Under the Clean Air Act, the EPA is required to review and, if appropriate, revise the air quality criteria and national ambient air quality standards (NAAQS) every five years. On April 3, 2012, the EPA published a final rule in which the Agency determined to retain the current secondary standards (welfare-based) for nitrogen oxides (NOx) and for sulfur oxides (SOx). On January 15, 2013, the EPA published a final rule in which the Agency retained the secondary standards for particulate matter. This review of the air quality criteria and secondary standards for ecological effects of SOx, NOx, and particulate matter includes the preparation of an Integrated Science Assessment, Risk/Exposure Assessment, and a Policy Assessment by the EPA, with opportunities for review by the EPA's Clean Air Scientific Advisory Committee and the public. These documents will inform the Administrator's proposed decision as to whether to retain or revise the standards. This proposed decision will be published in the Federal Register with an opportunity to provide public comment. The Administrator's final decisions will take into consideration these documents, CASAC advice, and public comment on the proposed decision.