Wis. Stat. § 285.49: Trading program for nitrogen oxide emissions credits.

The department shall establish or authorize air contaminant sources to participate in a market-based trading program for the purchase, sale and transfer of nitrogen oxide emissions credits for use in any state implementation plan under s. 285.11 (6) that requires reductions in nitrogen oxide emissions. To the extent allowed under federal law, the department shall allow nitrogen oxide emissions reductions by any source in this state, regardless of whether the source is subject to nitrogen oxide controls under a state implementation plan, to be purchased, sold or transferred under the trading program.