Many air quality problems involve the development of an air layer that begins formation at ground level and grows in height while traveling downwind from the origin. Two examples that directly occur in agriculture are dust plumes downwind from eroding fields and odor concentration downwind from line sources such as feedlots. Understanding the development of these boundary layers is important in the development of science-based solutions to these agriculture problems. Once an air mass leaves a source of dust, particulate matter, or odor-causing gas, the concentration of these constituents can be predicted as a function of the internal boundary layer that develops downwind. The rate at which the boundary layer grows depends on the magnitude of the change in elevation or surface roughness of the terrain downwind. When the wind encounters a change in surface conditions, a new profile shape will start to develop. An equation is presented that predicts the growth of internal boundary layers as a function of step change in elevation and aerodynamic roughness. Results show that the model can predict dispersion within cited literature data by explaining 88 percent of the variance about the mean with this prediction being highly significant (P=0.001). The model is simple, yet robust enough to include terrain roughness and man-made step changes such as buildings, fences and windbreaks. Concentration of odor downwind appears to be directly associated with the volume change of the plume contained within the internal boundary layer.