Elevated birch pollen episodes in Denmark: contributions from remote sources

The purpose of this study was to investigate the relationship between possible long-distance transport of birch pollen and episodes of elevated concentration in Denmark. By analysis of a twenty-six year (1980-2006) time-series of bi-hourly birch pollen counts from two sites (Copenhagen and Viborg) episodes of elevated counts (more than 100 grains) were identified in fewer than 2% of cases. Trajectory analysis showed that such episodes are primarily associated with long-distance transport from Eastern Europe and Scandinavia (43 and 33% of events, respectively); the lowest contribution originated from the British Isles. Long-term episodes (as in 1993 and 2006) occurred when atmospheric conditions favored long-distance transport from several source regions in succession.