Sponsored by NOAA's Office of Global Programs

This research examines the potential role of some types of mineral dust as heterogeneous ice nuclei at cirrus temperatures. Commercially available nanoscale powder samples of aluminum oxide, alumina-silicate and iron oxide 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 was used to condense sulfuric acid on the particles. The degree of acid coatings on the particles was determined by measuring their cloud condensation activity with a static thermal gradient diffusion chamber and applying Köhler theory for mixed particles. Measurements of ice nucleation were made using a continuous flow ice-thermal diffusion chamber (CFDC) operated to expose aerosols to temperatures between –45 and –60°C and a range of relative humidity above ice-saturated conditions. A clear size effect was indicated for ice formation by all of the particles as larger particles consistently nucleated ice at lower humidity than smaller particles at the same temperatures. Ice nucleation results from the minerals without a sulfuric acid coating indicate that relatively pure mineral oxide aerosols that are larger than about 100 n