Characterization of atmospheric trace gas and aerosol concentrations at forest sites in southern and northern Finland using back trajectories

The trace gas and aerosol concentrations as well as meteorological data (radiation, temperature, humidity) measured in Hyytiälä and Värriö, southern and northern Finland, respectively, were investigated with air mass analyses. The back trajectories of air masses arriving to the sites on the 925-hPa pressure level were calculated 96 hours backwards in time. Two trajectories per day, arriving at 00 UTC and 12 UTC, were computed. The studied time period covered December 1997 to August 1998, November 1998 to July 1999, and September 1999. The arriving air masses were divided into five sectors according to their origin: I = North–West (Arctic Ocean), II = North–East (Northern Russia, Kola Peninsula), III = South–East (Southern Russia, St. Petersburg), IV = South–West (Central Europe, Great Britain), and V Local, circulating air masses. The climatology of various properties of air masses originating from different sectors was studied. The analysis showed differences between typically clean (sectors I and II) and polluted (sectors III and IV) air masses. In air masses from sectors III and IV, the NOx concentrations were high during all seasons, but the O3 concentrations were high during the spring and summer seasons and low in winter. The highest SO2 concentrations arrived from sector III. In Värriö also the air from sector II was accompanied with higher SO2 concentrations and some very high peaks were observed in winter when air masses passed through the industrial area of Kola Peninsula. Nucleation events typically occurred in clean air masses and accumulation mode concentrations were higher in polluted air masses. Although there were some differences between Hyytiälä and Värriö, the overall behaviour was similar at both sites.