Nitrogen oxides
Environmental and health effects of
nitrogen oxides
Air quality standard
Measurement of nitrogen oxides
Nitrogen
oxides (NOx), a mixture of nitric oxide (NO) and nitrogen dioxide (NO2), are
produced from natural sources, motor vehicles and other fuel combustion processes.
Nitric oxide is colourless and odourless and is oxidised in the atmosphere to form nitrogen dioxide. Nitrogen dioxide is an odourous, brown, acidic, highly-corrosive gas that can affect our health and environment. Nitrogen oxides are critical components of photochemical smog — nitrogen dioxide produces the yellowish-brown colour of the smog.
Environmental and health effects
of nitrogen oxides
Nitrogen dioxide is harmful to vegetation, can fade and discolour fabrics,
reduce visibility, and react with surfaces and furnishings. Vegetation exposure
to high levels of nitrogen dioxide can be identified by damage to foliage, decreased
growth or reduced crop yield.
Indoor domestic appliances (gas stoves, gas or wood heaters) can also be significant sources of nitrogen oxides, particularly in areas that are poorly ventilated.
Nitric oxide does not significantly affect human health. On the other hand,elevated levels of nitrogen dioxide cause damage to the mechanisms that protect the human respiratory tract and can increase a person's susceptibility to, and the severity of, respiratory infections and asthma. Long-term exposure to high levels of nitrogen dioxide can cause chronic lung disease. It may also affect sensory perception, for example, by reducing a person's ability to smell an odour.
Air quality standard
Some studies suggest that asthmatics may experience increased airway resistance
at levels as low as 0.3 ppm, but results are not conclusive. The EPP(Air)
goal of 0.16 ppm (one-hour exposure period) is intended to protect sensitive
individuals such as children and asthmatics. 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 1hour goal and exposure at these levels do not generally increase respiratory symptoms.
Measurement of nitrogen oxides
Nitrogen oxides are measured using a technique that is based on a
"chemilumiscent" reaction, which is a chemical reaction that emits
energy in the form of light. Essentially the reaction is the oxidation
of nitric oxide (NO) to nitrogen dioxide (NO2) by ozone
(O3).
NO + O3> NO2+ O2
It is an exothermic reaction which produces NO2 in an activated state. When the NO2 molecules return to normal from the activated state some energy is emitted in the form of a small amount of light. Since one NO molecule is required to form one NO2 molecule, the intensity of the chemiluminescent reaction is directly proportional to the NO concentration in the sample. The analyser measures the amount of light emitted by the reaction and converts this to a concentration.
The chemiluminescent reaction only occurs between O3 and NO, therefore the measurement of NO2 is performed by diverting the ambient air stream through a converter which reduces any NO2 present to NO before entering the reaction cell. The differences between NO levels in the undiverted and diverted gas streams is the amount of NO2. Nitrogen oxides are also measured by DOAS instrumentation.
Last updated: 07 April 2006
