State Implementation Plan

As we have discussed previously, the federal Clean Air Act (CAA) addresses what is often termed “interstate transport.” That is the phenomenon in which emissions from factories, power plants, motor vehicles and many other emission sources are transported by prevailing winds across state lines, sometimes over great distances. The CAA looks to states, first and foremost, to include control measures in implementation plans to reduce emissions that travel into other states. The statutory objective is to prohibit “significant contributions” by upwind states to violations of national ambient air quality standards (NAAQS) in downwind states. Although states have primary responsibility, EPA sometimes has invoked its CAA authority to establish federally enforceable requirements to address significant contributions when it concludes upwind states have not taken sufficient steps. In 2016, EPA adopted its most recent set of regulatory interstate transport controls in a rulemaking action called the “Cross-State Air Pollution Rule Update”—or the “CSAPR Update” for short. On September 13, the US Court of Appeals for the DC Circuit issued a decision in closely-watched litigation involving challenges to the CSAPR Update. (The case is Wisconsin v. EPA, No. 16-1406.) While upholding this EPA regulation in most respects, the court ruled in favor of a challenge that concerns the timing of upwind-state emission controls.
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