We use time series of hourly records of ozone for a whole year (2006) collected by the European AirBase network to analyse the area of representativeness of monitoring stations and find, for similar class of stations (urban, suburban, rural), large heterogeneity and high sensitivity to the density of the network and to the noise of the signal. This suggests the mere station classification to be not a suitable method to help select the pool of stations used in model evaluation. Therefore a novel, more robust technique is developed consisting in studying the spatial properties of the associativity of the spectral component of the ozone time series, in an attempt to determine the level of homogeneity.