Tropospheric carbon dioxide concentrations at a northern boreal site in Finland: basic variations and source areas

Diurnal and annual variations of CO₂, O_3, SO₂, black carbon and condensation nuclei and their source areas were studied by utilizing air parcel trajectories and tropospheric concentration measurements at a boreal GAW site in Pallas, Finland. The average growth trend of CO₂ was about 2.5 ppm yr⁻¹ according to a 4-yr measurement period starting in October 1996. The annual cycle of CO₂ showed concentration difference of about 19 ppm between the summer minimum and winter maximum. The diurnal cycle was most pronounced during July and August. The variation between daily minimum and maximum was about 5 ppm. There was a diurnal cycle in aerosol concentrations during spring and summer. Diurnal variation in ozone concentrations was weak. According to trajectory analysis the site was equally affected by continental and marine air masses. During summer the contribution of continental air increased, although the southernmost influences decreased. During daytime in summer the source areas of CO₂ were mainly located in the northern parts of the Central Europe, while during winter the sources were more evenly distributed. Ozone showed similar source areas during summer, while during winter, unlike CO₂, high concentrations were observed in air arriving from the sea. Sulfur dioxide sources were more northern (Kola peninsula and further east) and CO₂ sources west-weighted in comparison to sources of black carbon. Source areas of black carbon were similar to source areas of aerosols during winter. Aerosol source area distributions showed signs of marine sources during spring and summer.