ABSTRACT
This chapter begins with a discussion on characteristics of common air pollutants. Air pollutants can be in the form of gas, liquid, or solid. They can be organic, inorganic, or microbial species. Many terms are being used to describe the state of an air pollutant. Air is a mixture of gases. Gas is a state of matter that has no fixed shape and volume. It should be mentioned that an air pollutant can be a combination of two or more pollutants. A primary air pollutant is an air pollutant emitted directly from a source, such as an industrial stack or a vehicle exhaust pipe; examples include carbon monoxide and nitric oxide. In the atmosphere it has the same physical and chemical forms as when it was emitted. A secondary air pollutant is not directly emitted from a source, but formed from chemical or photochemical reactions among the primary air pollutants in the atmosphere.
