Understanding Air Quality
Source: NOAA. Everyone wants and deserves clean air to breathe, and the U.S. Clean Air Act has established national air quality goals for the protection of human health and welfare. Tens of billions of dollars are invested each year to reduce air pollution. The results have been impressive. In the more than three decades since the passage of the Clean Air Act, emissions of air pollutants have declined while the Nation's Gross Domestic Product has more than doubled. Despite these efforts and significant progress, the United States still faces challenges in air quality:
- Almost a third of the population lives in areas where air pollution levels exceed the U.S. Environmental Protection Agency's health-based standards for air quality.
- Tens of thousands of people die each year as a direct result of exposure to high levels of air pollution; many more suffer adverse health impacts.
- Crop yields and forest productivity are adversely impacted by exposure to air pollution.
- Vistas in some of the most pristine areas of the country are often obscured by manmade haze.
The problems are complex and knowledge of the underlying atmospheric processes and sources that control pollution formation and distribution is needed to guide the development of policy and management strategies.
The Science of Air Quality
Source: NOAA. The quality of the air is the result of a complex interaction of many factors that involve the chemistry and motions of the atmosphere, as well as the emissions of a variety of pollutants from sources that are both natural and anthropogenic. The major pollutant usually associated with poor air quality is ozone. Ozone is naturally present in the atmosphere, but in elevated amounts it is damaging to the living tissue of plants and animals. Ozone is a "secondary" pollutant, meaning that it is formed by the chemical conversion in the atmosphere of other atmospheric species ("precursors"). The ozone precursors are the nitrogen oxides ("NOx", from the burning of fossil fuel, lightning, and other sources) and volatile organic compounds ("VOCs," from fuel burning, natural emissions of vegetation, and other sources). In the presence of sunlight, a series of chemical reactions in the atmosphere creates ozone. Meteorology plays an important role in the conditions that are ripe for making high amounts of ozone; hot, stagnant days cause more ozone to be produced from the NOx and VOC precursors.
Another important aspect of air quality is the presence of fine particles, or "particulate matter" (PM). Fine particles can be either directly emitted ("primary") pollutants or they can be formed within the atmosphere. For example, particles that are directly emitted into the atmosphere include soot particles from burning vegetation (which can be both a natural and a human-caused source), sea-salt spray, blowing dust, and volcanic ash. Other particles can be generated within the atmosphere, such as those arising from chemical conversion of the nitrogen oxides, volatile organic compounds, or sulfur-containing gases emitted from fuel burning, volcanic eruptions, or other sources.
Other components important in air quality include carbon monoxide, sulfur dioxide, NOx and VOCs (ozone precursors, mentioned above), and air toxics such as benzene, mercury and other hazardous air pollutants.