Measurement results
Comparison to EU values

The measurements were performed as campaigns, with three as "full" campaigns and nine as "reduced" campaigns. The number of samples in each of the classes in the two different types of campaigns are presented in table below, which is valid for both NO₂ and SO₂. VOC was measured at 9 or 10 sites during the three "full" campaigns and during one of the "reduced" campaigns.

Number of sampling sites in each of the classified environments during the campaigns.

Classes	Number of sampling sites, "full" campaigns	Number of sampling sites, "reduced" campaigns
A B C D	5 30-34 24-29 15-18	1-2 8-10 3-9 3-6

Background measurements of NO₂ and SO₂ were made in the outskirts of Tallinn at two or four different geographical directions during most of the campaigns.

Measurement results

The results from the different classified environments and from background measurements are presented as a total for all 12 campaigns in table below, and for each campaign event in Appendix. There is a falling average concentration in the order A, B, C, D and background sites, but the differences are not as pronounced as for NO₂. The maximum measured monthly means do not follow the classification of sites, but the highest minimum concentration was observed at an A site, while the minimum concentration in all other classes were comparable.

Results from all campaigns, monthly averages of SO₂, µg/m³.

Class	Max	Average	Min	Number of results
A B C D Background	15 21 19 17 12	10 8 7 6 4	4 1 1 1 1	32 173 134 94 26

The average, maximum and minimum levels of SO₂ for each month and the four site classes are illustrated in figures below . As for NO₂, the darker bars represent the "full" campaigns.




Also for SO₂ the highest average air concentrations were measured at the most traffic exposed sites, although the differences in average concentrations between A and D sites were considerably smaller than for NO₂. On the other hand greater relative variations in average concentration levels between different measurement periods were found.

Measured levels of SO₂ generally show a seasonal trend with higher levels in winter months and lower during summer. The lowest levels of SO₂ (2-3 mg/m³) were thus observed during July 1996. In July 1996, with predominantly westerly winds, the lowest average levels of NO₂ were also observed. The highest average SO₂ concentrations, above 10 mg/m³, were, however, measured in April 1996 at all classes of sites. In that month NO₂ levels were also amongst the highest. There is no obvious explanation to this. Generally higher levels of SO₂ would be expected during the colder part of the year. Examining the meteorological conditions, as presented in table of meteorological conditions, April of 1996 is the only month where the predominant wind direction was from the north. During this month the background result north of Tallinn was almost twice as high as any other background result, 12 µg/m³. The elevated levels of SO₂ and among the highest of NO₂ might be due to shipping in the Gulf of Finland or long-distance transport of air from other source areas north of Tallinn.

In figures below the geographical patterns of SO₂ levels at AB and CD sites respectively for the periods with the largest number of samplers (April and August 1996, January 1997) are shown. In the figures the variation in concentrations between these campaigns can be seen, and it is also evident that the differences between the site categories were relatively small. In some areas higher SO₂ concentrations were observed at CD sites than at sites in classes A and B.






There are large areas in Tallinn with private houses using wood, turf-briquettes and coal for heating, while other areas are dominated by apartment buildings using central heating. These areas were all covered as C-sites in the measurements. Selecting results from C-sites in areas with private houses separately from areas with apartment buildings, results from the "full" campaigns (April 1996, August 1996 and January 1997) show that the total average of SO₂ from private house areas, 7.7 µg/m³ were a little higher than in areas with apartment buildings, 6.7 µg/m³. In April and January only, when heating is on, the levels of SO₂ in the private house areas were clearly higher. In August, when heating is not necessary, the levels of SO₂ were a little lower in the private house areas than in the apartment building areas.

Comparison to EU limit values

The measurement results, as an average for all campaigns, are well below the EU annual limit value of 20 µg/m³ (Appendix). This limit value is set on the basis of protection of ecosystems, and should not be applied in the close vicinity of sources. The highest average value calculated on all campaign events, as well as only on winter months, was 10 µg/m³ at the A-sites, that is at 50% of the limit value. Since the limit value is not intended to be used close to sources, a comparison between the assessment threshold values and results from D-sites or background values could be made. In this comparison the average values from the measurements, 6 and 4 µg/m³ at D-sites and background sites respectively (table), are below the lower assessment threshold of 8 µg/m³.