3. From 2 years following the adoption of the implementing acts referred to in paragraph 7 of this Article, modelling applications or indicative measurements shall be used in addition to fixed measurements to assess the ambient air quality in all zones where the level of pollutants exceeds a relevant limit value or target value set out in Annex I.
The modelling applications or indicative measurements referred to in the first subparagraph shall provide information on the spatial distribution of pollutants. Where modelling applications are used, they shall also provide information on the spatial representativeness of fixed measurements and they shall be carried out as often as appropriate but at least every 5 years.