Determination of the evaporation coefficient of D&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O

W S Drisdell,R C Cohen,C D Cappa,R J Saykally,J D Smith

doi:10.5194/acp-8-6699-2008

Abstract

Abstract. The evaporation rate of D2O has been determined by Raman thermometry of a droplet train (12–15 μm diameter) injected into vacuum (~10-5 torr). The cooling rate measured as a function of time in vacuum was fit to a model that accounts for temperature gradients between the surface and the core of the droplets, yielding an evaporation coefficient (γe) of 0.57±0.06. This is nearly identical to that found for H2O (0.62±0.09) using the same experimental method and model, and indicates the existence of a kinetic barrier to evaporation. The application of a recently developed transition-state theory (TST) model suggests that the kinetic barrier is due to librational and hindered translational motions at the liquid surface, and that the lack of an isotope effect is due to competing energetic and entropic factors. The implications of these results for cloud and aerosol particles in the atmosphere are discussed.

Highlights

The evaporation and condensation rates of liquid water are of fundamental importance to many chemical, biological, and atmospheric processes
Some of the variation in older literature is likely due to impurities in or on the surface of the water samples used in the experiments; we note this fact hints that impurities will be important determinants of evaporation and condensation rates in mixed systems, a notion supported by field measurements of droplet growth rates (Feingold and Chuang, 2002; Ruehl et al, 2008)
It is generally accepted that condensation and evaporation occurring faster than 10% of the gas kinetic limit results in thermodynamic control over droplet growth while slower rates result in kinetic control over these growth rates (Chuang et al, 1997; Laaksonen et al, 2005)

Summary

Introduction

The evaporation and condensation rates of liquid water are of fundamental importance to many chemical, biological, and atmospheric processes. Cles on which cloud droplets condense (CCN) (IPCC, 2007; Laaksonen et al, 2005; McComiskey and Feingold, 2008; Lohmann et al, 2007). This variation is in part due to differing values for water evaporation and condensation kinetics and their relation to particle growth rates in these models (Laaksonen et al, 2005). It is generally accepted that condensation and evaporation occurring faster than 10% of the gas kinetic limit results in thermodynamic control over droplet growth while slower rates result in kinetic control over these growth rates (Chuang et al, 1997; Laaksonen et al, 2005)

Methods

Results

Discussion

Conclusion