Review of the Secondary National Ambient Air Quality Standards for Oxides of Nitrogen and Oxides of Sulfur

Under the Clean Air Act, EPA is required to review and, if appropriate, revise the air quality criteria for the primary (health-based) and secondary (welfare-based) national ambient air quality standards (NAAQS) every 5 years. On October 11, 1995, EPA published a final rule not to revise either the primary or secondary NAAQS for nitrogen dioxide (NO2). On May 22, 1996, EPA published a final decision that revisions of the primary and secondary NAAQS for sulfur dioxide (SO2) were not appropriate at that time, aside from several minor technical changes. On December 9, 2005, EPA initiated the current periodic review of NO2 air quality criteria with a call for information in the Federal Register (FR). On May 3, 2006, EPA initiated the current periodic review of SO2 air quality criteria with a call for information in the FR. Subsequently, the decision was made to review the oxides of nitrogen and the oxides of sulfur together, rather than individually, with respect to a secondary welfare standard for NO2 and SO2. This decision derives from the fact that NO2, SO2, and their associated transformation products are linked from an atmospheric chemistry perspective, as well as from an environmental effects perspective, most notably in the case of secondary aerosol formation and acidification in ecosystems. This review includes the preparation of an Integrated Science Assessment (ISA), Risk/Exposure Assessment (REA), and a Policy Assessment Document (PAD) by EPA, with opportunities for review by EPA's Clean Air Scientific Advisory Committee and the public. This review was be limited to only the secondary standards; the primary standards for SO2 and NO2 were reviewed separately.