By Robynn Andracsek, PE; Providence Engineering and contributing editor

For almost 20 years, the Environmental Protection Agency (EPA) has tried to regulate the effects of air pollution from one state causing issues in a different, downwind state. From the Clean Air Interstate Rule (CAIR) to the current version of the Cross-State Air Pollution Rule (CSAPR), EPA has encountered numerous legal setbacks when executing the Clean Air Act (CAA) to meet this goal.

Based on the highly successful Acid Rain Program, CSAPR uses a trading program to assign allowances and set caps on pollution within an airshed. The public comment window for the latest version of CSAPR (the “Good Neighbor” provisions) recently closed, providing an opportunity to step back and see the larger goals, issues and purposes of this decades-long effort to implement pollution transport regulation.

The foundation of the CAA is the establishment of National Ambient Air Quality Standards (NAAQS) for criteria pollutants such as nitrogen dioxide (NO2), sulfur dioxide (SO2), ozone, and particulate matter less than 2.5 micron in diameter (PM2.5).

A network of ambient air monitors across the country measures the concentration of each pollutant and compares it to these numerical standards. Counties with air quality worse than the NAAQS can be designated as “non-attainment” and required to set strict limits on large projects (major modifications) at existing sources as well as on new large sources of air pollution.

Power generation usually bears the brunt of these regulations. That’s because a single control device on a large stationary source is more impactful than addressing thousands of smaller sources. The CAA requires EPA to reevaluate the NAAQS every five years, leading to simultaneous review of multiple standards. Note that the problem pollutant is corrected by controlling both the NAAQS pollutant and its precursors. So, for example, NOx plus sunlight equals ozone; NOx and SO2 create PM2.5.

Problems arise when it comes to grandfathering existing power plants into the original 1970 CAA. A bedrock rule in the CAA is what is known as Prevention of Significant Deterioration (PSD). This requires a case-by-case analysis of the latest control devices for new and modified large sources of pollution. CAIR and CSAPR are newer ways to address PSD-regulated pollutants.

EPA has used PSD lookbacks and power plant enforcement initiatives to compel older power plants to install controls, shift fuels, or shut down. There are fewer and fewer remaining power plants that are uncontrolled or still burning fossil fuels. EPA assumed power plants would go through PSD permitting and install required controls by roughly 1980-1990. However, power plant operators learned how to use “routine maintenance” as a way to keep old plants operating.

EPA’s Acid Rain Program effectively used a NOX and SO2 cap-and-trade program, which all but eliminated acid rain but did not cause reductions large enough to achieve compliance with low NAAQS limits. The Acid Rain Program addresses the direct effects of NOx and SO2 emissions; it has reduced emissions by 87% and 92%, respectively, since 1995.

However, as EPA gained a fuller understanding of the health effects of fine particulate matter, transport rules tackled how precursors like NOx and SO2 form PM2.5. The proposed Good Neighbor CSAPR provisions pull in non-electric generation sources and broaden EPA’s influence over interstate emissions.

LEFT: This map illustrates the interconnected, regional nature of interstate air pollution transport. The map shows “linkages” between pollution from upwind states and receptors in downwind states that had problems meeting the 2008 ozone NAAQS. These linkages were identified in the context of the Cross-State Air Pollution Rule (CSAPR) Update. RIGHT: Upwind States Contributing Above 1% to Downwind States in 2023 for the 2015 Ozone NAAQS (Source: EPA).

The CAA contains a “good neighbor” provision that requires states to address interstate transport of air pollution that affects downwind states’ ability to attain and maintain NAAQS. CSAPR is meant as a replacement to CAIR. But legal issues with these rules have included linking pollutant impacts to sources, compliance timing, and imposing a federal solution before allowing states to develop their own solution.

Regulation by litigation

The District of Columbia Circuit Court struck down the CAIR rule because EPA’s approach violated the plain language of the federal CAA by setting regionwide caps with no state-specific quantitative contribution determinations or emissions requirements.

In short, not connecting upwind states’ emission reductions to the measurable non-attainment of downwind states was fundamentally flawed. The DC Circuit Court remanded the CSAPR Update to the EPA to address its holding that the rule unlawfully allows significant contribution to continue beyond downwind attainment deadlines.

Legal issues with transport and greenhouse gas rules include methods of forcing power plants to shift generation to cleaner fuels or to renewables. In general, this has not been upheld in court with many proposed greenhouse rules never making it into law (for example, the Clean Power Plan and Affordable Clean Energy rule).

In West Virginia v. Environmental Protection Agency, decided on June 30, the Supreme Court held that the CAA did not grant EPA authority to devise emissions caps based on the generation-shifting approach the agency took in the Clean Power Plan.

Efforts to regulate greenhouse gases are related to the transport rules given that the Good Neighbor CSAPR echoes the generation-shifting provisions of the newly struck down Clean Power Plan; it is likely that EPA will modify the proposed Good Neighbor CSAPR.

Additionally, a significant number of comments on Good Neighbor CSAPR were received from both utility and industrial sources that are new to this type of regulation (such as natural gas pipelines, cement plants, steel mills, glass plants, chemical plants, pulp/paper industries). Comments included errors in the modeling analysis, incorrect assumptions on feasible and existing controls for power plants, and the cost of controls inside a state versus controlling upwind sources.

The best (and least likely) solution is for Congress to amend the CAA to specifically address transport control methods. However, a more probable and problematic outcome is to continue regulating through litigation.

As you follow the evolution of Good Neighbor CSAPR to determine its effects on your facilities, remember that EPA will continue to work to implement this type of regulation…until it sticks.

About the Author: 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

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