Section 50.18 - National primary ambient air quality standards for PM[2.5](a) The national primary ambient air quality standards for PM2.5 are 12.0 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 to 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 primary annual PM2.5 standard is met when the annual arithmetic mean concentration, as determined in accordance with appendix N of this part, is less than or equal to 12.0 [MICRO]g/m3.(c) The primary 24-hour PM2.5 standard is 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. 78 FR 3277 , Jan. 15, 2013