> Since the 1990s, SO2 emissions have dropped 40%, and according to the Pacific Research Institute, acid rain levels have dropped 65% since 1976.
...although it seems debatable whether it is more effective than plain regulation.
> However, although it reduced emissions by 40%, the US Acid Rain Program has not reduced SO2 emissions as much as the conventional regulation applied in the European Union (EU), which reduced SO2 emissions by more than 70%. Therefore, the effectiveness of the emissions trading element as a mechanism has been criticised, ...
https://en.wikipedia.org/wiki/Acid_Rain_Program
> Since the 1990s, SO2 emissions have dropped 40%, and according to the Pacific Research Institute, acid rain levels have dropped 65% since 1976.
...although it seems debatable whether it is more effective than plain regulation.
> However, although it reduced emissions by 40%, the US Acid Rain Program has not reduced SO2 emissions as much as the conventional regulation applied in the European Union (EU), which reduced SO2 emissions by more than 70%. Therefore, the effectiveness of the emissions trading element as a mechanism has been criticised, ...