Ice Nucleation onto Arizona Test Dust at Cirrus Temperatures: Effect of Temperature and Aerosol Size on Onset Relative Humidity

The University of Toronto Continuous Flow Diffusion Chamber (UT-CFDC) was used to Study ice formation onto monodisperse Arizona Test Dust (ATD) particles. The onset relative humidity with respect to ice (RHi) was measured as a function of temperature in the range 251-223 K for 100 nm ATD particles. It was found that for 0.1% of the particles to freeze, water saturation was required at all temperatures except 223 K where particles activated at RHi below water saturation. At this temperature, where deposition mode freezing is Occurring, we find that the larger the particle size, the lower the onset RHi. We also demonstrate that the total number of particles present may influence the onset RHi observed. The surface area for ice activation, aerosol size, and temperature must all be considered when reporting onset values of ice formation onto ATD mineral dust particles. In addition, we calculate nucleation rates and contact angles of ice germs with ATD aerosols which indicate: that there exists a range of active sites on the Surface with different efficiencies for activating ice formation.