Nitrogen dioxide

Nitrogen dioxide (NO₂) belongs to a family of highly reactive gases called nitrogen oxides (NOx). These gases form when fuel is burned at high temperatures, and come principally from motor vehicle exhaust and power plants. Described by the EPA as "a suffocating, brownish gas," nitrogen dioxide is a strong oxidizing agent that reacts in the air to form corrosive nitric acid, as well as toxic organic nitrates. It also plays a major role in the atmospheric reactions that produce ground-level ozone (or smog).

Chemistry

NO₂ is formed in most combustion processes using air as the oxidant. At elevated temperatures nitrogen combines with oxygen to form nitric oxide:

O2 + N2 → 2 NO

Nitric oxide can be oxidized in air to form nitrogen dioxide, although at normal atmospheric concentrations this is a very slow process:

2 NO + O2 → 2 NO₂

The most important sources of NO₂ are internal combustion engines, power stations and, to a lesser extent, pulp mills. Butane gas heaters and stoves are also sources. The excess air required for complete combustion of fuels in these processes introduces nitrogen into the combustion reactions at high temperatures and produces nitrogen oxides (NOx).

Health and Environmental Effects

Nitrogen dioxide can irritate the lungs and lower resistance to respiratory infections such as influenza. Continued or frequent exposure to concentrations that are typically much higher than those normally found in the ambient air may cause increased incidence of acute respiratory illness in children. EPA's health-based national air quality standard for NO₂ is 0.053 ppm (measured as an annual arithmetic mean concentration). Nitrogen oxides contribute to ozone formation and can have adverse effects on both terrestrial and aquatic ecosystems. Nitrogen oxides in the air can significantly contribute to a number of environmental effects such as acid rain and eutrophication in coastal waters like the Chesapeake Bay. Eutrophication occurs when a body of water suffers an increase in nutrients that leads to a reduction in the amount of oxygen in the water, producing an environment that is destructive to fish and other animal life.

Studies have noted cities with high nitrogen dioxide (NO₂) concentrations have death rates four times higher than those with low NO₂ concentrations, suggesting a potential correlation.

Aging coal plants "grandfathered" in after passage of the Clean Air Act have been particularly linked to large quantities of harmful emissions.

Regulation

In November 2009, the EPA proposed the Transport Rule, the first time since 1971 that the agency had recommended tightening controls on sulfur dioxide (SO₂) to protect public health. Acting under federal court order, the Obama administration proposed new air-quality rules on July 6, 2010, for coal-burning power plants. The pollutants being singled out in the new rule making — sulfur dioxide and nitrogen oxides — are easily carried by the wind and affect states and cities far downwind from the plants where they are produced, and react in the atmosphere to form fine particulates and ground-level ozone (smog). The proposed regulation would apply to power plants in 31 states east of the Rockies, with the exception of the Dakotas, Vermont, New Hampshire and Maine. The proposed rules will replace the EPA's 2005 Clean Air Interstate Rule.

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.

Resources

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