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Prior observations have documented the process of cloud cleansing, through which cloudy, polluted air from a continent is slowly transformed into cloudy, clean air typical of a maritime environment. During that process, cloud albedo changes gradually, followed by a sudden reduction in cloud fraction and albedo as drizzle forms and convection changes from closed to open cellular. Experiments in a cloud chamber that generates a turbulent environment show a similar cloud cleansing process followed by rapid cloud collapse. Observations of (1) cloud droplet size distribution, (2) interstitial aerosol size distribution, (3) cloud droplet residual size distribution, and (4) water vapor supersaturation are all consistent with the hypothesis that turbulent fluctuations of supersaturation accelerate the cloud cleansing process and eventual cloud collapse. Decay of the interstitial aerosol concentration occurs slowly at first then more rapidly. The accelerated cleansing occurs when the cloud phase relaxation time exceeds the turbulence correlation time.

Plain Language Summary Initially, polluted clouds can slowly transform into clean clouds when there is no significant source of aerosol particles. The rate at which this aerosol removal occurs, which can be thought of as cloud cleansing, undergoes a sudden acceleration when a cloud becomes sufficiently clean. Cloud chamber experiments show that random, turbulent fluctuations become large when a cloud becomes sufficiently clean, and these large fluctuations in temperature and the concentration of water vapor cause rapid activation of aerosol particles and rapid growth of cloud droplets. In the atmosphere these large droplets would be expected to aid in initiating the formation of precipitation by collisions and coalescence of droplets.