Nitrogen Oxides in the United States

Nitrogen Oxides (NOx) in Environmental Law

One of many combinations of nitrogen and oxygen. Nitrogen oxides are regulated under the Clean Air Act in provisions concerning both acid deposition and National Ambient Air Quality Standards. Uncontrolled emissions of nitrogen oxides, such as sulfur dioxide, can combine with water in the atmosphere to form acid rain. They also precede the formation of ozone at ground level and give smog its yellowish color. Nitrogen dioxide is one of the pollutants regulated under the National Ambient Air Quality Standards, but if an air quality control region is having difficulty with ozone, it must put additional controls on nitrogen oxides as well as volatile organic compounds.

The sources of nitrogen oxides are many: industry, boilers used in power plants, and internal combustion engines. Motor vehicles [see mobile sources] can be a major contributor to nitrogen oxide levels in a municipal area. See also nonattainment areas.
Based on “Environment and the Law. A Dictionary”.