# 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

## TL;DR

A 2010 change in U.S. air quality standards for sulfur dioxide (SO2) led to reduced emissions from coal power plants, especially during peak hours.

## Contribution

The study quantifies how a policy shift in SO2 regulation affected emissions from coal-fired power plants.

## Key findings

- SO2 emissions during peak hours dropped by 31.6% at the 99th percentile after the policy change.
- Adding ambient SO2 monitors led to a 12.4% reduction in peak emissions.
- Plants in prioritized counties showed less responsiveness to the policy change.

## Abstract

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 PM2.5 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.