Section 203.1170  Excessive Concentration

"Excessive concentration" is defined for the purposes of determining good engineering practice stack height under Section 203.1200(a)(3) and means:

a)         For sources seeking credit for stack height exceeding that established under Section 203.1200(a)(2), 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 that individually is at least 40% in excess of the maximum concentration experienced in the absence of such downwash, wakes, or eddy effects and that contributes to a total concentration due to emissions from all sources that is greater than an ambient air quality standard. For sources subject to this Part, 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 that individually is at least 40% in excess of the maximum concentration experienced in the absence of the downwash, wakes, or eddy effects and greater than an ambient air increment under 35 Ill. Adm. Code 204.900. The allowable emission rate to be used in making demonstrations of excessive concentration shall be prescribed by the NSPS that is applicable to the source category unless the owner or operator demonstrates that this emission rate is infeasible. When those demonstrations are approved by the Agency, an alternative emission rate shall be established in consultation with the source owner or operator.

b)         For sources seeking credit for increases in existing stack heights up to the heights established under Section 203.1200(a)(2), either:

1)         A maximum ground-level concentration due in whole or part to downwash, wakes or eddy effects as provided in subsection (a), except that the emission rate specified by the SIP (or, in the absence of such a limit, the actual emission rate) shall be used; or

2)         The actual presence of a local nuisance caused by the existing stack, as determined by the Agency; and

c)         For sources seeking credit for a stack height determined under Section 203.1200(a)(2) when the Agency requires the use of a field study or fluid model to verify good engineering practice stack height, for sources seeking stack height credit based on the aerodynamic influence of cooling towers, and for sources seeking stack height credit based on the aerodynamic influence of structures not adequately represented by the equations in Section 203.1200(a)(2), a maximum ground-level concentration due in whole or part to downwash, wakes or eddy effects that is at least 40% in excess of the maximum concentration experienced in the absence of such downwash, wakes, or eddy effects.

(Source:  Added at 49 Ill. Reg. 6237, effective April 23, 2025)