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Nitrogen Oxides (NOx)

Sources

Nitrogen oxides are produced in combustion processes, partly from nitrogen compounds in the fuel, but mostly by direct combination of atmospheric oxygen and nitrogen in flames. Nitrogen oxides are produced naturally by lightning, and also, to a small extent, by microbial processes in soils. Man-made emissions of nitrogen oxides dominate total emissions in Europe, with the UK emitting about 2.2 million tonnes of NO2 each year. Of this, about one-quarter is from power stations, one-half from motor vehicles, and the rest from other industrial and domestic combustion processes. Unlike emissions of sulphur dioxide, emissions of nitrogen oxides are only falling slowly in the UK, as emission control strategies for stationary and mobile sources are offset by increasing numbers of road vehicles.

Atmospheric chemistry and transport

The primary pollutant, directly emitted, is nitric oxide (NO), together with a small proportion of nitrogen dioxide (NO2). NO is oxidised by ozone in the atmosphere, on a time scale of tens of minutes, to give NO2. In rural air, away from sources of NO, most of the nitrogen oxides in the atmosphere are in the form of NO2. NO and NO2 are collectively known as NOX because they are rapidly inter-converted during the day. NO2 is split up by UV light to give NO and an O atom, which combines with molecular oxygen (O2) to give ozone. Therefore, during the day NO, NO2 and ozone exist in a quasi-equilibrium which depends on the amount of sunlight. Eventually, NO2 is oxidised to nitric acid (HNO3, vapour) which is absorbed directly at the ground, is converted into nitrate-containing particles, or dissolves in cloud droplets. At night, different oxidation processes convert NO2 to nitrates.

Although nitric acid is rapidly absorbed on contact with surfaces (cloud droplets, soil or vegetation), the other nitrogen oxides are removed only rather slowly, and may travel many hundreds of km before their eventual conversion to nitric acid or nitrates. Consequently, emissions in one country will be deposited in others. The UK exports about three-quarters of its emissions of NOX.

Issues

Nitrogen dioxide has the potential to cause human respiratory problems at high concentrations, and the long-term UK target concentration for human exposure is 21 ppb NO2 (1 ppb is one part in a thousand million by volume). Concentrations rarely reach this concentration outside cities, but in several places with high traffic density this concentration is currently exceeded as an annual average. Nitrogen oxides are not directly harmful to plants at concentrations below 14 ppb.

Nitrogen oxides are also one of the precursors for photochemical ozone formation. Stringent control of NOX emissions will be required if regional (and hemispheric) ozone concentrations are to be reduced to levels which are harmless to vegetation.

Nitrate containing particles and nitric acid contribute to wet and dry deposition of nitrogen in areas both close to and remote from sources. Deleterious effects of deposited nitrogen on natural N-limited terrestrial ecosystems have been reported from across Europe, and N deposition also makes a significant contribution to the productivity of phytoplankton blooms in the surface waters of the Atlantic Ocean and North Sea. Although some of this 'excess' nitrogen comes from ammonia, control of NOX emissions is required to reduce the amount of nitrogen transported over long distances.

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