Willial, an emissions trading scheme in the US was (and still is) very successful in reducing the amount of sulphur dioxide (SO2) emitted from coal fired power stations at low cost.
"An early example of an emission trading system has been the SO2 trading system under the framework of the Acid Rain Program of the 1990 Clean Air Act in the U.S. Under the program, which is essentially a cap-and-trade emissions trading system, SO2 emissions were reduced by 50% from 1980 levels by 2007.[84] Some experts argue that the cap-and-trade system of SO2 emissions reduction has reduced the cost of controlling acid rain by as much as 80% versus source-by-source reduction.[10][85]"
