Index Geophysics

Exploring an approximation for the homogeneous freezing temperature of water droplets

In this work, based on the well-known formulae of classical nucleation theory (CNT), the temperature <i>T</i>_{<i>N</i>_c = 1} at which the mean number of critical embryos inside a droplet is unity is derived from the Boltzmann distribution function and explored as an approximation for homogeneous freezing temperature of water droplets. Without including the information of the applied cooling rate <i>γ</i>_cooling and the number of observed droplets <i>N</i>_{total_droplets} in the calculation, the approximation <i>T</i>_{<i>N</i>_c = 1} is able to reproduce the dependence of homogeneous freezing temperature on drop size <i>V</i> and water activity <i>a</i>_w of aqueous drops observed in a wide range of experimental studies for droplet diameter &gt; 10 µm and <i>a</i>_w &gt; 0.85, suggesting the effect of <i>γ</i>_cooling and <i>N</i>_{total_droplets} may be secondary compared to the effect of <i>V</i> and <i>a</i>_w on homogeneous freezing temperatures in these size and water activity ranges under realistic atmospheric conditions. We use the <i>T</i>_{<i>N</i>_c = 1} approximation to argue that the distribution of homogeneous freezing temperatures observed in the experiments may be partly explained by the spread in the size distribution of droplets used in the particular experiment. It thus appears that the simplicity of this approximation makes it potentially useful for predicting homogeneous freezing temperatures of water droplets in the atmosphere.

O, Kuan-Ting

Wood, Robert

2018-09-19

Text

application/pdf

10.5194/acp-16-7239-2016

https://acp.copernicus.org/articles/16/7239/2016/

eISSN: 1680-7324

eng