Once limit values and alert thresholds have been set, ambient air quality shall be assessed throughout the territory of the Member States, in accordance with this Article.

In accordance with the criteria referred to in Article 4 (3), and in respect of the relevant pollutants under Article 4 (3), measurement is mandatory in the following zones:

• agglomerations as defined in Article 2 (10),

• zones in which levels are between the limit values and the levels provided for in paragraph 3, and

• other zones where levels exceed the limit values.

The measures provided for may be supplemented by modelling techniques to provide an adequate level of information on ambient air quality.

A combination of measurements and modelling techniques may be used to assess ambient air quality where the levels over a representative period are below a level lower than the limit value, to be determined according to the provisions referred to in Article 4 (5).

Where the levels are below a level to be determined according to the provisions referred to in Article 4 (5), the sole use of modelling or objective estimation techniques for assessing levels shall be possible. This provision shall not apply to agglomerations in the case of pollutants for which alert thresholds have been fixed according to the provisions referred to in Article 4 (5).

Where pollutants have to be measured, the measurements shall be taken at fixed sites either continuously or by random sampling; the number of measurements shall be sufficiently large to enable the levels observed to be determined.