EPA has issued a final rule retaining the existing NAAQS for carbon monoxide (CO). The final rule contains revisions to the CO monitoring network, requiring near road CO monitoring in CBSAs with a population of 2.5 million or more beginning in 2015. Near road CO monitoring will be required in CBSAs with a population of 1 million or more beginning in 2017. Only one monitor per CBSA will be required. The CO near road monitors will generally be co-located with a near road NO2 monitors, though EPA is allowing flexibility for states/locals to use an alternative location on a case by case basis.
The final rule and fact sheet are available here - http://www.epa.gov/airquality/carbonmonoxide/actions.html.