Nitrogen Oxides

Nitrogen oxides are produced by combustion of all fossil fuels including coal- and gas-fired power stations and motor vehicles. (1)

Whereas fuel itself can produce some nitrogen (for example, oil and coal contain around 0.5 – 1.5% of nitrogen, and natural gas contains less than that (2)), most of nitrogen production comes from the reaction of atmospheric nitrogen and oxygen within the combustion chamber. (3)

The two main nitrogen oxides are nitric oxide (NO), or nitrogen monoxide, and nitrogen dioxide (NO2) the sum of which is equal to NO X.

Nitric oxide (NO) is a colorless gas. (4)

Nitrogen dioxide (NO2) is a gas of reddish-brown color with a distinct sharp, biting odor. (5)

Combustion of fuels always produces both NO2 and NO.

But almost 90% of the NOX combustion product is in the form of NO which is then oxidized to nitrogen dioxide (NO2) in the air (6, 7).

Therefore, only a small percentage of NO2 found in the atmosphere is directly emitted there in this form. The rest has been formed as a result of chemical reactions in the atmosphere itself. (8)

Anthropogenic Sources of Nitrogen Oxide Emissions

Road transport (motor vehicles) is by far the largest contributor of nitrogen emissions (in contrast, it contributes a very small proportion to sulfur dioxide emissions).

Bus Pollution in Guatemala
Photo: Doron Laor

For example, based on 1998 UK figures, road transport contributed nearly half of all nitrogen emissions, followed by contributions from electric power generating plants which only contributed around 20% of total nitrogen emissions. (9)

Below is a breakdown of the significant sources of emissions of nitrogen oxides (based on sources of the UK emissions) (10):

• Road transport
• Other Transport
• Energy Production
  
  • Electric power generation
  • Petroleum refining
  • Other combustion
• Combustion for industry use
• Production processes
• Extraction and distribution of fossil fuels.

Natural Sources of Nitrogen Oxide Emissions

Nitric oxide (NO) is also emitted by soils but there is very little data available for the proper assessment of this area.

Some estimates suggest that the soil production of NO in the UK may be around 2 – 5% of its production from fossil fuel combustion. (11)

Effects of Nitrogen Dioxide (NO2) Emissions

When inhaled, nitrogen dioxide becomes a serious air pollutant which may (12):

• Cause pulmonary edema (accumulation of excessive fluid in the lungs)
• Be part of acid rain (destroying fish and plant life in lakes, damaging surfaces of buildings etc)
• Contribute to photochemical smog.

Last Update: October 2008