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What are the problems with nitrogen oxides in car emissions?

By Loveday Hedgcock

Nitrous Oxide (N₂O) is used in hospitals as laughing gas so why are people so worried about Nitrogen oxide emissions from cars?

laughing gas pic
Satirical cartoon showing a Royal Institution demonstration of Laughing Gas with Humphry Davy and Count Rumford, entitled "Living Made Easy" and subtitled "Prescription for Scolding Wives" 1830

Nitrogen oxides dissolve in water vapour in the atmosphere to form nitric acid which falls as acid rain causing acidification of ecosystems. Nitrogen dioxide reacts with volatile organic compounds such as unburned hydrocarbon fuel in the presence of sunlight to form ground level ozone which has health effects such as damage to the lungs, shortness of breath, coughing and increased risk of lung infections as well as irritating the eyes.

Particulate matter can be formed by a similar process and small particles of soot, from incomplete combustion are already present in the exhaust gases. Fine particles (2.5μm and smaller) are especially harmful to health as they are small enough to pass into the bloodstream when they are inhaled. Smog is the combination of ground level ozone and air-born particulate matter.

Nox pic

Nitrogen oxide emissions can be minimised by using petrol rather than diesel. Catalytic converters can be used to reduce emissions. In these, Platinum or Rhodium is used to catalyse the reduction of nitrogen oxides back to nitrogen and oxygen, however use of these catalysts can cause N₂O (laughing gas) to be produced. While this is a greenhouse gas and causes depletion of the ozone layer, emissions of N₂O from cars only make up a very small percentage of overall emissions.

Written by Loveday Hedgcock


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