In cases where zones attain the long-term objectives, the number of sampling points for ozone shall, in combination with other means of supplementary assessment such as air quality modelling and co-located nitrogen dioxide measurements, be sufficient to examine the trend of ozone pollution and check compliance with the long-term objectives.

The number of stations located in agglomerations and other zones may be reduced to one-third of the number specified in the Table in Part 4. Where information from fixed measurement stations is the sole source of information, at least one monitoring station shall be kept. If, in zones where there is supplementary assessment, the result of this is that a zone has no remaining station, adequate assessment of concentrations of ozone against long-term objectives shall be ensured through co-ordination with the number of stations in neighbouring zones.

The number of rural background stations shall be 1 per 100,000 km².