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the May 1999 draft permit, ADEC first concluded that SCR was the most stringent emission-control technology that was both technically and economically feasible. That technology should have been designated BACT absent considerations justifying a conclusion that SCR was not achievable in this case. ADEC, however, selected Low NOx as BACT based on Cominco's emissions-offsetting suggestion. In September and December 1999, ADEC again rejected SCR as BACT but no longer relied on that suggestion. Rather, ADEC candidly stated that it aimed to support Cominco's project and its contributions to the region. ADEC's selection of Low NOx thus rested squarely and solely on SCR's "disproportionate cost." EPA rightly concluded that ADEC's switch from finding SCR economically feasible in May 1999 to finding SCR economically infeasible in September 1999 had no factual basis in the record. ADEC forthrightly conceded it was disarmed from reaching a judgment on SCR's economic impact on the mine by Cominco's refusal to provide relevant financial data. No record evidence suggests that the mine, were it to use SCR, would be obliged to cut personnel or raise zinc prices. Having acknowledged that it lacked information needed to judge SCR's impact on the mine's operation, profitability, or competitiveness, ADEC could not simultaneously proffer threats to the mine's operation and competitiveness as reasons for declaring SCR economically infeasible. Nor has ADEC otherwise justified its choice. To bolster its assertion that SCR was too expensive, ADEC invoked cost figures discussed in four BACT determinations made in regard to diesel generators used for primary power production. ADEC itself, however, had previously found SCR's per-ton cost to be well within what ADEC and EPA consider economically feasible. No reasoned explanation for ADEC's retreat from this position appears in the permit ADEC issued. ADEC's basis for selecting Low NOx thus reduces to a readiness to support Cominco's project and its contributions to the region. This justification, however, hardly meets ADEC's own standard of a source-specific economic impact that demonstrates SCR to be inappropriate as BACT. ADEC's justification that lower aggregate emissions would result from Cominco's agreement to install Low NOx on all its generators is also unpersuasive. The final PSD permit did not offset MG-17's emissions against those of the mine's six existing generators. As ADEC recognized in September and December 1999, a State may treat emissions from several pollutant sources as falling under one "bubble" for PSD permit purposes only if every pollutant source so aggregated is part of the permit action. In December 1999, however, only MG-17 figured in the permit action. Pp. 496-501. (2) This decision does not impede ADEC from revisiting its BACT determination. In letters and orders throughout the permitting process and at oral argument, EPA repeatedly acknowledged that ADEC
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