| Abstract |
In this document, the terms NO(sub 2) and NO(sub x) and SO(sub s) and SO(sub x) are not interchangeable. The terms NO(sub x) (oxides of nitrogen) and SO(sub x) (sulfur oxides) refer to the listed Criteria Air Pollutants for which EPA has regulatory authority under Sections 108 and 109 of the Clean Air Act (CAA), and for which criteria must be developed and reviewed every 5 years. In this review, NO(sub x) refers to all oxides of nitrogen (conventionally referred to as NO(sub y) in the scientific community), not simply the sum of NO and NO(sub 2) (conventionally referred to as NO(sub x) in the scientific community). The oxides of nitrogen compounds in the ambient air are nitric oxide (NO), nitrogen dioxide (NO(sub 2)), nitrous oxide (N(sub 2)O), nitrogen trioxide (NO(sub 3)), dinitrogen trioxide (N(sub 2)O(sub 3)), dinitrogen tetroxide (N(sub 2)O(sub 4)), dinitrogen pentoxide (N(sub 2)O(sub 5)); other ambient oxides of nitrogen include nitric acid (HNO(sub 3)), peroxyacetylnitrate (PAN), and other organic compounds such as nitrites, nitrates, nitrogen acids, and N-nitroso compounds (EPA, 1993). The terms NO(sub 2) and SO(sub 2) refer to the specific air quality indicators (pollutant species) specified by the current standards whose concentrations are monitored to determine whether the NAAQS is being met in a given location. |
