The EU daughter directives specify the minimum number of fixed sampling points to assess compliance with limit values for protection of human health, where fixed measurement is the sole source of information.

The minimum number of sites depends on the population of the zone or agglomeration and the expected level of concentrations. For a city of about 1 million inhabitants, the minimum number of fixed sampling points should be four, if concentrations are expected to exceed the upper assessment threshold. Two sites would be required where air pollution levels are between upper and lower assessment threshold.

For agglomerations the stations should cover:

street-side,
urban background (roof top) and 
regional background. 

The first site (if only one is available) should be in an area where you would expect the highest concentrations. In many urban areas of Europe today this may be in a busy street canyon. In some regions it may be downwind from a major industrial source.

The following considerations are cited from the EU Daughter Directives and relates to fixed measurement points directed at the protection of human health (Macro scale siting):

Sampling should be sited to

1	Provide data on the areas within zones and agglomerations where the highest concentrations occur to which the population is likely to be directly or indirectly exposed for a period which is significant in relation to the averaging period of the limit value(s);
2	Provide data on levels in other areas within the zones and agglomerations, which are representative of the exposure of the general population.
3	Avoid measuring very small microenvironments in their immediate vicinity. As a guideline, a sampling point should be sited to be representative of air quality in a surrounding area of no less than 200 m2 at traffic-orientated sites and of several square kilometers at urban-background sites.