ABSTRACT
The Clean Air Act requires EPA to set national ambient air quality standards for criteria pollutants. Currently, nitrogen oxides and five other major pollutants are listed as criteria pollutants. The sum of nitric oxide (NO) and NO2 is commonly called nitrogen oxides or NOx. Other oxides of nitrogen including nitrous acid and nitric acid are part of the nitrogen oxide family. While EPA’s National Ambient Air Quality Standard (NAAQS) covers this entire family, NO2 is the component of greatest interest and the indicator for the larger group of nitrogen oxides. NOx is also a precursor for ozone formation. There is a NAAQS for ozone as well. NOx is generally regulated as a precursor to ozone rather than for ambient NO2 reasons.
