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

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Based on the Environmental Protection Agency's (EPA's) review of the air quality criteria and national ambient air quality standards (NAAQS) for oxides of nitrogen (N oxides), oxides of sulfur (SO X ), and particulate matter (PM), the Environmental Protection Agency (EPA) proposes to revise the existing secondary sulfur dioxide (SO 2 ) standard to an annual average, averaged over three consecutive years, with a level within the range from 10 to 15 parts per billion (ppb). Additionally, the Agency proposes to retain the existing secondary standards for N oxides and PM, without revision. The EPA also proposes revisions to the data handling requirements for the proposed secondary SO 2 NAAQS.