What are Nitrogen Oxides?

Nitrogen oxides (NOx) consist of nitric oxide (NO), nitrogen dioxide (NO2) and nitrous oxide (N2O) and are formed when nitrogen (N2) combines with oxygen (O2). Their lifespans in the atmosphere range from one to seven days for nitric oxide and nitrogen dioxide, to 170 years for nitrous oxide.

Nitric oxide has no colour, odour, or taste and is non-toxic. In the air it is rapidly oxidized to nitrogen dioxide.

Nitrogen dioxide is a reddish-brown gas with a pungent, irritating odour. It absorbs light and leads to the yellow-brown haze sometimes seen hanging over cities. It is one of the important components of smog.

Nitrous oxide is a colourless, slightly sweet-smelling, non-toxic gas which occurs naturally in the atmosphere. Man-made nitrous oxide is used as the anaesthetic commonly called "laughing gas".


Nitrogen oxides occur naturally and also are produced by man's activities. In nature, they are a result of bacterial processes, biological growth and decay, lightning, and forest and grassland fires. The primary source of man-made nitrogen oxides is from the burning of fossil fuels.

Of the nitrogen oxides emitted, most is nitric oxide, some is nitrous oxide and less than 10 per cent is nitrogen dioxide. The amount of nitrogen dioxide emitted varies with the temperature of combustion; as temperature increases so does the level of nitrogen dioxide. Agriculture also plays a role in nitrogen oxide emissions with the use of fertilizers contributing nitrous oxide to the atmosphere.

Alberta contributes about 23 per cent of the 1887 kilotonnes of nitrogen oxides emitted annually in Canada.

In Alberta the oil and gas industry is responsible for about 41 per cent of nitrogen oxide emissions, transportation (including planes, trains and automobiles) 25 per cent, electrical utilities 17 per cent, and other industrial, commercial and residential users the remaining 17 per cent.


Nitric oxideby itself is non-toxic, but it is readily converted in the air to nitrogen dioxide. At high concentration levels, nitrogen dioxide is potentially toxic to plants, can injure leaves and reduce growth and yield. In combination with either ozone (O3) or sulphur dioxide (SO2), nitrogen dioxide may cause injury at even lower concentration levels. As one of the components of smog, nitrogen dioxide is known to irritate the lungs and increase susceptibility to respiratory infections.

Nitrous oxide is a greenhouse gas. As well it contributes to ozone depletion in the stratosphere, as discussed in the fact sheets Greenhouse Effect and Ozone - Stratospheric and Ground-Level.

Nitrogen oxides are an important component in acid deposition, as discussed in the fact sheet Acid Deposition (Acid Rain).

What Has Been Done So Far

Air quality objectives for nitrogen dioxide were set many years ago by the federal and provincial governments. The objective for nitrogen dioxide concentrations, averaged over one hour, is 0.21 parts per million (by volume) in air. The concentration of nitric oxide and nitrogen dioxidein the air is carefully monitored on a regular basis.

During the years 1985 to 1989 Calgary exceededthe air quality objective for nitrogen dioxide four times. Edmonton, Fort Saskatchewan and Fort McMurray did not exceed the objective.

National emission guidelines for new power plants were adopted in 1981. In 1988 Alberta required that all new natural gas-fired compressor engines use low nitrogen oxides technology. These engines produce about one-quarter of the emissions of older engines.

In 1988, Canada and 24 other countries signed a United Nations Economic Commission for Europe (UN-ECE) protocol agreeing to freeze nitrogen oxide emissions at the 1987 level and to accomplish this by 1994.

The Canadian Council of Ministers of the Environment (CCME) subsequently asked for a comprehensive plan to further manage nitrogen oxides and volatile organic compounds (VOCs) emissions to the year 2005. Following a national workshop in 1989, a draft Management Plan for NOx and VOCs was published in March 1990. A second workshop was held in April 1990 and the first edition of the plan is expected by October 1990. A second edition of the plan, describing Canada's progress, is to be published in 1995.

One measure proposed by CCME to limit nitrogen oxides emissions is the adoption of proposed California auto emission standards, the most stringent in the world. If implemented, by 1994 all new cars sold in Canada would have to meet these standards.

Further Information

Air quality issues - greenhouse gases, acid deposition (acid rain) and smog - cannot be addressed in isolation. Their complex inter-relationships make achieving the goal of clean air for the futre a challenge for individuals, industry and governments alike. The Clean Air Strategy for Alberta is providing an opportunity for Albertans to participate in meeting that challenge.

To assist Albertans in participating in the Clean Air Strategy for Alberta, the Alberta government has prepared a series of fact sheets and a glossary. Their purpose is to help Albertans understand the magnitude of the environmental and economic considerations, the complexity of the science, the potential requirements for changes in lifestyle, and the challenges facing individuals, industry and government.

List of Fact Sheets

An Overview, Greenhouse Effect, Acid Deposition (Acid Rain), Ozone - Stratospheric and Ground-Level, Carbon Dioxide, Sulphur Oxides, Methane, Nitrogen Oxides, Volatile Organic Compounds, Chlorofluorocarbons and Halons, Energy Efficiency, Policy Instruments, Glossary, Renewable Energy

Other information on nitrogen oxides (NOx):

Management Plan for Nitrogen Oxides and Volatile Organic Compounds. First Edition. Draft. March 1990. (Summary).

Nitrogen Oxides Emissions for Alberta 1987. Alberta Environment. February 1990.

Energy-Related Nitrogen Oxide Emissions in Alberta 1988-2005. Draft. Energy Resources Conservation Board / Alberta Environment. August 1990.

Understanding Automobile Emissions. Environment Canada/Petroleum Association for Conservation of the Canadian Environment.

Clean Air Strategic Alliance, 9th Floor, Sterling Place, 9940 - 106 Street, Edmonton, Alberta, Canada, T5K 2N2, ph. 403/427-9793, fx. 403/422-3127, em:

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