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Heterogeneous ice nucleation in atmospheric clouds

Heterogeneous ice nucleation in atmospheric clouds

Clouds are an integral part of the Earth system and influence the dynamics and radiative balance of the atmosphere. Ice formation is catalysed by particles in the atmosphere (ice nuclei) and directly influences cloud processes such as precipitation/sedimentation. However, there is large uncertainty in heterogeneous ice nucleation rates due to a lack of empirical observations, both in situ (they are extremely challenging to make) and in the laboratory. Furthermore, there is a complete lack of ice nucleation measurements for volcanic clouds.

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Movie showing pure water drop freezing experiment

Taking this as motivation, a series of experiments were carried out in the Michigan Technological University Cloud Physics Laboratory between 2004-2005 to quantify heterogeneous ice nucleation rates in liquid water drops. In a novel approach developed by Dr. R. Shaw, Y. Mi and Dr Adam Durant, single-particle freezing experiments were performed using volcanic ash particles (previously collected at the ground) as ice nuclei to investigate how composition and surface area affect water drop freezing temperature. The experiments provided new insight into: (1) the kinetics of the heterogeneous ice nucleation mechanism; (2) contact nucleation, with the realisation that drop surface-based ice nucleation can be initiated by the ice nucleus from within the drop volume ; and (3) volcanic cloud ice nucleation and suppression of particle sedimentation during rise in the column (through inhibition of the Bergeron process).

A recent Nature News and Views article, Water: Ins and outs of ice nucleation discusses our findings.