This paper consists of three parts. Part I is gravity wave growth, saturation and decay with height; part II reflection of gravity waves from critical layer using realistic background atmosphere and background wind; part III perturbation treatment of minor species' response to gravity waves. In part I by using Newtonian cooling and Rayleigh friction approximations and by considering only the average effects of turbulence on gravity waves we have derived an optical potential, with which we have studied the propagation of gravity waves and their reflections at every height level. We have found that reflections from higher level due to viscosity and heat conduction is so small that no ducting can be sustained. part II is the continuation of He Fan's work. In our work we adopt the same two parameter optical potential to model the gravity wave--critical layer interaction but we relaxed the conduction of isothermalness of the background and the linearity of the wind profile and we use the more realistic wind models, so our results should be more meaningful. We have found that the reflection coefficients of gravity waves from critical layer range from 5% to 25%, which should be measurable. In part III we develop a perturbation scheme with which it is possible to calculate the minor species response to any order in the linear gravity wave, including a secular component of the response which leads to wave-induced diffusion of minor species. Calculations to third order over a wide range of wave parameters show that the nonlinear effects can be substantial. A result is that care must be taken when analyzing data from minor species fluctuations, so that frequencies due solely to the nonlinear nature of the minor species response are not attributed to gravity waves.