Section 50.7 - 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 65 [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 micrograms per cubic meter.(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 65 micrograms per cubic meter.62 FR 38711, July 18, 1997, as amended at 69 FR 45595 , July 30, 2004