Introduction A wide variety of air pollutants may be emitted from many sources during refinery operations. This chapter discusses briefly the properties and sources of these pollutants as an introduction to the detailed descriptions of the methods of control contained in the following chapters. The methods employed to control refinery emissions depend on whether they occur as gases@ vapors@ aerosols@ or dusts. These states are defined as follows: Gases are fluids above their boiling points. They have neither shape .nor volume and tend to expand indefinitely (for example@ methane@ nitrous-oxide@ and carbon monoxide). Vapors are fluids which exist like gases in the atmosphere but at temperatures below their boiling points (for example. mercaptans and phenolic compounds) . Aerosols are suspensions of finely divided solid or liquid particles in air or gas@ such as smoke@ fumes@ and mists. a. Smoke is a product of the incomplete combustion of hydrocarbons and other carbonaceous matter consisting of particles ranging from 0.001 to 1.0 micron in diameter. b. Fumes consist of solid particles less than 1 micron in diameter commonly formed by the condensation of vapors of solid matter after volatilization from the molten state. Fumes may be generated by sublimation@ distillation@ calcination@ and chemical reaction whenever such processes create airborne particles. c. Mists and Fogs consist of particles formed by the condensation of vapors. In mists@ particle sizes range from 40 to 500 microns. In fogs@ the particles are less than 40 microns in diameter. 4. Dusts consist of solid particles blown into the air by wind@ released by other natural forces@ or generated by mechanical processes such as demolition or the crushing@ grinding@ and conveying of solids. Usually dusts are coarser than aerosols.