Thanks to Clean Air Act Amendments of 1990It's pretty obvious to anyone who looks back far enough that air quality regulations have had a huge impact, at least when it comes to pollutants other than greenhouse gases. The U.S. Energy Information Administration (EIA) has just released some data that further supports the point, showing that since the Clean Air Act was amended in 1990 to fight acid rain, ozone depletion, and control 189 toxic pollutants, sulfur dioxide (SO2) and nitrogen oxides (NOx) emissions from the U.S. power industry have declined significantly. But it didn't happen by accident, and countries with huge air quality issues like China should take note and get their own version of the Clean Air Act as soon as possible.
The decline in emissions is due primarily to an increasing number of coal-fired units retrofitted with flue-gas desulfurization (FGD), or scrubbers, to coal plants switching to lower sulfur coal, and to selective catalytic reduction (SCRs), selective noncatalytic reduction (SNCR), or low NOx burners to limit NOx emissions. In recent years the decreased use of coal for electric power generation because of cheaper natural gas has also played a significant role in the SO2 and NOx emissions declines. (source)
While after 2008 a lower usage of coal and low natural gas prices have been big factors to explain the reduction, they are not the only ones, and if we look before 2007, we can see that most of the reduction was due to coal plants employing strategies to reduce emissions, not a reduction in overall coal usage.
Obviously, our goal should be to replace polluting power sources with clean ones, starting with the very worst power plants and going down the list from there.