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Another “Time Lag” in the Activation of Atmospheric Ice Nuclei

G. A. IsaacDept. of Agricultural Physics, McGill University, Montreal, Quebec, Canada

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R. E. DouglasDept. of Agricultural Physics, McGill University, Montreal, Quebec, Canada

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Abstract

If most atmospheric ice nuclei activate by contacting or immersing themselves inside a water droplet, then ice nucleus cloud chambers may seriously underestimate the concentration of these particles. Ice nuclei diffuse toward cloud droplets due to Brownian and turbulent motion. In a natural cloud of 0.5 gm m−3 with a drop concentration of 1000 cm−3, ten times more particles 50–104 Å in radius enter the drops in 1 hr than collide in 2 min. In an ice nucleus counter with a similar cloud, a normal air sample residence time of under 2 min would not successfully model a stable atmospheric cloud, which might last several hours. In a specific time, the fraction collected by the above mechanisms increases with liquid water content and cloud drop concentration. Before ice nucleus counter measurements are used for theoretical computations, some attempt should be made to adjust the concentration to the parameters existing in the particular cloud under consideration. The relative proportion of nuclei which activate with or without collision with cloud drops should at least be estimated in the laboratory.

Abstract

If most atmospheric ice nuclei activate by contacting or immersing themselves inside a water droplet, then ice nucleus cloud chambers may seriously underestimate the concentration of these particles. Ice nuclei diffuse toward cloud droplets due to Brownian and turbulent motion. In a natural cloud of 0.5 gm m−3 with a drop concentration of 1000 cm−3, ten times more particles 50–104 Å in radius enter the drops in 1 hr than collide in 2 min. In an ice nucleus counter with a similar cloud, a normal air sample residence time of under 2 min would not successfully model a stable atmospheric cloud, which might last several hours. In a specific time, the fraction collected by the above mechanisms increases with liquid water content and cloud drop concentration. Before ice nucleus counter measurements are used for theoretical computations, some attempt should be made to adjust the concentration to the parameters existing in the particular cloud under consideration. The relative proportion of nuclei which activate with or without collision with cloud drops should at least be estimated in the laboratory.

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