Opinion ID: 501652
Heading Depth: 2
Heading Rank: 3

Heading: Modeling Adjustments for Complex Terrain

Text: 171 Plume impaction occurs when a stream of exhaust gases hits a higher hill or mountain downwind before dispersion, causing high concentrations on the mountainside. 719 F.2d at 452. To take account of this phenomeon, the agency gave credit in its 1982 Final Regulations for the amount of its stack necessary to ensure that violations [would] not occur on the mountain as a result of the amount of the mountain's height that is above GEP height. Id. This court found much to commend EPA's action from a policy perspective, but concluded that Sec. 123 did not permit such an adjustment. Id. at 455. The court therefore reversed the agency's attempt to reduce emissions limitations by [credit for] so much of the stack height as needed to avoid plume impaction. Id. at 456. 172 Pursuant to this court's holding on the matter, the agency included no provision for plume impaction credit in its 1985 regulations. Industry petitioners recognize that the agency is precluded from reintroducing plume impaction credits. Alabama Power Brief at 45. However, they maintain that unless the agency adjusts its complex terrain screening models to account for the existence of plume impaction, sources will be treated as if they were violating NAAQS in mountainous terrain when in fact they have avoided such violations by building appropriately tall stacks. Id. at 45-47. They therefore urged the agency during the rulemaking to make adjustments to its overpredictive complex terrain screening models. Id. at 45; see also J.A. 557-81. 173 EPA rejected the proposal because it found that 174 [m]anipulation ... of modeling parameters to avoid predicting theoretical plume impaction where actual stacks have been constructed above GEP would be tantamount to granting the same impaction credit that was invalidated by the [Sierra Club ] court. 50 Fed.Reg. 27,904/2. 175 The actual meaning of EPA's remark is not fully clear to us. In a currently ongoing rulemaking EPA is revising its Guideline on Air Quality Models and is evaluating the merits of the Rough Terrain Display Model advocated by industry. 50 Fed.Reg. at 27,904/2. The outcome of this rulemaking will presumably delineate what the agency believes is permissible and what is not. There being no final decision on the subject, we may not intervene.