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.
Emission Sources and Trends
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.
Emissions from electricity generation - NOx emissions from electricity generation are fairly constant from 1970 onwards until 1990. During the early 1990s the increased use of gas in electricity generation displaced coal and oil (DECC, 2009). The cleaner fuel and more modern power stations led to a significant reduction in NOx emissions from the sector until 2000. From 2000, the absolute level of gas used for electricity generation remained fairly constant, and increased demand was met by coal-fired power stations. Since 2006, coal use (and the total amount of fuel used in electricity generation) has substantially decreased (DECC, 2009).
NOx emissions from road transport - the road transport sector has provided a significant contribution to the downward trend in UK emissions. Emissions from road transport currently make the largest contribution to the UK total, accounting for some 33% in 2010 (Defra, 2011). The first petrol cars with three-way catalysts were introduced in 1992, and this resulted in a significant reduction in NOx emissions. Emission limits for diesel cars and light goods vehicles came into effect in 1993/94. Limits on emissions from heavy goods vehicles (HGVs) first came into effect in 1988 leading to a gradual reduction in emission rates as new HGVs penetrated the fleet. The introduction of these standards has had a substantial impact on NOx emissions from the road transport sector compared with the 1990s (RoTAP, 2012).
Further information and data on emissions can be found at: http://naei.defra.gov.uk/overview/pollutants?pollutant_id=6
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 (O3). 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 (RoTAP, 2012).
Measured NO2 concentrations show the predominance of traffic and urban sources, with the largest concentrations in the large conurbations and adjacent to the motorway network, with annual mean concentrations in excess of 10 ppb in these areas.
It is likely that the strongest effect of emissions of nitrogen oxides across the UK is through their contribution to total nitrogen deposition. However, direct effects of gaseous nitrogen oxides, may also be important, especially in areas close to sources (e.g. roadside verges). The critical level for all vegetation types from the effects of NOx has been set to 30 µg/m3. Experimental evidence suggests that moderate concentrations of NOx may produce both positive and negative growth responses, with the potential for synergistic interactions with sulphur dioxide (SO2) being very important. There is substantial evidence to suggest that the effects of NO2 are much more likely to be negative in the presence of equivalent concentrations of SO2. At the same time the ratio of SO2 to NO2 has decreased greatly in urban areas of the UK over the past 30 years.
One important effect of NOx may be its influence on insect populations; there is evidence of improved performance of insect pests on plants grown in moderate concentrations of NO2 and SO2 (Dohmen et al.,1984)
Nitrogen oxides are also one of the precursors for photochemical ozone formation (see ozone overview for information on ozone impacts).