The measurements have been performed during 12 campaign periods. The campaigns have been run as "full" campaigns (3 times at all sites) or at a reduced level (including 20% of the stations). The sampling periods were monthly for SO₂ and NO₂, while for VOC four weekly samples were used for monthly averages. During most of the campaigns sampling was also performed in background areas, in addition to those given in table. This was made in order to be able to evaluate the difference between conditions within the city to those in the very outskirts of the urban area. Sampling was made at two to four background locations in different geographical directions from Tallinn.