The Clouds of Venus: II. An Investigation of the Influence of Coagulation on the Observed Droplet Size Distribution

William B. Rossow Geophysical Fluid Dynamics Program, Princeton University, Princeton, N.J. 08540

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Abstract

Using an approximate numerical technique, we investigate the influence of coagulation, sedimentation and turbulent motions on the observed droplet size distribution in the upper layers of the Venus clouds. If the cloud mass mixing ratio is <10−5 at 250 K or the eddy diffusivity throughout the cloud is >106 cm2 s−1, then coagulation is unimportant. In this case, the observed droplet size distribution is the initial size distribution produced by the condensation of the droplets. We find that all cloud models with droplet formation near the cloud top (e.g., a photochemical model) must produce the observed droplet size distribution by condensation without subsequent modification by coagulation. We find, however, that neither meteoritic or surface dust can supply sufficient nucleating particles to account for the observed droplet number density. If, on the other hand, the cloud droplets are formed near the cloud bottom, the observed droplet size distribution can be produced solely by the interaction of coagulation and dynamics; all information about the initial size distribution is lost. The eddy diffusivity is ∼5×105 cm2 s−1. If droplet formation occurs near the cloud bottom, then the lower atmosphere of Venus is oxidizing rather than reducing.

Abstract

Using an approximate numerical technique, we investigate the influence of coagulation, sedimentation and turbulent motions on the observed droplet size distribution in the upper layers of the Venus clouds. If the cloud mass mixing ratio is <10−5 at 250 K or the eddy diffusivity throughout the cloud is >106 cm2 s−1, then coagulation is unimportant. In this case, the observed droplet size distribution is the initial size distribution produced by the condensation of the droplets. We find that all cloud models with droplet formation near the cloud top (e.g., a photochemical model) must produce the observed droplet size distribution by condensation without subsequent modification by coagulation. We find, however, that neither meteoritic or surface dust can supply sufficient nucleating particles to account for the observed droplet number density. If, on the other hand, the cloud droplets are formed near the cloud bottom, the observed droplet size distribution can be produced solely by the interaction of coagulation and dynamics; all information about the initial size distribution is lost. The eddy diffusivity is ∼5×105 cm2 s−1. If droplet formation occurs near the cloud bottom, then the lower atmosphere of Venus is oxidizing rather than reducing.

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