Ice Formation Processes in Upper Tropospheric Cloud Conditions

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Demott/Abstract The objective of this research are to determine how the chemical composition of atmospheric aerosols influences the formation of cirrus clouds. The Hypotheses to be tested are that ice formation by homogeneous freezing nucleation occurs in agreement with theory for solution droplets existing below 100% relative humidity at temperatures below -35 degrees Celsius, and that the insoluble components of partially- soluble atmospheric aerosols readily serve as heterogeneous freezing nuclei under less stringent conditions. Systematic laboratory measurements to temperatures as cold as -60 degrees Celsius will be made using a continuous flow diffusion chamber to determine the onset solute concentration conditions for homogeneous freezing nucleation in solution droplets formed on sulfate aerosols of atmospheric relevance. Measurements will be repeated for aerosols including well-characterized insoluble aerosol components. Validation measurements for distributions of soluble and partially-soluble aerosols will be made in a cloud chamber which simulates adiabatic air parcel ascent. The laboratory data will be used together with existing data on upper tropospheric aerosol and ice nuclei composition to numerically investigate ice formation in cirrus clouds. The results are expected to be relevant to the role of aerosols in influencing the radiative properties of upper tropospheric clouds through controls on cloud microphysical composition.