The problem of air pollution occurs in the earth's atmosphere. The chemical composition and the physical characteristics of the atmosphere are helpful for interpretation of the pollution cloud. The atmosphere is a mixture of gases. The concentration of each gas varies from trace levels to very high levels. Nitrogen and oxygen are the main constituents. Some constituents such as water vapor vary in space and time.

The earth’s atmosphere can be divided into several layers, depending on the temperature profile. The four major layers are:

    troposphere

    stratosphere

    mesosphere

    thermosphere.

The discussion in this course will be limited to troposphere, and stratosphere.