The Finnish Global Atmosphere Watch (GAW) station Pallas-Sodankylä has been operating since 1994. Upper-air soundings, climatological and other meteorological measurements are made at the Sodankylä observatory (67°22' N, 26°39' E), and tropospheric air composition and related boundary layer meteorological measurements at Pallas. The distance between the two sites is 125 km. The measurement programme was divided between these two sites because of the unsuitable location of the Sodankylä observatory (near a town, inside inversion layer during winters) for the tropospheric air composition measurements.
There are five measuring stations at the Pallas area: an automatic weather stations (AWS), Laukukero (68°04'N, 24°02'E, 765 m above sea level), and four stations measuring air composition. The main station, Sammaltunturi (67°58'N, 24°07'E, WMO index number 05821), is on a top of a fjeld (an Arctic hill), at the height of 565 m above sea level (a.s.l.), and ca. 300 m above the surrounding area. Matorova (68°00'N, 24°14'E) lies about six kilometres ENE of Sammaltunturi at of 340 m a.s.l. Kenättärova station (67°59'N, 24°15'E, 347 m a.s.l.) is situated 1.4 km south of the Matorova station. It lies with spruce forest, and is used to measure greenhouse gas fluxes. The newest station, Lompolojänkkä (68°00'N, 24°13'E, 269 m a.s.l.), is measuring green house gas fluxes in an aapa mire. The Laukukero and Sammaltunturi stations are within the Pallas-Yllästunturi National Park, inside the northern boreal forest zone. The stations are maintained by FMI together with the Finnish Forest Research Institute (METLA).
The Sammaltunturi station resides on a top of the second southernmost fjeld in a 50 km long north and south chain of fjelds. The tree line is some 100 m below the station. The vegetation on the fjeld top is sparse, consisting mainly of low vascular plants, moss and lichen. The highest fjelds in the chain are from 600 to 800 m a.s.l. Otherwise the region is hilly (250 - 400 m a.s.l.), forested and partly swampy with some rather large lakes (ca 250 m a.s.l.). In the north, north-east and SSW there are fjelds within 3-6 kilometres from the station with heights from 600 to 800 m a.s.l. The sectors 180-330° and 100-130° are very open.
One of the main criteria for selecting the Pallas area as the site for tropospheric measurements was the absence of large local and regional pollution sources. The distance to the nearest town, Muonio, with some 2500 inhabitants, is 19 km to the west. The second nearest town, Kittilä (6000 inhabitants) is 46 km to the south-east.
Due to its northern location Pallas area experiences about 7 week long polar day in summer and 3.5 week polar night in winter. The ground is covered with snow from the middle of October to late May (ca 220 days), maximum snow cover depth (ca 1 m) occurs during April. Annual amount of precipitation at the Pallas region is 400 - 500 mm, and mean cloud cover is 60 - 70 %. Due to its elevation, the station is from time to time within the cloud cover. The wind speed is very rarely less than 0.5 m/s. The small seasonal and diurnal variation of radon concentration indicates that the station is very rarely inside the surface inversion layer (Paatero et al., 1999). However, direct micrometeorological CO2 flux measurements over a forest east of Sammaltunturi show that in summer during some nights respiration from the forest ecosystem may increase CO2 concentration at the fjeld top site, but during daytime the concentrations are well mixed (Tuovinen et al., 1999).
The ongoing measurements (June 2001) at Pallas are listed above. The programme is divided so that most of the continuous measurements are made at Sammaltunturi and all the continuously running sample collectors at Matorova. The Sammaltunturi site is unsuitable for filter sampling because of severe frost formation during the winter months.
From October 1996 to March 1998 CO2 was measured in co-operation with the Air Quality Research Branch of Meteorological Service of Canada. Since August 1998 FMI has monitored CO2 concentration with its own NDIR setup. Measurements of POPs (persistent organic pollutants) and mercury at Matorova are done in cooperation with the Swedish Environmental Research Institute, IVL. The measurements belong to the AMAP programme and they started at the beginning of 1996.
The old station building has been replaced by a new measuring station (see photos) during summer 2001. The station is a 120 m2 building consisting of a large room for housing analytical equipment and separate rooms for pumps, storage and office. Two 7 m high towers are attached to either end (east and west) of the building for air sampling. These towers are electrically warmed to prevent clogging of the inlets by ice and snow. The east tower is the main tower, with a line made of acid resistant stainless steel tubing with an outer diameter of 60 mm (ID 54 mm). The air is continuously drawn through this line with a pump with nominal flow rate of 150 m3/h. Thus the residence time inside the sampling line is short, a few seconds, before the monitor connections. All the monitors take their sample air from this manifold, except for the radon monitor, which take their air from the second tower. Also some aerosol monitors have their own inlets/sample lines. The exhaust air is led some 50 meters north of the station building.
The tables of the currently ongoing measurement programmes (see above) contain also information about the method/instrumentation with which the measurements are made. The continuous CO2 measurement is carried out with a Li-Cor NDIR analyser using one reference gas and three calibration gases. These gases are calibrated against WMO/CCL standards.