acid deposition The falling of acids and acid-forming compounds from the atmosphere to the arth's surface. Acid deposition is commonly known as acid rain, a term that refers only to wet deposition of droplets of acids and acid-forming compounds.

acid rain See acid deposition. 

air pollution One or more chemicals in high enough concentrations in the air to (1) harm humans, other animals, vegetation, or materials or (2) alter climate. Excess heat and noise are also considered forms of air pollution. Such chemicals or physical conditions are called air pollutants. See primary pollutant, secondary pollutant.

atmosphere Whole mass of air surrounding the earth. See stratosphere, troposphere. 

buffer Substance that can react with hydrogen ions in a solution and thus hold the acidity or pH of a solution fairly constant. See pH.

dust dome Dome of heated air that surrounds an urban area and traps pollutants, especially suspended particulate matter. See also urban heat island.

dust plume Elongation of a dust dome by winds that can spread a city's pollutants hundreds of kilometers downwind.

industrial smog Type of air pollution consisting mostly of a mixture of sulfur dioxide, suspended droplets of sulfuric acid formed from some of the sulfur dioxide, and a variety of suspended solid particles. Compare photochemical smog.

inversion See temperature inversion. 

PANs Peroxyacyl nitrates. Group of chemicals found in photochemical smog. 

Photochemical smog Complex mixture of air pollutants produced in the lower atmosphere by the reaction of hydrocarbons and nitrogen oxides under the influence of sunlight. Especially harmful components include ozone, peroxyacyl nitrates (PANs), and various aldehydes. Compare industrial smog.

primary pollutant Chemical that has been added directly to the air by natural events or human activities and occurs in a harmful concentration. Compare secondary pollutant.

Radiation temperature inversion Temperature inversion that typically occurs at night in which a layer of warm air lies atop a layer of cooler air nearer the ground as the air near the ground cools faster than the air above it. As the sun rises and warms the earth's surface, the inversion normally disappears by noon and disperses the pollutants built up during the night. See temperature inversion. Compare subsidence temperature inversion.

Secondary pollutant Harmful chemical formed in the atmosphere when a primary air pollutant reacts with normal air components or other air pollutants. Compare primary pollutant. 

smog Originally a combination of smoke and fog but now used to describe other mixtures of pollutants in the atmosphere. See industrial smog, photochemical smog.

stratosphere Second layer of the atmosphere, extending about 17-48 kilometers (11-30 miles) above the earth's surface. It contains small amounts of gaseous ozone (O3), which filters out about 95% of the incoming harmful ultraviolet (UV) radiation emitted by the sun. Compare troposphere.

Subsidence temperature inversion Inversion of normal air temperature layers when a large mass of warm air moves into a region at a high altitude and floats over a mass of colder air near the ground. This keeps the air over a city stagnant and prevents vertical mixing and dispersion of air pollutants. See temperature inversion. Compare radiation temperature inversion. 

Temperature inversion Layer of dense, cool air trapped under a layer of less dense, warm air. This prevents upward-flowing air currents from developing. In a prolonged inversion, air pollution in the trapped layer may build up to harmful levels. See radiation temperature inversion, subsidence temperature inversion.

thermal inversion See temperature inversion. 

troposphere Innermost layer of the atmosphere. It contains about 75% of the mass of earth's air and extends about 17 kilometers (11 miles) above sea level. Compare stratosphere