Trajectories

Three-dimensional ten-day backward trajectories arriving at 300 m above ground level (agl) every 6 hours have been calculated for the four sites using the PC version 4.5 (March 2002 release) of the Hybrid Single-Particle Lagrangian Integrated Trajectory Model (HYSPLIT4, 2002; Draxler and Hess, 1998). We assume that the chosen arrival height is within the mixed layer for all four sites, so that there is effective coupling between transport and surface measurements. NCEP/NCAR reanalysis data have been used as meteorological data input to the model. The resolution of the reanalysis data archive is 6 hours on a 2.5$^\circ \times$2.5$^\circ$ global grid. The output of the trajectory model is hourly trajectory endpoints indicating the geographical location and the height of the air parcel. The position errors for ten-day trajectories are rather large, of the order of 1000 km at the end of the period (Stohl, 1998, for a review). It is supposed that the use of the analysed wind field can significantly reduce the trajectory uncertainty (Lin et al., 2001). Moreover, the use of a large number of trajectories in the statistics computations will minimize to a certain degree the random errors generated by interpolation, truncation and wind data.

Other trajectory data have been also available, but for periods not fully covering the concentration data sets: (1) three-dimensional 4-day back trajectories arriving at 950 hPa at Sevettijärvi every 6 or 8 hours, calculated by the TRADOS model of the Finnish Meteorological Institute, (2) three-dimensional 5-day back trajectories calculated once per day (1200 UT) and for up to 6 arrival levels (960, 900, 850, 800, 700, 500 hPa) for Sde Boker by the German Weather Service, and (3) three-dimensional 10-day back trajectories arriving at 300 m agl at Birkenes and Skreådalen every 3 hours, computed with the FLEXTRA model (Stohl et al., 1995) and using ECMWF analysed wind fields.