Description: This Lecture describes how pollutants mix in the atmosphere. Three cases are considered: confined mixing, unconfined mixing, and unconfined mixing with wind. In a confined volume, the concentration of pollutant in the air depends on the volume and the mass of the air present in the volume. Unconfined mixing is also known as diffusion, in which the pollutant disperses through the air from the source over time. When wind is considered, the pollutant disperses from the source in the direction of the wind. The change in temperature with height in the atmosphere is also discussed.