§ 50.21 National secondary ambient air quality standards for sulfur oxides (sulfur dioxide).
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Verbatim text below is from the Electronic Code of Federal Regulations (eCFR), a public-domain U.S. government work. Always verify the current version with the eCFR before relying on it for any legal matter.
Full Text
2(a) The level of the annual secondary national ambient air quality standard for oxides of sulfur is 10 parts per billion (ppb), measured in the ambient air as sulfur dioxide (SO) by a reference method based on appendix A-1 and appendix A-2 of this part, or by a Federal Equivalent Method (FEM) designated in accordance with part 53 of this chapter.
2(b) The annual secondary standard is met when the 3-year average of the annual SO concentration is less than or equal to 10 ppb, as determined in accordance with appendix T of this part.
[89 FR 105785, Dec. 27, 2024]
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