Criteria Pollutants

Criteria pollutants are those pollutants that are common and found all over the United States. EPA uses these "criteria pollutants" as indicators of air quality. Each of the criteria pollutants will be discussed in detail below.

Figure: Sources of CO

Carbon Monoxide (CO), is a colorless, odorless gas formed when carbon in fuel is not burned completely. Motor vehicle exhaust contributes about 60 percent of all CO emissions nationwide (Latest Finding on National Air Quality 2002). Other non-road engines and vehicles (such as construction equipment and boats) contribute about 22 percent of all CO emissions nationwide. Higher levels of CO generally occur in areas with heavy traffic congestion. In cities, 95 percent of all CO emissions may come from motor vehicle exhaust. Other sources of CO emissions include industrial processes (such as metals processing and chemical manufacturing), residential wood burning, and natural sources such as forest fires. Woodstoves, gas stoves, cigarette smoke, and unvented gas and kerosene space heaters are sources of CO indoors. The highest levels of CO in the outside air typically occur during the colder months of the year when CO automotive emissions are greater and nighttime inversion conditions are more frequent. In inversion conditions the air pollution becomes trapped near the ground beneath a layer of warm air.

Sulfur Oxides (SOx) are colorless gases formed by burning sulfur. SOx gases are formed when fuel containing sulfur, such as coal and oil, is burned, and when gasoline is extracted from oil or metals are extracted from ore. Sulfur dioxide (SO2) is the criteria pollutant that is the indicator of sulfur oxide concentrations in the ambient air. SO2 dissolves in water vapor to form acid, and interacts with other gases and particles in the air to form sulfates and other products that can be harmful to people and their environment. Over 65% of SO2 released to the air, or more than 13 million tons per year, comes from electric utilities, especially those that burn coal. Other sources of SO2 are industrial facilities that derive their products from raw materials like metallic ore, coal, and crude oil, or that burn coal or oil to produce process heat. Examples are petroleum refineries, cement manufacturing, and metal processing facilities. Also, locomotives, large ships, and some nonroad diesel equipment currently burn high sulfur fuel and release SO2 emissions to the air in large quantities.

Figure: Sources of SOx

Nitrogen Oxides (NOx), is the generic term used to describe the sum of NO, NO2 and other oxides of nitrogen. NOx is a group of highly reactive gases that play a major role in the formation of ozone. Many of the nitrogen oxides are colorless and odorless. However, one common pollutant, nitrogen dioxide (NO2) along with particles in the air can often be seen as a reddish-brown layer over many urban areas. Nitrogen oxides form when fuel is burned at high temperatures, as in a combustion process. The primary sources of NOx are motor vehicles, electric utilities, and other industrial, commercial, and residential sources that burn fuels.

Ozone (O3) is a gas composed of three oxygen atoms. It is a colorless compound that has an electric-discharge-type odor. It is a unique criteria pollutant in that it is exclusively a secondary pollutant. It is not usually emitted directly into the air, but at ground level is created by a chemical reaction between oxides of nitrogen (NOx) and volatile organic compounds (VOCs) in the presence of heat and sunlight. The concentration of ozone in a given locality is influenced by many factors, including the concentration of NO2 and VOCs in the area, the intensity of the sunlight, and the local weather conditions. Ozone and the chemicals that react to form it can be carried hundreds of miles from their origins, causing air pollution over wide regions. Figures: Lead emission sources 1970 & 1997

Ozone has the same chemical structure whether it occurs miles above the earth or at ground level and can be "good" or "bad," depending on its location in the atmosphere. "Good" ozone occurs naturally in the stratosphere and forms a layer that protects life on earth from the sun's harmful rays or ultraviolet radiation. In the earth's lower atmosphere, or troposphere, ground-level ozone is considered "bad". Ozone is the most prevalent chemical found in photochemical air pollution, or smog.

Figure:Lead emission sources 1970 & 1997

Lead (Pb) is a metal found naturally in the environment as well as in manufactured products. Because of unique physical properties that allow it to be easily formed and molded, lead has been used in many applications. The major sources of lead emissions have historically been motor vehicles (such as cars and trucks) and industrial sources. Due to the phase out of leaded gasoline, metals processing is the major source of lead emissions to the air today. The highest levels of lead in air are generally found near lead smelters. Other stationary sources are waste incinerators, utilities, and lead-acid battery manufacturers.

Particulate Matter is the general term used for a heterogeneous mixture of solid particles and liquid droplets found in the air, including dust, dirt, soot, smoke, and liquid droplets. Particles can be suspended in the air for long periods of time. Some particles are large or dark enough to be seen as soot or smoke. Others are so small that individually they can only be detected with an electron microscope. PM can be a primary or secondary pollutant. "Primary" particles, such as dust or black carbon (soot) are directly emitted into the air. They come from a variety of sources such as cars, trucks, buses, factories, construction sites, tilled fields, unpaved roads, stone crushing, and burning of wood. "Secondary" particles are formed in the air from the chemical change of primary gaseous emissions. They are indirectly formed when gases from burning fuels react with sunlight and water vapor. These can result from fuel combustion in motor vehicles, at power plants, and in other industrial processes. PM2.5 describes the "fine" particles that are less than or equal to 2.5 µm in diameter. PM10 refers to all particles less than or equal to 10 µm in diameter (about one-seventh the diameter of a human hair).

More information on criteria pollutants can be found at http://www.epa.gov/air/urbanair/6poll.html