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a time frame as possible so that long-term trends can be properly assessed.
Measurements for compliance purposes (i.e. the comparison of concentrations with air
quality standards) should not just
be limited to urban areas, as limit values should not
be exceeded anywhere. Concerning exposure and health considerations, urban
areas
combine a large fraction of the population and elevated concentrations; nevertheless,
rural areas should not be excluded from a measuring network.
In the case of ozone,
for example, rural concentrations can be high downwind of large emission point
sources. A number of models are available for estimating ambient concentrations of
air
pollutants, most of which are based on the Gaussian air dispersion model.
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