Freezing Temperature Of Water Droplets Research Articles

Exploring an approximation for the homogeneous freezing temperature of water droplets

Abstract. In this work, based on the well-known formulae of classical nucleation theory (CNT), the temperature TNc = 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 γcooling and the number of observed droplets Ntotal_droplets in the calculation, the approximation TNc = 1 is able to reproduce the dependence of homogeneous freezing temperature on drop size V and water activity aw of aqueous drops observed in a wide range of experimental studies for droplet diameter > 10 µm and aw > 0.85, suggesting the effect of γcooling and Ntotal_droplets may be secondary compared to the effect of V and aw on homogeneous freezing temperatures in these size and water activity ranges under realistic atmospheric conditions. We use the TNc = 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.

Investigations on biogenic ice nuclei in the Arctic atmosphere

Air samples collected at several heights above the Arctic Ocean contained at least two types of bacteria and five different fungal spores which could elevate the freezing temperature of water droplets. It was observed that the freezing temperature increased logarithmically with the number of cells per drop up to a maximum temperature dependent upon species. Bacteria were more effective than fungal spores but, unlike the spores, they lost their activity under laboratory growth conditions.Maximum concentration of nearly 10 microbial cells per liter occurs in clouds and 1 per liter in air outside clouds. However, numbers and varieties of microbial cells in excess of those found near sea ice surface level were observed up to 7 km.