The Environmental Protection Agency (EPA) is finalizing particulate matter (PM) emission standards and test procedures applicable to certain classes of engines used by civil subsonic jet airplanes (engines with rated output of greater than 26.7 kilonewtons (kN)) to replace the existing smoke standard for those engines. The EPA is adopting these standards under our authority in the Clean Air Act (CAA). These standards and test procedures are equivalent to the engine standards adopted by the United Nations' International Civil Aviation Organization (ICAO) in 2017 and 2020 and will apply to both new type design aircraft engines and in-production aircraft engines. The EPA, as well as the U.S. Federal Aviation Administration (FAA), actively participated in the ICAO proceedings in which the ICAO requirements were developed. These standards reflect the importance of the control of PM emissions and U.S. efforts to secure the highest practicable degree of uniformity in aviation regulations and standards. Additionally, the EPA is migrating, modernizing, and streamlining the existing regulations into a new part in the Code of Federal Regulations. As part of this update, the EPA is also aligning with ICAO by applying the smoke number standards to engines less than or equal to 26.7 kilonewtons rated output used on supersonic airplanes.