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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