The Environmental Protection Agency (EPA) is approving a State Implementation Plan (SIP) revision submitted by the State of Montana. On July 13, 2011, the Governor of Montana's designee submitted to EPA a second 10-year maintenance plan for the Billings area for the carbon monoxide (CO) National Ambient Air Quality Standard (NAAQS). This maintenance plan addresses maintenance of the CO NAAQS for a second 10- year period beyond the original redesignation. EPA is also approving an alternative monitoring strategy for the Billings CO maintenance area, which was submitted by the Governor's designee on June 22, 2012.