§50.20. National primary ambient air quality standards for PM 2.5 .

The national primary ambient air quality standards for PM2.5 are 9.0 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 PM2.5 (particles with an aerodynamic diameter less than or equal to a nominal 2.5 micrometers) by either:

The primary annual PM2.5 standard is met when the annual arithmetic mean concentration, as determined in accordance with appendix N to this part, is less than or equal to 9.0 µg/m 3.

The primary 24-hour PM2.5 standard is met when the 98th percentile 24-hour concentration, as determined in accordance with appendix N to this part, is less than or equal to 35 µg/m 3.