Temperature

The rate of change of temperature with altitude has a substantial effect on mixing of air pollutants. The diurnal (daily) and seasonal solar cycles will essentially control the temperature profile of the lower atmosphere.

During the day, the temperature in the lower atmosphere typically decreases with height, the rate depending on the amount of energy received from the sun. As evening approaches, solar heating decreases and the earth's surface experiences a net heat loss to the atmosphere. As the ground cools, the lower levels of the atmosphere will also cool. The vertical temperature profile of the atmosphere is now the reverse of the daytime situation. This increasing temperature with height is referred to as a temperature inversion. Temperature inversions will generally last longer during the winter and fall than in the spring and summer seasons. In winter, temperature inversions can last all day. The combination of a strong temperature inversion and light winds may lead to a layer of cold, stagnant air near the ground. Pollutants emitted from low-level sources, such as vehicles, are trapped in this layer of air. A persistent temperature inversion over a long period of time may lead to increased concentrations of air pollutants in the lower atmosphere from low level sources.

The region of air that extends from the earth's surface to the base of the temperature inversion is referred to as the mixing layer. This layer of air is relatively well mixed due to heating from the sun and from human sources. The depth of the mixing layer defines the volume of air in which air pollutants can be mixed. The lower the depth of the mixing layer, the less volume that is available to disperse air pollutants. A persistent lack of a mixing layer or shallow mixing depth may lead to episodes of high pollution concentrations. The mixing layer is especially important in urban locations where large quantities of pollutants are released near ground level.