As required by the Clean Air Act (CAA), Colorado submitted, and EPA approved, a State Implementation Plan (SIP), which described how Colorado will attain and maintain compliance with CAA requirements.
In 2018, EPA designated the Denver Metro-Front Range area as nonattainment for ozone, which required Colorado to submit a revised SIP for approval. Under the CAA, EPA must review a state’s SIP revisions, approve them if consistent with the CAA and EPA’s rules, and reject the revisions if not. If rejected, the state must prepare and submit new SIP revisions.
Challenge to EPA’s Approval of Colorado’s SIP Revisions
In 2020, Colorado submitted a revised SIP, which EPA approved in 2022. The Center for Biological Diversity filed a Petition in the 10th Circuit claiming EPA’s approval of the SIP was unlawful for several reasons.
Wrong to Exclude “Temporary Emissions” from Major Source Determination
A divided 10th Circuit panel agreed with the Petitioner’s argument that Colorado’s proposed SIP did not comply with an EPA regulation implementing the CAA because the SIP excluded “temporary emissions,” such as those from construction or exploration, in determining if a facility is a major source. Therefore, EPA’s approval was in error.
EPA argued that the regulation, which was silent on the issue of excluding “temporary emissions,” was ambiguous, so that any reasonable interpretation by EPA was valid. The majority held that the silence on “temporary emissions” made the regulation unambiguous and prohibited EPA from excluding all temporary emissions in determining if a facility is a major source.
Dissent Said Colorado’s Revisions Were Adequate
The dissent agreed that EPA’s regulation was unambiguous but argued that when all of Colorado’s relevant regulations are compared to EPA’s regulations, the Colorado regulations met the CAA standard for approval because, despite using different words, they are “as or more stringent” than EPA’s.
To see the majority opinion and the dissent https://www.ca10.uscourts.gov/sites/ca10/files/opinions/010110921453.pdf