Laboratory studies of ice nucleation by aerosol particles in upper tropospheric conditions

Ice formation in sulfate, sulfuric acid and black carbon/sulfate aerosol particles under upper tropospheric conditions was studied using a continuous flow thermal diffusion chamber. No clear difference in the homogeneous freezing conditions (temperature, relative humidity) as a function of degree of liquid sulfate neutralization was found, consistent with most other studies. Results support that homogeneous freezing nucleation cannot alone explain observed conditions for cirrus cloud formation. Some types of black carbon (soot) associated with sulfates in mixed particles will induce freezing in preference to the homogeneous process, but only at quite large particle sizes. Small soot produced from burning a particular jet fuel did not show heterogeneous ice nucleation activity until water saturation conditions were exceeded at upper tropospheric temperatures.