By Robynn Andracsek, PE; Providence Engineering and contributing editor and Leslie Fifita, Providence Engineering

In a major impact to coal-fired boilers and other power plants, EPA’s January 2023 proposed rule change will lower the National Ambient Air Quality Standards (NAAQS) for fine particulate. Exactly how will this impact your facility?

First, a little background. The Clean Air Act requires the Environmental Protection Agency (EPA) to differentiate “clean” air from “dirty” by establishing NAAQS. EPA’s science policy workshop provides guidance on the level of air cleanliness required to protect the most sensitive portions of the population: elderly, asthmatics and children. NAAQS are reviewed approximately every five years and are established for six criteria pollutant at different averaging periods. Particulate matter is divided into fine particulate less than 2.5 microns in diameter (PM2.5) and coarse particulate less than 10 microns in diameter (PM10). EPA last reviewed the particulate matter NAAQS in 2012.

EPA explains in the proposal that “Fine particles, sometimes called soot, can penetrate deep into the lungs and can result in serious health effects that include asthma attacks, heart attacks and premature death – disproportionately affecting vulnerable populations including children, older adults, those with heart or lung conditions, as well as communities of color and low-income communities throughout the United States.”

The proposed rule, currently on a 60-day public comment period, would lower the annual average PM2.5 NAAQS from 12 micrograms per cubic meter (µg/m3) to between 8 and 11 µg/m3, with the likely final NAAQS at 9 or 10 µg/m3. The 24-hour average PM2.5 and both the annual and 24-hour PM10 average NAAQS would not be changed.

EPA monitors the air quality across the country. Figures 1-3 show which counties are currently nonattainment with the 12 µg/m3 NAAQS and would likely become non-attainment under either 9 or 10 µg/m3.

For a closer, more interactive look at these maps, see the following links:

PM2.5 Annual NAAQS is 12 ug/m3 (Current)
PM2.5 Annual NAAQS is 10 ug/m3 (Current)
PM2.5 Annual NAAQS is 9 ug/m3 (Current)

A lowering of the NAAQS, which will theoretically be more protective of public health, will hugely affect utilities. The annual PM2.5 standard is already strict and difficult to comply with when conducting dispersion modeling. A model predicts the air quality of a facility plus its neighboring sources plus a background concentration to represent impacts from farther away upwind facilities. In many parts of the country, background concentrations are already at 9 or 10 µg/m3, leaving little to no airshed available for new facilities or increases in emissions at existing facilities.

The effects are numerous and significant:

  1. In counties which will be classified as nonattainment under the revised NAAQS, the state will be required to develop a plan to bring the area into compliance. New rules will specifically target existing facilities to lower emissions and retrofit control devices. Power plants will need to dust off their life expectancy analyses for each plant and factor in these impacts. New facilities and new projects which trigger major permits will be required to install controls to meet the Lowest Achievable Emission Rate (LAER) instead of Best Available Control Technology (BACT) controls. The main difference between LAER and BACT is that LAER does not allow for cost considerations.
  2. A lowering of the NAAQS is yet another regulatory incentive to switch away from coal combustion, a critical baseload fuel. Every time a new rule targets power plants, more boilers switch fuels or cease operation. Again, this impacts a power plant’s life cycle analysis.
  3. When a new major facility is constructed near an existing power plant, the dispersion model will include the emissions of the neighboring power plant. The modeling results can frequently indicate that the power plant is out of compliance with the new standards even though the power plant did not make any changes or modify its permit. The result could force the power plant to revise their emissions, stacks, limits, permits, etc. until a dispersion model indicates compliance.
  4. Even gas-fired plants will be impacted. The trick will be to avoid Prevention of Significant Deterioration (PSD) or Non-attainment New Source Review (NNSR) permitting, which is easier for simple-cycle turbines and harder for combined-cycle turbine or boilers.

A lowering of the annual PM2.5 NAAQS has been expected for years and was delayed due to avoidance by the last EPA administration. At this point, it is inevitable. To prepare, utilities should know the air quality where each plant is located, preemptively conduct dispersion modeling, and update their long term forecasts for utilization.

About the Authors: Robynn Andracsek, PE, is a Senior Air Quality Engineer at Providence Engineering and Environmental Group LLC with 26 years of experience in air permitting for utilities and district energy facilities.  Providence is an employee-owned, multidisciplinary engineering and environmental consulting firm. Our work has taken us across the United States and beyond in support of our governmental and industrial clients’ goals and challenges all the while holding an unwavering dedication to our founding principles – to take care of our clients, make a little money, and have fun while doing it. Her email address is

Leslie Fifita is a Senior Air Modeler at Providence Engineering and Environmental Group LLC with 22 years of experience in air quality consulting.  She focuses on AERMOD modeling in support of air permit applications for large industrial clients.  She also manages the software team (Providence/ORIS) which provides air dispersion modeling software tools, training, and cloud-based modeling solutions.  Her email address is

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