Influence of local, regional and global sources on air composition



According to the ozone (O3) and sulphur dioxide (SO2) measurements and the EMEP-model (EMEP = Co-operative programme for monitoring and evaluation of long-range transmission of air pollutants in Europe) calculations of the long range transport of the pollutants, the station is located in one of the cleanest area on the European continent (Mylona, 1993). There is no heavy industry within couple hundreds kilometres, and the population density is low, less than 2 inhabitants/km2.

The measured yearly averages of SO2 concentrations are about 1 g/m3. The highest hourly sulphur dioxide concentration in 1991 - 1994 was 56 µg/m3, but the concentration is generally low, 90 % of the hourly values are below 2.5 µg/m3. The occasional short episodes of high sulphur dioxide concentrations are mainly due to the long range transport from mining and metallurgical industry in the Kola Peninsula, 300 km north-east of Pallas (Tuovinen et al., 1993). Most of the long-range transport episodes take place in winter and spring.

In addition to the use of trajectories to characterise the airshed of the station, the interpretation of the time series of the trace gas measurements depends on a number of other parameters that can be used to identify the influence of local sources or the long range transport of anthropogenic material to the station (Hopper et al., 1994 and Worthy et al., 1994). Fresh combustion products contain an abundance of small particle aerosol, whereas, aged aerosols tend to be much larger. A condensation nuclei (CN) counter may be used to monitor the small particle concentration and an aethalometer which gives an estimate of the amount of black carbon (BC) in the airmass may be used to identify long range transport events. It should be noted that there are other aerosols that are included on the filter that cannot be distinguished from black carbon by the aethalometer. Nevertheless, it is a good indicator of large particle aerosols and it is commonly referred to as an instrument measuring black carbon. For the sake of brevity, only one example is given in here.

As can be seen in the figure, when the trajectories are out of the north or north-west both the CN counts and BC levels are low. Several events are apparent in both the CN and BC levels which are correlated with similar events in the CO2 record. While the station is on a hill top above the tree line, it is quite possible that the inversion is above the station and, therefore, the measurements would be influenced by local combustion products on a diurnal basis. In such cases, there should be a large increase in the CN counts without a corresponding rapid increase in the signal from the aethalometer. Similarly a large change in CN without an change in BC could also indicate sulphur dioxide to sulphate conversion, especially if the air is coming out of the Kola Peninsula. Care must be taken to mark or flag such events in the record, perhaps using radon, black carbon and condensation nuclei. There were extended periods with little evidence of anthropogenic influence indicating that the recovery rate of "baseline" data from the record will be quite high.

CO2 concentrations have been measured in continuous basis at Sammaltunturi since 1996. The concentrations during 1997-2001 can be seen here. In order to examine the seasonal cycle the concentration data were selected during the afternoon when the atmospheric mixing is greatest. Monthly medians were calculated for the period of record, the long term trend was subtracted and a harmonic curve fitted to the detrended seasonal cycle. Because of the rather short record, the double humps in the winter part of the seasonal cycle are not considered significant but the amplitude of 16-18 µmole per mole is in agreement with other high latitude stations (Alert, Canada and Barrow, Alaska). According to trajectory analysis the sources of high CO2 are mainly located in northern parts of the Central Europe (see the figure). The analysis was made using method developed by Stohl (1996, 1998). The measurements were taken from July and August 1997-1999 at 14:00 local time.




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Last updated 13.8.2001