In 2007, China required some coal power plants to install Continuous Emissions Monitoring Systems (CEMS) to track pollutants given off by those stations. In 2014, the Chinese government implemented tighter emissions standards for coal plants. Have those two regulations worked to reduce pollution?

The answer is yes, but with a few caveats.

In a paper published last week, researchers compared troves of CEMS readings to emissions readings from a NASA-owned satellite. They found that while Chinese regulations definitely lowered sulfur dioxide (SO₂) levels, there’s some evidence that CEMS data shows an overly rosy picture of how large the drop was. Coal plants in key regions were given tighter standards than those elsewhere, and those plants facing tighter standards appeared to miss their targets more often than not.