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 others are ozone, lead, carbon monoxide, sulfur oxides, and particulate matter. The law also requires EPA to periodically review the standards and revise them if appropriate to ensure that they provide the requisite amount of health and environmental protection and to update those standards as necessary.
All areas presently meet the current (1971) NO2 NAAQS, with annual NO2 concentrations measured at area-wide monitors well below the level of the standard (53 ppb). Annual average ambient NO2 concentrations, as measured at area-wide monitors, have decreased by more than 40% since 1980. Currently, the annual average NO2 concentrations range from approximately 10-20 ppb.
EPA expects NO2 concentrations will continue to decrease in the future as a result of a number of mobile source regulations that are taking effect. Tier 2 standards for light-duty vehicles began phasing in during 2004, and new NOx standards for heavy-duty engines are phasing in between 2007 and 2010 model years. Current air quality monitoring data reflects only a few years of vehicles entering the fleet that meet these strict NOx standards.