Montana Sulphur & Chemical Company v. United States Environmntal Protection Agency.

Montana Sulphur & Chemical Company seeked review of the EPA's final rule which partially disapproved a proposed revision to Montana's State Implementation Plan (SIP) governing sulfur dioxide (SO2) emissions and of the EPA's April 2008 final rule promulgating a Federal Implementation Plan (FIP) for the State of Montana's. Petitioner argued that EPA improperly disapproved the state SIP by ignoring actual air monitoring data. According to the company, “EPA’s reliance on dispersion modeling ‘provides an unrealistic picture of reality because it assumes that every single facility is always emitting as much pollution as it can possibly emit during every hour of the modeling run’ and under ‘worst case’ weather conditions.” Petitioner made the same argument with respect to the FIP and added that EPA exceeded a two-year time limit to promulgate the FIP. The court disagreed, ruling that the Clean Air Act “expressly recognizes modeling as an appropriate regulatory tool” and that EPA “reasonably interpreted the Clean Air Act to require continuous limits on emissions.” Finally, the court held that EPA did not act arbitrarily or capriciously with respect to the SIP or the FIP. Montana Sulphur had argued that the EPA acted arbitrarily and capriciously because it used outdated modeling in the FIP. The Court said further, "The EPA's continued use of the ISC model was not arbitrary or capricious. The model was properly employed at both the time of the state SIP and the proposed FIP. The FIP did not replace the entire SIP, but only the limited portions the EPA had disapproved; because use of a different model could have yielded results that did not comport with the remainder of the SIP, it was not arbitrary or capricious for the EPA to continue with the existing model despite some later technological improvements."