Concerning Organic Aerosol Components and Ice Nucleation in the Upper Troposphere

We have obtained evidence in recent field studies indicating an association of the presence of organic aerosol components with impedance of homogeneous freezing nucleation of ice at upper tropospheric conditions. It is important to understand any direct relation between certain organic compounds and ice nucleation because of implications for affecting the conditions of formation, the ice phase composition and ultimately the radiative properties of upper tropospheric clouds. Laboratory studies are being conducted to examine the impact of certain organic compounds on homogeneous freezing nucleation, either as pure species or in mixture with ammonium sulfate. We will present the field evidence for impacts of organic compounds on ice nucleation, discuss some ways that these substances could affect homogeneous ice nucleation and summarize the results of ongoing studies of ice formation by certain organic acid/ammonium sulfate mixtures.