Laboratory investigation of aerosol coating and capillarity effects on particle ice nucleation in deposition and condensation modes

In this work, the ice nucleating effectiveness of Arizona Test Dust (ATD), bare and coated with NaCl, not examined in previous published papers, was investigated. The ATD, NaCl coated ATD, and NaCl aerosols were generated by nebulizing water suspensions of bare ATD, ATD-NaCl and NaCl solution. Dried aerosols were sampled on black-gridded cellulose nitrate membrane filters for ice nuclei assessment through a Dynamic Filter Processing Chamber device (DFPC). After water evaporation ATD-NaCl droplets gave rise to ATD particles coated with NaCl. Processing of filters was performed at -22 °C and-18 °C, under sub- and super-saturated conditions (water saturation ratio Sw < 1 and Sw > 1, respectively). Additional experiments were performed by modifying the storage conditions of the filters to simulate variation of atmospheric conditions, i.e. air temperature and relative humidity. For this purpose, the filters were housed in Petri dishes, with silica gel, at +10 °C and-10 °C. As silica gel can desorb water from the aerosol capillary at low temperatures, storage conditions with and without silica gel can simulate the aerosol pre-activation atmospheric process. The highest activated fraction (AF) and iceactive surface site density (ns) values were obtained for ATD at -22 °C and Sw=1.02. Results showed a decrease in the AF and ns between bare and ATD coated with NaCl both at Sw=1.02 and Sw=0.96 and for filters stored in the presence of silica gel. The results obtained show the great influence exerted by NaCl coated insoluble aerosol on the ice nucleation.