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The upper haze (UH) of Venus is variable on the order of days and it ispopulated by two particle modes. We use a 1D microphysics and verticaltransport model based on the Community Aerosol and Radiation Model forAtmospheres to evaluate whether interaction of upwelled cloud particles andsulfuric acid particles nucleated in situ on meteoric dust are able to generatethe two size modes and whether their observed variability are due to cloud topvertical transient winds. Nucleation of photochemically produced sulfuric acidonto polysulfur condensation nuclei generates mode 1 cloud droplets that thendiffuse upwards into the UH. Droplets generated in the UH from nucleation ofsulfuric acid onto meteoric dust coagulate with the upwelled cloud particlesand cannot reproduce the observed bimodal size distribution. The mass transportenabled by cloud top transient winds are able to generate a bimodal sizedistribution in a time scale consistent with observations. Sedimentation andconvection in the middle and lower clouds causes the formation of large mode 2and mode 3 particles. Evaporation of these particles below the clouds creates alocal sulfuric acid vapor maximum that causes upwelling of sulfuric acid backinto the clouds. If the polysulfur condensation nuclei are small and theirproduction rate is high, coagulation of small droplets onto larger droplets inthe middle cloud may result in sulfuric acid "rain" below the clouds once everyfew Earth months. Reduction of the polysulfur condensation nuclei productionrate destroys this oscillation and reduces the mode 1 particle abundance in themiddle cloud by two orders of magnitude, though it better reproduces thesulfur-to-sulfuric-acid mass ratio in the cloud and haze droplets. In generalwe find satisfactory agreement between our results and observations, thoughimprovements could be made by incorporating sulfur microphysics.

Bimodal Distribution of Sulfuric Acid Aerosols in the Upper Haze of Venus