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, aw. Additional analyses are presented on the dependence of diatom surface area and aqueous volume on heterogeneous freezing temperatures, ice nucleation rates, ωhet, ice nucleation rate coefficients, Jhet, 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, Δaw, 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 aw from 0.8 to 0.99, ωhet~0.11+0.06−0.05 s−1, Jhet~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.
Highlights
Aerosol particles play an important role in the radiative balance of our Earth’s climate by directly scattering and absorbing short wave and long wave radiation (Charlson et al, 1992; Andreae and Crutzen, 1997; Ramanathan et al, 2001; Anderson et al, 2003; McComiskey et al, 2008)
Homogeneous and heterogeneous freezing of ice from micrometer-sized aqueous NaCl droplets with and without diatoms have been analyzed in the temperature range of 180 to 260 K and for water activities of 0.8 to 0.99
The experimentally derived homogeneous ice nucleation rate coefficients were in agreement with predictions of the water-activity based theory and can be employed to further constrain that theory
Summary
Aerosol particles play an important role in the radiative balance of our Earth’s climate by directly scattering and absorbing short wave and long wave radiation (Charlson et al, 1992; Andreae and Crutzen, 1997; Ramanathan et al, 2001; Anderson et al, 2003; McComiskey et al, 2008). Freezing temperatures of micrometer sized aqueous sea salt and NaCl aerosol particles occurs homogeneously below 235 K and follows the water activity based homogeneous ice nucleation theory (Koop et al, 2000b,a). We extend these previous studies to determine nucleation dependency on aw, ice nucleation rate coefficients, and the stochastic behavior of the nucleation process.
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