Sources of Nitrogen Dioxide

6. Nitrogen dioxide is a gas produced by the reaction of nitrogen and oxygen generally in a two stage reaction which initially results in the formation of nitric oxide. Nitric oxide, once emitted into the atmosphere, combines further with oxygen atoms, usually derived from ozone, to form nitrogen dioxide. This means that there tends to be an inverse relationship between ozone and nitrogen dioxide. For instance, in a city centre nitric oxide produced by vehicles reacts with and thus removes ozone, generating nitrogen dioxide, while later the nitrogen dioxide acts as a chemical source under the influence of sunlight for the formation of further ozone. These reactions are discussed in the Panel's report on Ozone. Another important mechanism by which nitric oxide is converted to nitrogen dioxide, occurs in the wintertime, during stagnant, cold weather conditions and involves oxidation by atmospheric oxygen. This mechanism is believed to be responsible for the unprecedented nitrogen dioxide concentrations observed in London during December 1991, Manchester in 1992 and Birmingham in 1992 and 1994. Oxides of nitrogen are also produced naturally by certain soil bacteria, and the soil and vegetation growing on it may make a contribution to air concentrations of these gases in some rural areas.
7. The main sources of oxides of nitrogen in the United Kingdom are shown in Table 1. Currently the greatest contribution is from motor vehicles, at over 1 million tonnes per annum. About half a million tonnes are contributed by non-nuclear power stations and the rest, save for about three percent from domestic sources, is derived mainly from industrial activity. Generally, ground level concentrations of nitrogen dioxide outdoors are influenced more by emissions from motor vehicles than by sources such as power stations which disperse pollutants from tall stacks Of the transport sources, petrol combustion in cars is currently responsible for a greater proportion than diesel, though this relationship is changing with the progressive introduction of the catalytic converter into petrol vehicles. The overall emissions of oxides of nitrogen into the air in the United Kingdom are estimated to have increased slowly from 1970 to 1989 and to have declined to 1970 levels since then (Figure 1).
8. In terms of the exposures of individuals to nitrogen dioxide, there are two important sources, combustion outdoors and combustion within the home. Since most people in the United Kingdom spend more time indoors than out, it follows that indoor levels can be the major determinant of an individual's exposure, even though domestic emission of oxides of nitrogen comprises only a small proportion of the total national emissions. The major indoor source is cooking with gas, although kerosene heaters and cigarette smoke can also contribute significantly. In homes with gas cookers concentrations of nitrogen dioxide may be higher than outdoors, and much higher in kitchens for short periods of time when the cookers are in use. In homes that do not have gas cookers or other un-flued combustion appliances, concentrations of nitrogen dioxide are strongly influenced by outdoor concentrations. This report is primarily concerned with exposure of individuals to nitrogen dioxide in outdoor air. The Panel, however, recognise that both sources will contribute to overall personal exposure. This has been discussed in a recent publication on indoor air quality by the Institute for Environment and Health.

Table 1. Estimated United Kingdom emissions of oxides of nitrogen by emission source, 1994, thousand tonnes per year

Source	Estimated Emissions*	Percentage of Total**
Power Stations (fossil fuelled)	526	24
Domestic Commercial/public service	69 34	3 2
Refineries Iron and Steel Other industrial Combustion	45 48 128	2 2 6
Non Combustion Processes	5	-
Extraction and distribution of Fossil Fuels	109	4
Road Transport: petrol diesel	653 442	29 20
Other Transport	151	7
Waste Treatment and Disposal	4	-
Agriculture	3	-
Total	2218	100

*     Rounded to the nearest thousand tonnes

**    Rounded to nearest 1%

Source: Digest of Environmental Protection and Water Statistics, Number 18 London:HMSO. 1996