To evaluate the large scale concentrations of nitrogen oxides, it is useful to measure at

least NO2 and O3 in rural background air. Such measurements are made in most

countries in Europe within the EMEP programme. In many areas in Europe, urban

monitoring networks to be supported by rural background sites to separate rural

background concentrations from local urban pollution for abatement planning.

For monitoring in urban areas, it is necessary to distinguish between urban

background and roadside. "Roadside" sites are those close to the traffic sources.

"Urban background" are sites at some distance from streets, mainly influenced by the

total pollution load in the urban air rather than by a few specific sources. Distinction

between the two types of sites is of importance for the estimation of population

exposure. Most people spend the majority of their time outside exposed to air

pollution concentrations more like urban background than those found at the roadside.

Addition measurements may be needed in residential areas which are not characterised

as urban background, but may be subject also to the influence of traffic and industrial


Many people are exposed for short periods to high concentrations of NO2 when

travelling by car, bus, bicycle or on foot. Smaller groups of the population may spend

considerable time in the street environment and thus be heavily exposed to traffic

exhaust, include. NO2. The difference between roadside and urban background is also

of great importance for the selection of control measures. Decreasing the general NO2

in urban background will require emission reductions, whereas improvement of air

quality at roadside may be obtained merely by changing traffic pattern.


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