DISCUSSION AND RECOMMENDATIONS

The measurement campaigns using diffusive samplers performed in Tallinn have given a lot of interesting information.

The relations between the results from the different site categories seem to be reasonable. As expected, air concentrations of NO2 were considerably higher at sites directly exposed to road traffic than at sites in near-by residential areas and in areas of "local background". For SO2 the difference in average levels between the four classes were not as pronounced as for NO2, but even for SO2 residential areas and areas of "local background" generally exhibited lower exposure levels than areas more exposed to traffic. However, at some individual measurement sites there were deviations from the general pattern.

The different concentration patterns for the two components are probably mainly due to the emission situation. NO2 is mostly generated by traffic, hence the highest concentrations will be found close to roads with high traffic intensity. The sources of SO2 can be assumed to be more widespread within the area studied, since a major part of the SO2 emissions originates from fossil fuel combustion in large power plants as well as in local residential heating devices.


Class B station (also VOC station V8)
 		The span in concentration between sites within one type category during one campaign period was less for A sites than for the others, probably at least partly due to the lower number of A sites.

Generally the SO2 levels were higher during winter months, while for NO2 the concentration levels were more even throughout the year. There was, however, a tendency of lower levels during summer months in the C and D points. The high concentrations of SO2 and NO2 observed in April 1996 might be due to meteorological conditions, where the prevailing wind direction was from the north, with addition of southwesterly winds. The background measurement site north of Tallinn showed the highest SO2 background value measured during that month. The lowest SO2 and NO2 levels were measured in July 1996, when the wind direction was from the west and southwest. However, to be able to evaluate temporal variations in concentrations more thoroughly, longer time series and more meteorological data are needed.

In contrary to what was expected, the values of VOC were lower during the only wintertime campaign (January-February campaign 1997) than during the three other campaigns. One probable explanation to this is the higher average wind speed in January/February 1997 than during the other three VOC measurement periods. A general negative correlation between VOC-levels and wind speed has been observed elsewhere.

A brief comparison to concentration levels of NO2 and SO2 in the two largest Swedish cities, Stockholm and Gothenburg, show higher levels of SO2 in Tallinn while the levels of NO2 are in the same order of magnitude.

From the results achieved so far it is obvious that the use of diffusive samplers, in combination with a careful selection of measurement sites, is a fruitful strategy in mapping geographical variations in levels of air pollutants in Tallinn, an area of complex emissions. An important support in air quality management has been generated during a relatively short time period and at a reasonable cost. All results in combination have given an excellent insight into the variations of air pollution levels in the region as a whole as well as in different typical environments.

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