Heterogeneous freezing of droplets with immersed mineral dust particles - measurements and parameterization

During the measurement campaign FROST (FReezing Of duST), LACIS (Leipzig Aerosol Cloud Interaction Simulator) was used to investigate the immersion freezing behavior of coated and uncoated Arizona Test Dust (ATD) particles with a mobility diameter of 300 nm. Particles were coated with succinic acid (C sub(4)H sub(6)O sub(4), sulfuric acid (H sub(2)SO sub(4), two different coating conditions), and ammonium sulfate ((NH sub(4)) sub(2)SO sub(4)). Ice fractions at temperatures between 233.15 K and 240.65 K were determined for all types of particles acting as IN (Ice Nuclei). In this temperature range, uncoated particles and those coated with C sub(4)H sub(6)O sub(4) or small amounts of H sub(2)SO sub(4) started to act as IN at higher temperatures compared to particles with larger amounts of H sub(2)SO sub(4) or (NH sub(4)) sub(2)SO sub(4) coatings. Although the latter two showed similar hygroscopic growth and droplet activation behavior, they differed in their ability to act as IN. ATD particles coated with (NH sub(4)) sub(2)SO sub(4) were the most inefficient IN. The ability of the investigated particles to act as IN was found not to be related to water activity for the freezing process investigated, however, in LACIS, the supercooled droplets were activated and highly diluted before the freezing occurred. Applying the measurement results, a parameterization, based on a simplified CNT (Classical Nucleation Theory) type nucleation rate expression, was developed. The simplified theory allows us to determine that thermodynamic changes at the surface are dominating the change in nucleating ability, rather than changes in surface area or kinetic effects.