The spreading or @S05487@ of a gaseous or liquid material. @E01888@ diffusion in the atmosphere is the process of transport of gases due to turbulent mixing in the presence of a composition @G02669@. Molecular diffusion is the net transport of molecules which results from their molecular motions alone in the absence of turbulent mixing; it occurs when the @C01227@ of a particular gas in a mixture differs from its equilibrium value. @E01888@ diffusion is the most important mixing process in the lower atmosphere, while molecular diffusion becomes significant at the lower pressures of the upper atmosphere.