No. 19-3220 ___________________________

United States of America

lllllllllllllllllllllPlaintiff - Appellee

Sierra Club

lllllllllllllllllllllIntervenor - Appellee

v.

Ameren Missouri

lllllllllllllllllllllDefendant - Appellant

------------------------------ Chamber of Commerce of the United States of America; American Chemistry Council; America's Power; Missouri Chamber of Commerce and Industry; National Association of Manufacturers; National Mining Association

lllllllllllllllllllllAmici on Behalf of Appellant(s) ____________

Appeal from United States District Court for the Eastern District of Missouri - St. Louis ____________

Submitted: December 16, 2020 Filed: August 20, 2021 ____________ Before SMITH, Chief Judge, LOKEN and MELLOY, Circuit Judges. ____________ SMITH, Chief Judge.

Ameren Missouri (“Ameren”) appeals an adverse judgment of the district court in a Clean Air Act (CAA) enforcement action brought by the United States of America, acting at the request of the Administrator of the United States Environmental Protection Agency (EPA) (hereinafter, EPA or “government”). Ameren argues that the district court erroneously found it liable for not obtaining permits for projects at its Rush Island Energy Center (“Rush Island”) and, as a result, assessed liability under the applicable federal regulations. In addition, Ameren maintains that the district court ordered legally flawed injunctions at both Rush Island and at a different plant, Labadie Energy Center (“Labadie”). We affirm the district court’s liability determination, but we reverse in part the remedial portion of its order concerning the Labadie plant and remand for further proceedings consistent with this opinion.

I. Background A. Statutory and Regulatory Background of the CAA “Congress enacted the Clean Air Act Amendments of 1970 seeking to guarantee the prompt attainment and maintenance of specified air quality standards.” Sierra Club v. Otter Tail Power Co., 615 F.3d 1008, 1011 (8th Cir. 2010) (quotations omitted). In enacting the CAA amendments, Congress “directed EPA to devise National Ambient Air Quality Standards (NAAQS) limiting various pollutants, which the States were obliged to implement and enforce.” Id. (quotation omitted). The New Source Performance Standards (NSPS) program was a key part of the CAA’s regulatory scheme. Id. The NSPS program “required EPA to develop technology- based performance standards designed to limit emissions from major new sources of

-2- pollution.” Id. (quotation omitted). Both newly constructed facilities and modified facilities with increased emissions constitute “[n]ew sources.” Id. “It is ‘unlawful for any owner or operator of any new source to operate such source in violation of’ applicable performance standards.” Id. (quoting 42 U.S.C. § 7411(e)).

The NSPS program, however, “did too little to ‘achieve the ambitious goals of the 1970 amendments.’” Id. (quoting Env’t Def. v. Duke Energy Corp., 549 U.S. 561, 567 (2007)). “Merely setting emissions limits failed to improve air quality in those areas that had already attained the minimum standards of the NAAQS because polluters had no incentive to diminish emissions below the established limits.” Id. As a result, in 1977, Congress amended the CAA “to add the ‘Prevention of Significant Deterioration’ (PSD) program, which seeks to ensure that the ‘air quality floor’ established by the NAAQS does