Bartels et al. (1993) describe the Market Clearing Auction (MCA), a computer-assisted smart auction designed to replicate the outcome of an efficient market in sulfur dioxide (SO2) emissions allowances created by the Clean Air Act Amendment of 1990. The system is a centralized marketplace for trading SO 2 emission allowances and accepts bids and offers for any combination of allowances and for streams of allowances covering more than one year.