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This rulemaking follows the International Civil Aviation Organization (ICAO) agreement on the international particulate matter (PM) emission standards for commercial aircraft engines in February 2019. Domestically the EPA proposed PM standards that would be at least as stringent as ICAO's standards, including replacing the existing smoke number visibility standard with a PM-based standard for certain classes of engines. The proposal also included new measurement procedures for PM emissions and visibility control that align with those of ICAO. In addition, we proposed to migrate the aircraft engine regulations from the existing 40 CFR part 87 to a new 40 CFR part 1031, and the new aircraft engine PM standards and requirements described earlier, if finalized as proposed, would be specified in this new part 1031.