Ice Nucleation by Mineral Dust/Sulfate Particles at Cirrus Temperatures

This research examines the role of some types of mineral dust as heterogeneous ice nuclei at cirrus temperatures. Commercially available nanoscale powder samples of iron oxide, aluminum oxide and aluminasilicate were atomized from suspensions, dried and selected at monodisperse sizes (50 to 200 nm) for use as surrogates for atmospheric mineral dust particles. A tube furnace with a linear temperature gradient is used to condense sulfuric acid on the particles. The degree of acid coatings on the particles is determined by measuring their cloud condensation activity with a static thermal gradient diffusion chamber and applying Kohler theory for mixed particles. Measurements of ice nucleation are made using a continuous flow ice-thermal diffusion chamber (CFDC) operated to expose aerosols to temperatures between -45 and -60degC and a range of relative humidity above ice-saturated conditions. Ice nucleation results from the minerals without a sulfuric acid coating indicate that relatively pure mineral oxide aerosols nucleate ice at lower relative humidity than that required to homogeneously freeze sulfuric acid drops of the same size. Also, a clear size effect is indicated for ice formation by these particles. Larger particles nucleate ice at lower humidity than smaller particles. Currently, the freezing nucleation behavior of the same mineral oxides coated with sulfuric acid is being investigated. A sample of reference Asian dust will also be examined for ice nucleation properties in the same manner as done for the manufactured particles. Quantitative results will be presented and implications for cirrus formation will be discussed.