Changes in the Temporal Targeting of the U.S. National Ambient Air Quality Standards (NAAQS) for SO 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 SO NAAQS change that altered policy targeting from peak hourly emissions to annually aggregated hourly emissions affected SO emissions from coal-fired power plants. Using data on electricity generating unit (EGU) characteristics, SO emissions, fuel prices, and PM NAAQS classifications from 2001 to 2019 from public data sources, we estimate that, after a county was classified under the 2010 SO standard, EGUs reduced SO 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 SO 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 SO 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 SO NAAQS change may have reduced SO emissions but may not have had an outsized impact on peak emissions despite policy guidance encouraging their control.