National primary ambient air quality standards for PM2.5.

§ 50.18 National primary ambient air quality standards for PM 2.5 . (a) The national primary ambient air quality standards for PM 2.5 are 12.0 micrograms per cubic meter (µg/m 3 ) annual arithmetic mean concentration and 35 µg/m 3 24-hour average concentration measured in the ambient air as PM 2.5 (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 PM 2.5 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 µg/m 3 . (c) The primary 24-hour PM 2.5 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 µg/m 3 . [78 FR 3277, Jan. 15, 2013]