Time series data older than 1990 often exhibit gaps of a certain length. As pollutant concentrations vary considerably throughout the year, theses gaps cannot be simply ignored. The data series were therefore completed as follows: the monthly averages were calculated, then the missing monthly averages were obtained by linear regression of data series from another measuring station exhibiting similar variation over time, and finally, providing there were at least at least four valid monthly averages available for the station, the yearly average was then calculated from these values. Isolated gaps in the data series of the stations of Basel-Biningen (NO2) and Dübendorf (SO2) were interpolated directly by linear regression of the yearly averages.The Bern measuring station had to be moved in 1997, though it remained along the same thoroughfare hemmed in by tall buildings. The resulting inconsistencies in the time series were corrected (see Annex 5 of the NABEL Annual Report).