The goal of the project is to determine the microphysical mechanisms which lead to the formation of ice crystals and clouds, thus, improving predictive understanding of the corresponding changes in the aerosol induced radiative forcing and in the distribution of the greenhouse gas water vapor. The formation of ice from water soluble organic compounds (WSOC) and water insoluble organic compounds (WISOC) containing aerosol particles will be studied. The response in ice nucleation after oxidation of the WSOC and WISOC by ozone will also be addressed. The organic species employed will cover a wide range of chemical and physical properties: WSOCs such as levoglucosan, surface active species including fatty acids and humic acids, WISOCs such as oleic acid and biogenic organic material including colloids, bacteria and bacterial cell wall debris, and viruses. Homogeneous ice nucleation rate coefficients from aqueous organic and inorganic/organic solution droplets will be derived. Heterogeneous ice nucleation rate coefficients induced by surface-active organic species, by solid organic compounds suspended in aqueous solutions, and by solid organic species will be determined. The effect of chemical aging by ozone on ice nucleation will be studied by controlled ozone exposure of the particles prior to the formation of ice. The results of these experiments will allow the derivation of atmospheric ice particle production rates which can be parameterized for inclusion in climate models allowing improved predictive understanding of ice formation in the atmosphere.