In this chapter, the structure and composition of the present-day atmosphere are described. The structure is defined in terms of the variation of pressure, density, and temperature with height. Pressure and density are controlled by gases in the air, the most abundant of which are molecular nitrogen and oxygen. The temperature structure is controlled by the distribution of gases that absorb ultraviolet (UV) and thermal-infrared (IR) radiation. Pressure, density, and temperature are interrelated by the equation of state. Gases in the air include fixed and variable gases. In the following, the structure of the atmosphere in terms of pressure, density, and temperature variations with height is discussed. The main constituents of the air are then examined in terms of their sources and sinks, abundances, health effects, and importance with respect to different air pollution issues.

AIR PRESSURE AND DENSITY STRUCTURE

Air consists of gases and particles, but the mass of air is dominated by gases. Of all gas molecules in the air, more than 99 percent are molecular nitrogen or oxygen. Thus, oxygen and nitrogen are responsible for the current pressure, temperature, and density structure of the Earth's atmosphere. Air pressure, the force of air per unit area, can be calculated as the summed weight of all gas molecules between a horizontal plane and the top of the atmosphere and divided by the area of the plane. Thus, the more oxygen and nitrogen present, the greater the air pressure.