The introduction of the Clean Air Interstate Rule in 2005 has led to a big reduction in pollution from eastern US coal power plants, data from NASA's Aura satellite shows.
Levels of sulfur dioxide - which contributes to the formation of acid rain and can cause serious health problems - have almost halved since 2005. About two-thirds of sulfur dioxide pollution in American air comes from coal power plants.
And, after the introduction of the new legislation, which placed a cap on emissions, many power plants installed desulfurization devices and took other steps to limit the release of sulfur dioxide.
Scientists have previously used the satellite's Ozone Monitoring Instrument to observe sulfur dioxide levels within large plumes of volcanic ash and over heavily polluted parts of China. But this is the first time they have observed such subtle details over the much-less-polluted United States.
The secret is a new mathematical approach centered on averaging measurements within a 30-mile radius of a sulfur dioxide source over several years. It makes it possible to detect emissions at levels four times lower before.
"Now that we've confirmed that the technique works, the next step is to use it for other parts of the world that don't have ground-based sensors," says Nickolay Krotkov, a researcher based at NASA's Goddard Space Flight Center.
"The real beauty of using satellites is that we can apply the same technique to the entire globe in a consistent way."
The team also plans to use a similar technique to monitor other important pollutants that coal power plants release, such as nitrogen dioxide, a precursor to ozone.