Title IV of the Clean Air Act, as amended by Clean Air Act Amendments of 1990, (``Act'') authorizes the Environmental Protection Agency (``EPA'' or ``Agency'') to establish the Acid Rain Program. The purpose of the Acid Rain Program is to significantly reduce emissions of sulfur dioxide and nitrogen oxides from electric generating plants in order to reduce the adverse health and ecological impacts of acidic deposition (or acid rain) resulting from such emissions. This final rule is intended to promote participation in the title IV opt-in program by clarifying existing regulations, allowing a limited exception to the general rule of one designated representative for all affected units at a source, revising the conditions under which the Agency may cancel current-year allowance allocations, and allowing thermal energy plans to be effective on a quarterly basis.