DO STRATOSPHERIC AEROSOL DROPLETS FREEZE ABOVE THE ICE FROST POINT

Laboratory experiments are presented which show that liquid stratosphe ric aerosol droplets under polar winter conditions do not freeze for t emperatures higher than the water ice saturation temperature (frost po int). Calorimetric measurements of the freezing of supercooled H2SO4/H NO3/H2O bulk solutions with concentrations typical of the polar strato spheric aerosol exhibit very small freezing rates, which excludes the possibility of homogeneous freezing of the droplets for temperatures a bove the frost point. Even heterogeneous formation of H2SO4 and HNO3 h ydrates at these temperatures is a very inefficient process unless the stratosphere offers nuclei better suited for nucleation than those pr esent in the laboratory experiments, which appears to be unlikely. Onl y ice was found to be a potential nucleus suited for the formation of the hydrates, which could cause the hydrates to freeze at temperatures below the frost point.