Ice nucleation from aqueous NaCl droplets with and without marine diatoms

Abstract. Ice formation in the atmosphere by homogeneous and heterogeneous nucleation is one of the least understood processes in cloud microphysics and climate. Here we describe our investigation of the marine environment as a potential source of atmospheric IN by experimentally observing homogeneous ice nucleation from aqueous NaCl droplets and comparing against heterogeneous ice nucleation from aqueous NaCl droplets containing intact and fragmented diatoms. Homogeneous and heterogeneous ice nucleation are studied as a function of temperature and water activity, a_w. Additional analyses are presented on the dependence of diatom surface area and aqueous volume on heterogeneous freezing temperatures, ice nucleation rates, ω_het, ice nucleation rate coefficients, J_het, and differential and cumulative ice nuclei spectra, k(T) and K(T), respectively. Homogeneous freezing temperatures and corresponding nucleation rate coefficients are in agreement with the water activity based homogeneous ice nucleation theory within experimental and predictive uncertainties. Our results confirm, as predicted by classical nucleation theory, that a stochastic interpretation can be used to describe the homogeneous ice nucleation process. Heterogeneous ice nucleation initiated by intact and fragmented diatoms can be adequately represented by a modified water activity based ice nucleation theory. A horizontal shift in water activity, Δa_{w, het} = 0.2303, of the ice melting curve can describe median heterogeneous freezing temperatures. Individual freezing temperatures showed no dependence on available diatom surface area and aqueous volume. Determined at median diatom freezing temperatures for a_w from 0.8 to 0.99, ω_het~0.11^+0.06_−0.05 s⁻¹, J_het~1.0^+1.16_−0.61×10⁴ cm⁻² s⁻¹, and K~6.2^+3.5_−4.1 ×10⁴ cm⁻². The experimentally derived ice nucleation rates and nuclei spectra allow us to estimate ice particle production which we subsequently use for a comparison with observed ice crystal concentrations typically found in cirrus and polar marine mixed-phase clouds. Differences in application of time-dependent and time-independent analyses to predict ice particle production are discussed.