in the air undergo chemical reactions that change their size and hygroscopicity. Those properties in turn influence how airborne aerosols affect climate by scattering radiation and modifying the brightness and lifetime of clouds. Adding sulfate to those particles is a particularly important process. A research group led by Peter Hoppe, Eliza Harris, and Bärbel Sinha of the Max Planck Institute for Chemistry, in Germany, now pinpoints transition-metal ions as key oxidants that turn SO₂ into SO₄^2– on airborne mineral dust within clouds (Science 2013, DOI: 10.1126/science.1230911). The team studied so-called orographic clouds at Mount Schmücke, in Germany, where clouds form as air rises to go over the mountain. The researchers compared sulfur isotope ratios upwind and downwind of the clouds to determine whether SO₂ was oxidized by H₂O₂, O₃, or transition metals. They estimate that transition metals may account for about 1% of SO₄^2– production in urban locations but as much as 58% in rural areas. If incorporated into climate models, the results would decrease the amount and change the geographic distribution of aerosol-driven climate cooling.