(5)(b) a maximum ground-level concentration due to

(5)(b) a maximum ground-level concentration due to emissions from a stack due in whole or part to downwash, wakes, and eddy effects produced by nearby structures or nearby terrain features which individually is at least 40 percent in excess of the maximum concentration experienced in the absence of such downwash, wakes, or eddy effects and which contributes to a total concentration due to emissions from all sources that is greater than an ambient air quality standard. For sources subject to the Prevention of Significant Deterioration program, an excessive concentration alternatively means a maximum ground-level concentration due to emissions from a stack due in whole or part to downwash, wakes, or eddy effects produced by nearby structures or nearby terrain features which individually is at least 40 percent in excess of the maximum concentration experienced in the absence of such downwash, wakes, or eddy effects and greater than a prevention of significant deterioration increment. The allowable emission rate to be used in making demonstrations under this part shall be prescribed by the new source performance standard that is applicable to the source category unless the owner or operator demonstrates that this emission rate is infeasible. Where such demonstrations are approved by the U. S. Environmental Protection Agency or the Commission, an alternative emission rate shall be established in consultation with the source owner or operator. (2) For sources seeking credit after October 11, 1983, for increases in existing stack heights up to the heights established under Rule 1.9.A(5)(b) either a maximum ground-level concentration due in whole or part to downwash, wakes, or eddy effects as provided in Rule 1.9.A(5)(b) of this rule, except that the emission rate specified by the State implementation plan (or, in the absence of such a limit, the actual emission rate) shall be used, or the actual presence of a local nuisance caused by the existing stack, as determined by the Commission, and (3) For sources seeking credit after January 12, 1979, for a stack height determined under Rule 1.9.A(5)(b) where the U. S. Environmental Protection Agency or the Commission requires the use of a field study or fluid model to verify GEP stack height, for sources seeking stack height credit after November 9, 1984, based on the aerodynamic influence of cooling towers, and for sources seeking stack height credit after December 31, 1970, based on the aerodynamic influence of structures not adequately represented by the equations in Rule 1.9.A(5)(b) a maximum ground-level concentration due in whole or part to downwash, wakes, or eddy effects that is at least 40 percent in excess of the maximum concentration experienced in the absence of such downwash, wakes, or eddy effects. B. Stack Height Effect on Emission Limitations (1) The degree of emission limitation required of any source for control of any air pollutants shall not be affected by so much of any source's stack height that exceeds good engineering practice (GEP) or by any other dispersion technique, except as provided in (2) of this paragraph. (2) The provisions of Rule 1.9.B(1) shall not apply to stack heights in existence, or dispersion techniques implemented, prior to December 31, 1970, except where pollutants are being emitted from such stacks or using such dispersion techniques by sources, as defined in Section 111(a)(3) of the Clean Air Act, which were constructed, or reconstructed or for which major modifications, as defined pursuant to Rule 1.6. New Sources, were carried out after December 31, 1970. (3) If any existing source, after appropriate application of the preceding limitations and provisions, is found to exceed or potentially exceed an air quality standard or increment, as appropriate, when operating within previously established emission limitations, the emission limitations applicable to that source shall be modified so as to eliminate and prevent the exceedance. (4) If any new source or source modification, after appropriate application of the preceding limitations and provisions, is predicted to exceed an air quality standard, or increment, as appropriate, when considered as operating under emission limitations consistent with other Applicable Rules and Regulations, the emission limitations considered shall be deemed inadequate and different emission limits, based on air quality considerations, shall be made applicable. (5) If any source provides a field study or fluid modeling demonstration proposing a GEP stack height greater than that allowed by Rule 1.9.A(5)(a) and (b) then the public will be notified of the availability of the study and provided the opportunity for a public hearing before any new or revised emission limitation or permit is approved. (6) The actual stack height used or proposed by a source shall not be restricted in any manner by requirements of this paragraph.