1) Sulfur dioxide (SO₂). A colorless gas produced during combustion of sulfur-containing materials such as coal, oil, and biomass, and during smelting of sulfide metal ores. SO₂ is emitted mainly by large stationary sources such as fossil fueled power plants, metal smelters, and certain other industrial and commercial installations. Biomass burning is an important source of sulfur oxides in tropical regions of the world.

2) Nitrogen oxides (NO_X). Two colorless gases (NO and NO₂) produced in any high temperature process such as combustion of coal, oil, gasoline, and natural gas. NO_x’s are emitted by both stationary sources and transportation vehicles. In tropical countries burning of biomass is also an important source of NO_X.

3) Toxic elements. Heavy metals such as lead, cadmium, nickel, and mercury or fluorine and other toxic elements are released mainly by large metal smelters and by transportation vehicles using leaded gasoline.

4) Volatile organic compounds (VOC). A wide variety of carbon compounds ranging from such simple molecules as ethylene, gasoline, and cleaning and painting solvents to very complex molecules such as pesticides. VOC are emitted by many different, usually small stationary and mobile sources.

5) Carbon monoxide (CO). A colorless and odorless but highly toxic gas produced during incomplete combustion of coal, oil, and natural gas and incineration of garbage and other solid and liquid wastes. Carbon monoxide inhibits respiration in humans and other animals. It is of concern to society mostly in urban areas where it accumulates in stagnant air mainly from transportation vehicles.

6) Particulate matter (PM). A catch-all category of pollutants ranging from very coarse " fugitive dust" particles that cause soiling of textiles, windows, paints, etc., to very fine aerosol particles that cause atmospheric haze or are drawn into lungs where they induce respiratory disease. These substances are extremely diverse both chemically and physically. The larger particles range from almost pure carbon in the case of soot from oil burners to mineral dusts in the case of manufacturing facilities that process cement, asbestos, clay, ceramic, glass, textile, and other materials. The fine particles range from smoke to all sorts of sulfate, ammonium, organic, metallic, and other particles formed by condensation of gases, vapors, and other volatile substances in the atmosphere. Some of these particles have very remarkable and complex fine structures that are characteristic of the original sources of the emissions.

7) Ozone (O₃) and other photochemical oxidants. These substances include peroxyacetyl nitrate (PAN) and peroxyproprionyl nitrate (PPN). They occur only as secondary pollutants which are produced when NO_X and VOC interact with atmospheric oxygen in the presence of sunlight. Ozone is the most important of these three pollutants because it is much more abundant even though it is somewhat less toxic than PAN or PPN. These compounds are among the most toxic gases to which plants, animals, and humans are exposed in the environment.

8) Acid deposition. A variety of acidic and acidifying substances produced when gaseous SO₂, NO_X, HCI, and certain other airborne chemicals interact with oxygen, ammonia, and moisture in the air to give aqueous solutions or aerosols of sulfuric, nitric, and hydrochloric acids. Wet deposition of these substances occurs during all rain, snow, hail, dew, fog, cloud, or rime-ice events; dry deposition occurs at all times—by absorption or adsorption of gaseous SO₂, NO_X, HNO₃, and HCI and by impaction of sulfate, nitrate, and chloride aerosols on the surfaces of plants, soils, animals, microorganisms, surface waters, and materials. The acidic and acidifying substances in wet and dry deposition may be partially or completely neutralized by alkaline earth elements such as calcium, potassium, sodium, or ammonium ions. Acidification of ecosystems also occurs when ammonium sulfate aerosol and certain other ammonium compounds or ammonia itself are taken up by plants, animals, or microorganisms after deposition into ecosystems.