The present day composition of atmosphere is the product of a lengthy evolutionary process that began more than four billion years ago. It is composed of a mixture of many differentgases and suspended particles (Table shown below). As you can see from the table there are about 12 gases in the atmosphere. However, most of them are present in trace amounts. Nitrogen and oxygen are the major constituents while CO, is only 0.03%. Because of continual mixing of atmospheric gases, this composition is almost constant for approximately 15 krn height. We may travel. anywhere on the earth and be confident that we are breathing essentially the same type of air.
Atmosphere also contains minute liquid or solid particles in a suspended form which are known as aerosols. Most of these are found in the lower atmosphere (up to 80 km) near the earth's surface. They originate as a result of forest fires, wind erosion of soil, as sea salt crystals from ocean sprays as well as, from industrial and agricultural activities.