Ice nucleation by surrogates for atmospheric mineral dust and mineral dust/sulfate particles at cirrus temperatures

[摘要] This study examines the potential role of some types ofmineral dust and mineral dust with sulfuric acid coatings as heterogeneousice nuclei at cirrus temperatures. Commercially-available nanoscale powdersamples of aluminum oxide, alumina-silicate and iron oxide were used assurrogates for atmospheric mineral dust particles, with and withoutmultilayer coverage of sulfuric acid. A sample of Asian dust aerosolparticles was also studied. Measurements of ice nucleation were made using acontinuous-flow ice-thermal diffusion chamber (CFDC) operated to exposesize-selected aerosol particles to temperatures between -45 and -60°Cand a range of relative humidity above ice-saturated conditions. Pure metaloxide particles supported heterogeneous ice nucleation at lower relativehumidities than those required to homogeneously freeze sulfuric acidsolution particles at sizes larger than about 50 nm. The ice nucleationbehavior of the same metal oxides coated with sulfuric acid indicateheterogeneous freezing at lower relative humidities than those calculatedfor homogeneous freezing of the diluted particle coatings. The effect ofsoluble coatings on the ice activation relative humidity varied with therespective uncoated core particle types, but for all types the heterogeneousfreezing rates increased with particle size for the same thermodynamicconditions. For a selected size of 200 nm, the natural mineral dustparticles were the most effective ice nuclei tested, supportingheterogeneous ice formation at an ice relative humidity of approximately135%, irrespective of temperature. Modified homogeneous freezingparameterizations and theoretical formulations are shown to have applicationto the description of heterogeneous freezing of mineral dust-like particleswith soluble coatings.

[学科分类] 大气科学