A time-varying model of neutral gas expansion in a nonuniform environment is developed. The model includes diffusion in a multicomponent atmosphere, chemical reactions between the diffusing gases and the atmosphere, thermal diffusion effects, and transport due to altitude-dependent winds. The three-dimensional diffusion equation governing the neutral gas flow is solved numerically using Fourier transform and finite difference techniques. Examples of H2, OH, and CO2 diffusion illustrate the effects of chemical reactions and wind shears on the neutral expansion. The model may be applied to chemical releases which produce ionospheric depletions or luminescent trails.