Sarnia-Lambton Environmental Association

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Air Quality: Nitrogen Oxides (NOx)

Three oxides of nitrogen, or nitrogen oxides (NOX), are typically present in the atmosphere.

Nitric oxide (NO) is a colourless and odourless gas.
Nitrogen dioxide (NO₂) gas has a reddish-brown hue and produces a pungent and irritating odour.
Nitrous oxide (N₂O) is a commonly used an aesthetic, known as laughing gas.

N₂O is produced by microbiological processes, while the other gases are mainly by-products of fossil fuel combustion processes and significant air contaminants. The largest sources of NOX are motor vehicles, fossil fuel power generation plants and industrial processes. NO₂ transforms in air to form gaseous nitric acid and nitrates, which contribute to the formation of fine particulate matter. It also has a major role in the atmospheric reactions that produce ground-level ozone.

Nitrogen Oxides (NOx)

For more than 30 years, NO_X have been monitored by the SLEA at its Sarnia and Corunna stations, the locations where emission dispersion modelling predicts the highest local concentrations will occur. The monitoring method for NO_X, called chemiluminescence, was set by the U.S. Environmental Protection Agency and is based on the reaction of nitric oxide with an excess amount of ozone to produce a measurable emission of light energy. The amount of light energy produced is proportional to the concentration of NO in the air sample being tested.

In 2008, NO_X levels remained below the Ontario hourly and daily criteria. Annual mean levels of NO_X recorded at Sarnia and Corunna stations since 1987 are presented in Figure 4. During the past 20 years, local levels have declined significantly, although only gradually over the past decade. However, efforts underway to reduce industrial emissions of NO_X further could result in a renewed downward trend in levels during the next five years.