Section 50.13 - National primary and secondary ambient air quality standards for PM[2.5](a) The national primary and secondary ambient air quality standards for particulate matter are 15.0 micrograms per cubic meter ([MICRO]g/m3) annual arithmetic mean concentration, and 35 [MICRO]g/m3 24-hour average concentration measured in the ambient air as PM2.5 (particles with an aerodynamic diameter less than or equal to a nominal 2.5 micrometers) by either:(1) A reference method based on appendix L of this part and designated in accordance with part 53 of this chapter; or(2) An equivalent method designated in accordance with part 53 of this chapter.(b) The annual primary and secondary PM2.5 standards are met when the annual arithmetic mean concentration, as determined in accordance with appendix N of this part, is less than or equal to 15.0 [MICRO]g/m3.(c) The 24-hour primary and secondary PM2.5 standards are met when the 98th percentile 24-hour concentration, as determined in accordance with appendix N of this part, is less than or equal to 35 [MICRO]g/m3.(d) Until the effective date of the final Fine Particulate Matter National Ambient Air Quality Standards: State Implementation Plan Requirements rule to be codified at 40 CFR 51.1000 through 51.1016 , the 1997 annual PM2.5 NAAQS set forth in this section will continue in effect, notwithstanding the promulgation of the 2012 primary annual PM2.5 NAAQS under § 50.18 . The 1997 primary annual PM2.5 NAAQS set forth in this section will no longer apply upon the effective date of the final Fine Particulate Matter National Ambient Air Quality Standards: State Implementation Plan Requirements rule; except that for areas designated nonattainment for the 1997 annual PM2.5 NAAQS set forth in this section as of the effective date of the final Fine Particulate Matter National Ambient Air Quality Standards: State Implementation Plan Requirements rule, the requirements applicable to the 1997 primary annual PM2.5 NAAQS set forth in this section will apply until the effective date of an area's redesignation to attainment for the 1997 annual PM2.5 NAAQS pursuant to the requirements of section 107 of the Clean Air Act. The 1997 secondary annual PM2.5 NAAQS and the 1997 24-hour PM2.5 NAAQS shall remain in effect. The area designations and classifications with respect to the 1997 annual and 24-hour PM2.5 NAAQS remain codified in 40 CFR part 81 in order to provide information on where the 1997 primary annual PM2.5 NAAQS has been revoked and to facilitate the implementation of the 1997 secondary annual PM2.5 NAAQS and the 1997 24-hour PM2.5 NAAQS. 71 FR 61224 , Oct. 17, 2006, as amended at 81 FR 58149 , Aug. 24, 2016