A new temperature- and humidity-dependent surface site density approach for deposition ice nucleation

Deposition nucleation experiments with Arizona Test Dust (ATD) as a surrogate for mineral dusts were conducted at the AIDA cloud chamber at temperatures between 220 and 250 K. The influence of the aerosol size distribution and the cooling rate on the ice nucleation efficiencies was investigated. Ice nucleation active surface site (INAS) densities were calculated to quantify the ice nucleation efficiency as a function of temperature, humidity and the aerosol surface area concentration. Additionally, a contact angle parameterization according to classical nucleation theory was fitted to the experimental data in order to relate the ice nucleation efficiencies to contact angle distributions. From this study it can be concluded that the INAS density formulation is a very useful tool to describe the temperature-and humidity-dependent ice nucleation efficiency of ATD particles. Deposition nucleation on ATD particles can be described by a temperature-and relative-humidity-dependent INAS density function n(s) (T, S-ice) with ns(x(therm)) = 1.88 x 10(5) . exp(0.2659 . x(therm)) [m(-2)], (1) where the temperature- and saturation-dependent function x(therm) is defined as x(therm) = -(T - 273.2) + (S-ice - 1) x 100, (2) with the saturation ratio with respect to ice S-ice > 1 and within a temperature range between 226 and 250 K. For lower temperatures, x(therm) deviates from a linear behavior with temperature and relative humidity over ice. Also, two different approaches for describing the time dependence of deposition nucleation initiated by ATD particles are proposed. Box model estimates suggest that the time-dependent contribution is only relevant for small cooling rates and low number fractions of ice-active particles.