06-096-100 Me. Code R. § 134
"PM2.5" means particulate matter in the ambient air with an aerodynamic diameter less than or equal to a nominal 2.5 micrometers as measured by the reference method based on 40 C. F. R. Part 50, Appendix L and designated in accordance with 40 C. F. R. Part 53. PM2.5 emissions shall include gaseous emissions from a source or activity which condense to form particulate matter at ambient temperatures. Such condensable particulate matter shall be accounted for in applicability determinations and in establishing emissions limitations for PM2.5 in PSD permits. Compliance with emissions limitations for PM2.5 issued prior to this date shall not be based on condensable particulate matter unless required by the terms and conditions of the permit or the applicable implementation plan. Applicability determinations made prior to this date without accounting for condensable particulate matter shall not be considered in violation of this section unless the applicable implementation plan required condensable particulate matter to be included.
06-096 C.M.R. ch. 100, § 134