Abstract
Abstract. 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 ns(T, Sice) with ns(xtherm) = 1.88 ×105 · exp(0.2659 · xtherm) [m−2] , (1) where the temperature- and saturation-dependent function xtherm is defined as xtherm = −(T−273.2)+(Sice−1) ×100, (2) with the saturation ratio with respect to ice Sice >1 and within a temperature range between 226 and 250 K. For lower temperatures, xtherm 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.
Highlights
Aerosol particles interacting with clouds have a significant influence on the global climate by impacting cloud life cycles, precipitation formation and the global radiation budget
Deposition nucleation on Arizona Test Dust (ATD) particles was investigated with AIDA cloud chamber experiments, following expansion trajectories starting from ice-subsaturated conditions at about 250, 235 or 223 K
The aerosol surface area concentrations and cooling rates were varied for all expansion experiments, because one of the goals of this experimental study was to determine the relevance of timescales to the observed ice nucleation efficiencies
Summary
Aerosol particles interacting with clouds have a significant influence on the global climate by impacting cloud life cycles, precipitation formation and the global radiation budget. Interaction between clouds and aerosol particles may occur via the initiation of ice crystal formation within clouds. There are four heterogeneous ice nucleation modes involving insoluble aerosol particles (Vali, 1985). Immersion freezing occurs if a particle is already immersed within a cloud droplet when ice nucleation is initiated, whereas condensation nucleation happens shortly after or at the time of water condensation on the particle which acts as condensation and freezing nucleus at the same time. Water vapor is directly transformed into ice at the particle’s surface. Contact freezing may occur if a particle collides with a supercooled droplet
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
