A Classical-Theory-Based Parameterization of Heterogeneous Ice Nucleation by Mineral Dust, Soot, and Biological Particles in a Global Climate Model

TLDR

The parameterization models immersion, contact, and deposition ice nucleation for mineral dust, soot, bacteria, fungal spores, and pollen in mixed‑phase clouds from 0 °C to –38 °C, treating immersion freezing for insoluble, cloud‑droplet‑activated particles and allowing deposition and contact nucleation only for uncoated, unactivated aerosols. The model shows mineral dust as the dominant ice nucleation source, with soot contributing next, while biological particles have a negligible effect; ice nuclei concentrations below –20 °C correlate with coarse‑mode aerosol levels, overall agreement with in‑situ measurements is good, though the model predicts a stronger temperature dependence locally, and the simulated IN composition is 77 % dust, 23 % soot, and 10⁻⁵ % biological.

Abstract

Abstract An ice nucleation parameterization based on classical nucleation theory, with aerosol-specific parameters derived from experiments, has been implemented into a global climate model—the Community Atmosphere Model (CAM)-Oslo. The parameterization treats immersion, contact, and deposition nucleation by mineral dust, soot, bacteria, fungal spores, and pollen in mixed-phase clouds at temperatures between 0° and −38°C. Immersion freezing is considered for insoluble particles that are activated to cloud droplets, and deposition and contact nucleation are only allowed for uncoated, unactivated aerosols. Immersion freezing by mineral dust is found to be the dominant ice formation process, followed by immersion and contact freezing by soot. The simulated biological aerosol contribution to global atmospheric ice formation is marginal, even with high estimates of their ice nucleation activity, because the number concentration of ice nucleation active biological particles in the atmosphere is low compared to other ice nucleating aerosols. Because of the dominance of mineral dust, the simulated ice nuclei concentrations at temperatures below −20°C are found to correlate with coarse-mode aerosol particle concentrations. The ice nuclei (IN) concentrations in the model agree well overall with in situ continuous flow diffusion chamber measurements. At individual locations, the model exhibits a stronger temperature dependence on IN concentrations than what is observed. The simulated IN composition (77% mineral dust, 23% soot, and 10−5% biological particles) lies in the range of observed ice nuclei and ice crystal residue compositions.