A useful, brief article, Robert N. Stavins Lessons Learned from SO2 Allowance Trading,
The allowance trading program has had exceptionally positive welfare effects, with estimated benefits being as much as ten times greater than costs. It is notable that the majority of the benefits of the program are due mainly to the positive human health impacts of decreased local SO2 and particulate concentrations, not to the ecological impacts of reduced long-distance transport of acid deposition. This contrasts with what was assumed at the time of the program’s enactment in 1990.
…despite the uncertainties, market-based instruments for environmental protection — tradable permit systems in particular — now enjoy proven successes in reducing pollution at low cost.