"PM2.5" means any particulate matter in the atmosphere with an aerodynamic diameter less than or equal to a nominal 2.5 micrometers as measured by an approved reference method or equivalent method based on 40 C.F.R. Part 50, Appendix L, and designated in accordance with 40 C.F.R. Part 53.
Nev. Admin. Code § 445B.1348
NRS 445B.210