A NEW OPTICAL TECHNIQUE TO STUDY AEROSOL PHASE-TRANSITIONS - THE NUCLEATION OF ICE FROM H2SO4 AEROSOLS

Anew optical microscope technique has been developed to investigate ph ase transitions in micrometer-sized droplets. This technique has been used to study the nucleation of ice from aqueous H2SO4 aerosols 0-35 w t % in composition in the temperature range from 273 to 170 K. The aer osols were produced with a nebulizer and were deposited on a quartz pl ate, which was coated with a hydrophobic silane monolayer to minimize the effects of heterogeneous nucleation. More than 1200 aerosol partic les were monitored individually with the optical microscope, and their freezing temperatures and melting points were recorded. The observed freezing temperatures are lower than the ones from comparable aerosol studies reported in the literature, the differences in the freezing te mperature being up to 30 K, especially for the more concentrated aeros ols. No freezing was observed above 170 K for compositions greater tha n 27 wt %. A thermodynamic model has been used to apply the new freezi ng temperature data to the formation of clouds in the upper tropospher e and lower stratosphere. The results indicate that the homogeneous nu cleation of ice particles in cirrus clouds requires saturation ratios with respect to ice ranging from about 1.5 at 230 K to 1.6 at 205 K. I n addition, the formation of type II polar stratospheric clouds under volcanically perturbed conditions where H2SO4 is the main aqueous aero sol component at low temperatures is predicted to occur about 3 K belo w the ice frost point.