Atmospheric transport of carbon dioxide to a baseline monitoring station in northern Finland

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Interannual and seasonal variations in atmospheric transport to a baseline monitoring station at Pallas (67°58’N, 24°07’E) in northern Finland were examined. The transport was analysed through cluster analysis of three-dimensional 5-d back-trajectories during the period 1997–2003. The trajectory climatology shows that air mass advection from the north is most frequent—mostly at high wind speeds across the Arctic Basin and from northern Siberia—but during summer more stagnant flows from the Norwegian Sea are common as well. Western and Central Europe were found to be the second most important regions of influence for air arriving at Pallas, followed by atmospheric transport from west Russia and the Atlantic, respectively. The trajectory clusters were combined with measurements of carbon dioxide (CO2) in order to examine the linkage between atmospheric large-scale circulation and CO2concentration at Pallas. The Atlantic and Arctic air masses were associated with relatively small annual CO2 amplitudes at Pallas. In contrast, large concentration differences between the summer minimum and winter maximum were observed during periods of continental air mass transport from the south and the east. In particular the air masses originating from west Russia were associated with very low CO2 concentrations during summer, indicating high photosynthetic activity of the terrestrial biosphere in this region. We analysed how the vertical motion of the trajectories affects the observed CO2 at Pallas. The largest difference in CO2 concentration between air parcels moving at low and high altitudes, respectively, was found during air mass advection from Europe and west Russia. This was especially true during the winter months when large CO2 emissions in these areas, i.e. from fossil fuel combustion and the decomposition and respiration of the vegetation, in combination with stable stratification can give rise to very high CO2concentrations in air parcels transported close to the surface. The CO2 time-series from Pallas was compared with CO2 measurements made at the Mount Zeppelin station on Svalbard, illustrating the different characteristics—boreal and maritime, respectively—of the regions affecting the two monitoring sites.