Nitrogen is the most common part of the air we breathe: about 80% of the air is nitrogen. When air is heated, like in coal boilers, nitrogen atoms break apart and join with oxygen, forming nitrogen oxides (NOx) (rhymes with "socks"). NOx can also be formed from the atoms of nitrogen that are trapped inside coal. Coal combustion release oxides of nitrogen, which react with volatile organic compounds in the presence of sunlight to produce ground-level ozone, the primary ingredient in smog.
In atmospheric chemistry and air pollution and related fields, nitrogen oxides refers specifically to NOx (NO and NO2).
Health and environmental effects
In the air, NOx is a pollutant. Coal combustion release oxides of nitrogen, which react with volatile organic compounds in the presence of sunlight to produce ground-level ozone, the primary ingredient in smog. Asthma exacerbations have been linked specifically to exposure to ozone. Nitrogen oxide also contributes to fine particulate matter (PM), found in soot, which is also linked to a host of serious health effects.
Exposures to ozone and PM are both correlated with the development of and mortality from lung cancer. Recent research suggests that nitrogen oxides and PM2.5, along with other pollutants, are associated with hospital admissions for potentially fatal cardiac rhythm disturbances. Cities with high nitrogen dioxide (NO2) concentrations have death rates four times higher than those with low NO2 concentrations, suggesting a potential correlation.
The EPA Office of Air Quality Planning and Standards (OAQPS) has set National Ambient Air Quality Standards under the Clean Air Act for six principal pollutants, which are called "criteria" pollutants: sulfur dioxide, particulate matter, nitrogen oxides, ozone, lead, and carbon monoxide. After the EPA sets or revises each standard and a timeline for implementation, the responsibility for meeting the standard falls to the states. Each state must submit an EPA-approved plan that shows how it will meet the standards and deadlines. These state plans are known as State Implementation Plans (SIPs)." 
The Clean Air Transport Rule is a proposed rule of the EPA requiring significant reductions in sulfur dioxide (SO2) and nitrogen oxides (NOx) emissions that cross state lines. By 2014, the rule and other state and EPA actions would reduce power plant SO2 emissions by 71 percent over 2005 levels and NOx emissions by 52 percent, according to EPA estimates. The proposed rules will replace the EPA's 2005 Clean Air Interstate Rule, which was vacated in July 2008 by the D.C. Circuit Court. Acting under federal court order, the Obama administration proposed the new air-quality rules on July 6, 2010, for coal-burning power plants. The proposed rule would require NOx and SO2 emission reductions in 2012 and additional SO2 emission reductions in 2014.
The pollutants being singled out in the new rule making — sulfur dioxide and nitrogen oxides — react in the atmosphere to form fine particulates and ground-level ozone (smog). They are easily carried by the wind and affect states and cities far downwind from the plants where they are produced. The Transport Rule would apply to power plants in 31 states east of the Rockies, with the exception of the Dakotas, Vermont, New Hampshire and Maine.
Gina McCarthy, head of the EPA’s air and radiation office, said the new rules would reduce emissions of sulfur dioxide and nitrogen oxides by hundreds of thousands of tons a year and bring $120 billion in annual health benefits. Those benefits, Ms. McCarthy said, include preventing 14,000 to 36,000 premature deaths, 23,000 nonfatal heart attacks, 21,000 cases of acute bronchitis, 240,000 cases of aggravated asthma and 1.9 million missed school and work days. Additionally, the rule would substantially reduce unhealthy smog. The cost of compliance to utilities and other operators of power plants would be $2.8 billion a year, according to E.P.A. estimates.
The proposed regulation will require utilities operating coal-burning plants to install scrubbers and other technology to reduce emissions of the pollutants. Some companies may decide to retire older plants rather than invest in new control measures because other new rules under the Clean Air Act are expected in the coming years. The new rules do not address power plant emissions of carbon dioxide and five other pollutants that contribute to global warming. The Obama administration is moving forward with a plan to phase in regulation of such heat-trapping gases, a move that is being challenged in Congress and in the courts.
On August 21, 2012, a federal appeals court in Washington, D.C. ruled 2-1 against the Transport Rule. The court determined that the rule forced states to be accountable for pollution that was not theirs, potentially making some states responsible for reducing pollution in other states that may be primarily caused by third party states. Second, the court found that the way in which the rule mandates obligations on polluting states is in conflict with existing mandates under the Clean Air Act, as the EPA did not allow states the initial opportunity to implement the required reductions within their borders. The two judges in the majority were appointed by George W. Bush; the dissenter by Bill Clinton.
March 2011: New EPA Standards for Mercury and Air Toxics Proposed
On March 16, 2011, the U.S. Environmental Protection Agency (EPA) announced its proposed emissions standards to limit mercury, acid gases and other toxic pollution from power plants, to prevent an estimated 91 percent of the mercury in coal from being released to the air. The EPA estimates that there are approximately 1,350 units affected by the action, including 1,200 existing coal-fired units.
There are currently no existing national limits on the amount of mercury and other toxic air pollution released from power plant smokestacks. The 1990 Clean Air Act Amendments mandated EPA control toxic air pollutants, and the EPA took action to reduce mercury emissions from the highest-emitting sources, except power plants, as the Clean Air Mercury Rule passed under President George W. Bush was vacated by a court.
The proposed toxics rule would reduce emissions of heavy metals, including mercury (Hg), arsenic, chromium, and nickel, and acid gases, including hydrogen chloride (HCl) and hydrogen fluoride (HF). EPA is also proposing to revise the New Source Review performance standards (NSPS) for fossil-fuel-fired plants. This NSPS would revise the standards new coal- and oil-fired power plants must meet for particulate matter (PM), sulfur dioxide (SO2), and nitrogen oxides (NOx). The proposed standards should reduce mercury emissions from power plants burning coal and oil by 91 percent, acid gas pollution by 91 percent, direct particulate matter emissions by 30 percent, and sulfur dioxide (SO2) emissions by 53 percent, down to 2.1 million tons of annual SO2 emissions.
The EPA's proposed standards are projected to save as many as 17,000 lives every year by 2015; prevent up to 120,000 cases of childhood asthma symptoms and 11,000 fewer cases of acute bronchitis among children every year; avoid more than 12,000 emergency room and hospital visits annually; and prevent 850,000 lost work days every year. The monetized benefits from the improved health standards are estimated to be $59 billion to $140 billion annually, compared to annual compliance costs of approximately $10.9 billion. The EPA also projects that the proposed standards will create up to 31,000 short-term construction jobs and 9,000 long-term utility jobs.
Requirements of the new standards include:
- For all existing and new coal- and oil-fired electric utility steam generating units (EGUs), the proposed standards would establish numerical emission limits for mercury, PM, and HCl.
- For all existing and new oil-fired EGUs, the proposed toxics rule would establish numerical emission limits for total metals, HCl, and HF.
- Actions available to power plants to meet the emission limits include wet and dry scrubbers, dry sorbent injection systems, activated carbon injection systems, and baghouses, all part of Maximum Achievable Control Technology (MACT).
- The proposed standards would establish work practices, instead of numerical emission limits, to limit emissions of organic air toxics, including dioxin/furan, from existing and new coal- and oil-fired power plants.
- The proposed revisions to the NSPS would include revised numerical EGU emission limits for PM, SO2, and NOX.
- "Knocking the NOx Out of Coal" DOE, accessed September 2010.
- "Coal Power: Air Pollution," Union of Concerned Scientists, accessed September 2010
- United States Clean Air Act
- Alan Lockwood, Kristen Welker-Hood, Molly Rauch, Barbara Gottlieb,"Coal's Assault on Human Health" Physicians for Social Responsibility Report, November 2009
- "The Health Risks of Burning Coal for Energy" The Environmental Defense Fund, September 5, 2006
- "Deadly Power Plants? Study Fuels Debate" MSNBC.com, June 9, 2004
- "America's Biggest Polluters: Carbon Dioxide Emissions from Power Plants in 2007" Environment America, November 24, 2009
- "NAAQS" Sierra Club, accessed July 2010.
- "Air Transport" EPA, accessed Feb. 2011.
- John Broder, "New Rules Issued on Coal Air Pollution" New York Times, July 6, 2010.
- "Proposed Transport Rule Would Reduce Interstate Transport of Ozone and Fine Particle Pollution," EPA Fact Sheet, accessed July 8, 2010
- "EPA "Transport Rule" Struck Down by Court," American Spectator, August 21, 2012.
- "Fact Sheet: Proposed Mercury and Air Toxics Standards" EPA, accessed March 2011.
- John Walke, "A little background on the EPA’s new mercury and air toxics rule" Grist, March 16, 2011.
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