Judge Brett Kavanagh wrote for the Court:
Here, EPA’s Transport Rule exceeds the agency’s statutory authority in two independent respects. First, the statutory text grants EPA authority to require upwind States to reduce only their own significant contributions to a downwind State’s nonattainment. But under the Transport Rule, upwind States may be required to reduce emissions by more than their own significant contributions to a downwind State’s nonattainment. EPA has used the good neighbor provision to impose massive emissions reduction requirements on upwind States without regard to the limits imposed by the statutory text. Whatever its merits as a policy matter, EPA’s Transport Rule violates the statute. Second, the Clean Air Act affords States the initial opportunity to implement reductions required by EPA under the good neighbor provision. But here, when EPA quantified States’ good neighbor obligations, it did not allow the States the initial opportunity to implement the required reductions with respect to sources within their borders. Instead, EPA quantified States’ good neighbor obligations and simultaneously set forth EPA-designed Federal Implementation Plans, or FIPs, to implement those obligations at the State level. By doing so, EPA departed from its consistent prior approach to implementing the good neighbor provision and violated the Act.
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