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Overview of the Cross-State Air Pollution Rule (CSAPR)

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On July 6, 2011 the US Environmental Protection Agency (EPA) finalized the Cross-State Air Pollution Rule (CSAPR) to address air pollution from upwind states that crosses state lines and affects air quality in downwind states. Sulfur dioxide (SO2) and oxides of nitrogen (NOX) emissions react in the atmosphere and contribute to the formation of fine particle (soot) pollution. NOX also contributes to ground-level ozone (smog) formation. These emissions and the soot and smog they form can affect air quality and public health locally, regionally, and in states hundreds of miles downwind.  For more information on interstate transport of air pollution, see EPA’s interstate air transport page.

This rule requires certain states in the eastern half of the U.S. to improve air quality by reducing power plant emissions that cross state lines and contribute to smog and soot pollution in downwind states. These improvements help downwind areas attain and maintain EPA's health-based soot and smog air quality standards known as National Ambient Air Quality Standards (NAAQS).

The CSAPR replaced EPA’s 2005 Clean Air Interstate Rule (CAIR), following the direction of a 2008 court decision that required EPA to issue a replacement regulation.  CSAPR implementation began on January 1, 2015.

On September 7, 2016, the EPA revised the CSAPR ozone season NOX program by finalizing an update to CSAPR for the 2008 ozone National Ambient Air Quality Standards, known as the CSAPR Update. The CSAPR Update ozone season NOX program will largely replace the original CSAPR ozone season NOX program starting on May 1, 2017. The CSAPR Update will further reduce summertime NOX emissions from power plants in the eastern U.S.

To view information on affected units, emission reductions, pollution controls and monitoring, compliance, environmental, and air quality results under the CSAPR programs, see the progress report.

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Who is Affected?  

The CSAPR requires fossil fuel-fired electric generating units at coal-, gas-, and oil-fired facilities in 27 states to reduce emissions to help downwind areas attain fine particle and/or ozone NAAQS. 

CSAPR requirements have been promulgated to address interstate transport for the 2006 24-hour fine particulate NAAQS, 1997 annual fine particulate NAAQS, 1997 8-hour ozone NAAQS, and 2008 ozone NAAQS, resulting in the creation of several air quality-assured trading programs for states in the CSAPR region:

  • The CSAPR SO2 Group 1 trading program;
  • The CSAPR SO2 Group 2 trading program;
  • The CSAPR NOX annual trading program;
  • The CSAPR NOX ozone season Group 1 trading program;
  • The CSAPR NOX ozone season Group 2 trading program.

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CSAPR Framework

CSAPR provides a 4-step process to address interstate transport of certain air pollutants:

  1. Identifying downwind receptors that are expected to have problems attaining or maintaining clean air standards (i.e., NAAQS);
  2. Determining which upwind states contribute to these identified problems in amounts sufficient to “link” them to the downwind air quality problems;
  3. Identifying upwind emissions that significantly contribute to nonattainment or interfere with maintenance of a standard by quantifying appropriate upwind emission reductions and assigning upwind responsibility among linked states; and
  4. Reduce the identified upwind emissions via permanent and enforcable requirements (e.g., regional allowance trading programs).

EPA also issued a memo along with the 2015 ozone NAAQS, noting its belief that interstate transport for the 2015 ozone NAAQS can be addressed in a timely fashion using the CSAPR framework.

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How the CSAPR Trading Programs Works

EPA sets a pollution limit (emission budget) for each of the states covered by CSAPR.  Authorizations to emit pollution, known as allowances, are allocated to affected sources based on these state emissions budgets. The rule provides flexibility to affected sources, allowing sources in each state to determine their own compliance path. This includes adding or operating control technologies, upgrading or improving controls, switching fuels, and using allowances. Sources can buy and sell allowances and bank (save) allowances for future use as long as each source holds enough allowances to account for its emissions by the end of the compliance period.

CSAPR includes provisions to assure that each state will meet its pollution control obligations. These provisions ensure that each state eliminates the SO2 and NOX emissions that significantly contribute to downwind nonattainment or interfere with maintenance of NAAQS.

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