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Progress Report

Emission Reductions

Ozone Season Nitrogen Oxides (NOₓ) Figures

Source: EPA, 2017
Last updated: 05/2017

Related Figures

Ozone Season NOₓ Emissions from CSAPR Sources, 2005–2015
State-by-State Ozone Season NOₓ Emissions from CSAPR Sources, 2000–2015
Comparison of Ozone Season NOₓ Emissions and Heat Input for CSAPR Sources, 2000–2015
CSAPR Ozone Season NOₓ Trends

Key Points

Overall Results

  • Ozone season NOₓ emissions have declined dramatically under the ARP, NBP, CAIR, and CSAPR programs.
  • These reductions have occurred while electricity demand (measured as heat input) remained relatively stable, indicating that the emission reductions were not driven by decreased electric generation.
  • These emission reductions are a result of an overall increase in the environmental efficiency at affected sources as power generators installed controls, switched to lower emitting fuels, or otherwise reduced their ozone season NOₓ emissions while meeting relatively steady electricity demand.
  • Other programs—such as regional and state NOₓ emission control programs—also contributed significantly to the ozone season NOₓ emission reductions achieved by sources in 2015.

Ozone Season NOₓ Emissions Trends

  • Units in the CSAPR NOₓ ozone season program emitted 450,000 tons in 2015, a reduction of 1.8 million tons (80 percent) from 1990, 1.1 million tons lower (70 percent reduction) than in 2000 (before implementation of the NBP), 330,000 tons lower (42 percent reduction) than in 2005 (before implementation of CAIR), and 58,000 tons lower (11 percent reduction) than in 2014 (before implementation of CSAPR).
  • In 2015, CSAPR NOₓ ozone season program emissions were 28 percent below the regional emission budget of 628,392 tons.

Ozone Season NOₓ State-by-State Emissions

  • Between 2005 and 2015, ozone season NOₓ emissions from CSAPR sources fell in every state participating in the CSAPR NOₓ ozone season program.
  • Twenty-three states had emissions below their CSAPR 2015 allowance budgets, collectively by about 180,000 tons. Two states (Louisiana and West Virginia) exceeded their 2015 budgets by about 2,800 tons combined through use of excess allowances in other states.

Ozone Season NOₓ Emission Rates

  • In 2015, the average NOₓ ozone season emission rate fell to 0.10 lb/mmBtu. This indicates a 40 percent reduction from 2005 emission rates, with the majority of reductions coming from coal-fired units.
  • Although heat input has decreased slightly over the past 10 years, emissions have decreased dramatically since 2005, indicating an improvement in NOₓ emission rate. This is due in large part to greater use of control technology on coal-fired units and increased heat input at natural gas-fired units, which emit less NOₓ than coal-fired units.

Analysis and Background Information

Nitrogren oxides, or NOₓ, are made up of a group of highly reactive gases that are emitted from power plants and motor vehicles, as well as other sources. NOₓ emissions contribute to the formation of ground-level ozone and fine particle pollution, which cause a variety of adverse human health effects. NO₂, one form of NOₓ, is specifically linked to adverse health effects on the human respiratory system.

The CSAPR NOₓ ozone season program was established to reduce interstate transport of pollution during the ozone season (May 1 – September 30), the warm summer months when ozone formation is highest, and to help eastern U.S. counties attain the 1997 ozone standard.

In general, the states with the highest emitting sources of ozone season NOₓ emissions in 2000 have seen the greatest emission reductions under the CSAPR NOₓ ozone season program. Most of these states are in the Ohio River Valley and are upwind of the areas CSAPR was designed to protect. Reductions by sources in these states have resulted in important environmental and human health benefits over a large region.

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