Does buoyancy affect mass diffusion?
I am conducting a simulation involving replacing a gas in an enclosed space with a heavier gas. The process is done by slowly purging a light gas out the top of a container by introducing a heavier gas from the bottom. There will be mass transfer by diffusion of course, but I'm not sure if buoyancy will play a factor as well. Will the heavier gas tend to form a level layer since it is heavier (convective diffusion)? I believe so but am not sure? Any comments would be appreciated. Thank you!

After thinking more about it I'm quite sure it does play a role, and will have to be considered as well. So the full buoyancy model must be used to correctly model molecular diffusion. If anyone would like to back me up on this though, or disagree, that would be great!
Thanks again. 
So I've come to the conclusion The boissenisq approximation would be more appropriate and using the full buoyancy model will change my density and therefore give false results for velocity.
I forgot to mention I am mapping composition to temperature and running it as a temperature diffusion analysis since they are governed by the same equations. I matched experimental results perfectly for a 1D case, but 2D is taking an unreasonable amount of time to solve, months for a 10 hour transient simulation when I include buoyancy. Without buoyancy it was very fast but not physically accurate. If I run this as a particle trasport fluid will the solution calculate quicker, or will the solver essentially see the same thing? Any advice on this would be apreciated. Thanks 
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