Nitrogen oxides

Summary: The term nitrogen oxides refers to a group of compounds denoted by the chemical shorthand NO_x. Both are gases and combine into gases. Nitrogen oxides are among the greenhouse gases, and also contribute to acid rain.

Nitrogen is a vital element for living beings as a fundamental building block of organic compounds, and it is the most abundant gas in Earth’s atmosphere. Nitrogen oxides are a group of molecular compounds that combine different quantities of nitrogen and oxygen in a variety of structures. Of paramount importance in atmospheric chemistry are the two compounds nitric oxide (NO) and nitrogen dioxide (NO₂).

Nitric oxide (NO) is a colorless, odorless gas that can be isolated at room temperature and is reactive but stable. Nitrogen dioxide (NO₂) is a reddish-brown gas with a pungent odor; it also can be isolated at room temperature and is reactive but stable. Additionally, nitrous oxide (N₂O), a colorless gas with a sweet odor—known as laughing gas for its use as an anesthetic—reacts readily with oxygen to become NO, and is a significant source of this greenhouse gas.

Major anthropogenic releases of both NO and NO₂ include the burning of fossil fuels in automobile engines and in power plants. Automobile exhaust produces about three-quarters of such emissions. Additionally, nitrogen dioxide can be released by refineries and some manufacturing facilities in the industrial sector; and also by the residential sector from gas stoves and home heating units. The release of these nitrogen oxides stems from the oxidation of air, which is approximately 78 percent nitrogen. In nature, lightning strikes are abundant generators of nitrogen oxides, as are natural fires and biological processes in soil and water.

Harmful Effects

Specific concentrations of some nitrogen compounds, in particular NO₂, have proved toxic effects. High levels can even be fatal, while lower levels affect a range of body systems, especially lung tissue. It is also known that long-term exposure to some NO_x compounds weakens resistance to respiratory infections. For example, NO₂increases animal susceptibility to pneumonia, influenza, and other such threats. There is little difference in human risk factors from those of other higher animals. Respiratory disease risk increase in humans is confirmed in association with chronic low-level exposures of NO₂. Specific exposures and associated human health effects include the following:

Short-term exposure at concentrations greater than 3 parts per million (ppm) can measurably decrease lung function.

Concentrations less than 3 ppm can irritate lungs.

Concentrations as low as 0.1 ppm cause lung irritation and measurable decreases in lung function in asthmatics.

Long-term low-level exposures can destroy lung tissue, leading to emphysema.

Small levels of NO_x can cause nausea, irritation to the eyes and nose, fluid forming in lungs, and shortness of breath.

Breathing in high levels of NO_x can lead to rapid, burning spasms, visual impairment, swelling of the throat, reduced oxygen intake, a larger buildup of fluid in lungs, and death.

NO_x, plus other ground-level ozone, can cause other major respiratory problems at high levels.

It can react with aerosols from aerosol cans and also cause respiratory problems.

NO_x can cause visual impairment in areas affected by NO_x.

Nitrogen dioxide in particular is also harmful to the environment. It can damage vegetation, fade and discolor fabrics, reduce visibility, and react with surfaces and furnishings. Vegetation exposure to high levels of nitrogen dioxide can be identified by depleted foliage, decreased growth, and reduced crop yield.

The environmental effects of NO_x include the following:

• It helps form acid rain.
• It contributes to global warming.
• It hampers the growth of plants.
• It can react with other pollutants to form toxic chemicals.

Guidelines and Controls

Some studies suggest that people with asthma may experience increased airway resistance at levels as low as 0.3 ppm, but results are not conclusive. In Australia, for example, the Queensland government’s Environmental Protection (Air) Policy goal of 0.16 ppm (one-hour exposure period) is intended to protect sensitive individuals such as children and people with asthma. Exposure to levels of 0.50 ppm for one hour would be necessary before a majority of healthy people in a community were significantly affected. Typical outdoor nitrogen dioxide concentrations are well below the one-hour goal; exposure at these levels do not generally increase respiratory symptoms. The US Environmental Protection Agency (EPA) has set standards for NO_x pollution on every type of motor vehicle. For power plants and factories, the EPA has set quantitative limits on coal-burning emissions (which also helps to reduce the formation of acid rain).

The EPA has standards in place specifically for upwind states as well, and since NO_x travels by wind, the states that “push” emissions are affected by such rules. With a great bulk of NO_xemissions originating in the transportation sector, the EPA established standards for installation of catalytic converters on automobiles; the rules went into effect in 1975. Catalytic converters, acting on the hot exhaust fuels prior to their release from tailpipes, also precipitate out other greenhouse gases from the exhaust mix. Even with such measures employed for several decades, this element of pollution has still grown by an estimated 10 percent since the first substantial attempts were made to reduce it. More needs to be done. Among the measures that individuals can take to reduce NO_x emissions and their exposure to such emissions are the following:

• Reduce the amount a person drives alone: walk, bike, or carpool.
• Support efforts to use renewable, nonpolluting power sources.
• In homes with gas stoves, smokers, or gas heaters, open up the house; fresh air helps minimize exposure to NO_x.
• Replace older gas-powered appliances such as gas stoves and heaters.
• Do not smoke cigarettes.
• Do not burn any type of gas at high temperatures.
• Do not use products from aerosol cans.
• Avoid living in a large rural area or next to industrial parts of town.
• Avoid prolonged exposure to silos that contain silage.

Again, it is largely through properly informed land-management practice and fertilization campaigns; continued application and enforcement of progressively stronger transportation sector standards; and broad efforts to shift electricity production and other industrial sector energy sources from fossil fuel to low-nitrogen renewables that NO_x emissions can primarily be controlled and reduced.

Bibliography

Clean Air Technology Center. Nitrogen Oxides(NO_x): Why and How They Are Controlled. Research Triangle Park, NC: Office of Air Quality Planning and Standards, et al., 1999.

Dara, S. S. A Text Book of Environmental Chemistry and Pollution Control. New Delhi: S. Chand, 1993.

De, Anil K. Environmental Chemistry. New Delhi: New Age International, 1994.

Graham, J. A. Nitrogen Oxides. Geneva: World Health Organization, 1997.

"Ground-Level Ozone Pollution." US Environmental Protection Agency, 22 July 2024, www.epa.gov/ground-level-ozone-pollution. Accessed 5 Aug. 2024.