Changes in the Temporal Targeting of the U.S. National Ambient Air Quality Standards (NAAQS) for SO₂ Reduce Average and Peak Emissions from Coal Power Plants

The U.S. National Ambient Air Quality Standards (NAAQS) protect public health by limiting ambient pollutant concentrations, but effects at power plants are not well characterized. We estimate how a 2010 SO₂ NAAQS change that altered policy targeting from peak hourly emissions to annually aggregated hourly emissions affected SO₂ emissions from coal-fired power plants. Using data on electricity generating unit (EGU) characteristics, SO₂ emissions, fuel prices, and PM_2.5 NAAQS classifications from 2001 to 2019 from public data sources, we estimate that, after a county was classified under the 2010 SO₂ standard, EGUs reduced SO₂ emissions during daily maximum hours by 31.6% (95% CI [-0.381, -0.247]) at the 99th percentile and 34.4% (95% CI [-0.424, -0.253]) at the 50th percentile. After a nearby ambient SO₂ monitor was added to assess NAAQS compliance, hourly emissions fell by 12.4% (95% CI [-0.188, -0.054]) at the 99th percentile and 14.4% (95% CI [-0.228, -0.050]) at the 50th percentile. Our results also suggest the new standard had the same effect on peak hourly SO₂ emissions as median and that plants in counties prioritized for early designation were less responsive to the policy change. These results suggest that the 2010 SO₂ NAAQS change may have reduced SO₂ emissions but may not have had an outsized impact on peak emissions despite policy guidance encouraging their control.