Ice nucleation from aqueous NaCl droplets with and without marine diatoms

Abstract

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, a 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 −4.1 ×10 4 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.

Thermodynamic derivation of the activation energy for ice nucleation

“…For T > 218 K and a w = 0.9, J hom from all formulations agree within 3 orders of magnitude and within experimental uncertainty (Alpert et al, 2011;Larson and Swanson, 2006) of the measured rates. For T < 216 K, the models tend to predict higher than measured nucleation rates.…”

“…Experimental measurements also show a weaker dependency of J hom on T at a w = 0.9 than at a w = 1.0. This is evident in the data of Alpert et al (2011) and less evident for the correlation of Larson and Swanson (2006). For the latter the authors report a T uncertainty of about 1 K, which may account for the observed difference.…”

“…the activation energy. The correlation derived by Koop et al (2000) is also reproduced along with available experimental data (Alpert et al, 2011;Knopf and Rigg, 2011). For the latter, only data reported for T < 221 K are shown to avoid heterogeneous freezing effects.…”

“…For the latter the authors report a T uncertainty of about 1 K, which may account for the observed difference. Alpert et al (2011) reports an uncertainty in initial a w of 0.01, which explains the scatter of the data around T ∼ 218 K. The authors, however, do not report measurements of a w at the point of freezing. Knopf and Rigg (2011), who used a similar technique, discuss the possibility of a slight decrease in a w of their test solutions as T decreases, which would indicate a higher uncertainty in a w than reported by Alpert et al (2011).…”

“…Also presented are experimental results and empirical formulations obtained from the literature. NNF, this work NNF (a w =0.88) B14 CNT_A CNT_B Koop et al, 2000Knopf and Rigg, 2011Alpert et al, 2011Larson and Swanson, 2006 Figure 5. Homogeneous ice nucleation rate calculated using Eq.…”