D promulgated national ambient air quality standards (NAAQS) for air pollutants, which states must implement, maintain, and enforce through state implementation plans (SIP). Even after a SIP is approved, D may at a later time call for SIP revisions if the Administrator finds a SIP inadequate to attain or maintain the NAAQS, to meet the dictates of pollutant transport commissions, or 'to otherwise comply with any requirement of this chapter.' In October 1998 EPA issued a final rule mandating that 22 states and the District of Columbia revise their SIPs to mitigate the interstate transport of ozone. D mandated 'highly cost-effective controls,' namely, those controls D found capable of removing NOX at a cost of $ 2000 or less per ton. D promulgated a new, more stringent '8-hour standard' which limits ozone levels to 0.08 ppm, averaged over an 8-hour period. D had found that Ps were significant contributors to downwind pollution. Section 110(a)(2)(D)(I)(i) requires states from having emissions that contribute significantly to downwind air pollution issues. Section 110(a)(2)(D)(i)(I) applies only to states that 'contribute significantly' to nonattainment in a downwind state. The two types of state-specific modeling go by the names UAM-V and CAMx. D’s goal was to make modest reductions. D’s SIP addressed regional ozone attainment through reduction of NOX. Ps claimed that nothing in the text of the new section or any other provision of the statute spells out a criterion for classifying 'emissions activity' as 'significant.' Nor did D, under the then-existing provision, bind itself to any criterion. Further, given D's finding as to the cumulative effects of the pollutants that generate ozone, D might well be able to distinguish this case from the sulfur dioxide cases that the states have cited. D allowed Ps to pick the control measures to fit within the budgets of each state. Each P could adopt interstate trading to purchase NOX allowances from sources. States could also use allowances they have banked over the years. Ps claimed that D had acted contrary to existing precedent, it had considered costs of reduction and the budget program had intruded on the statutory right of states to adopt their own SIP.