On-line measurements of sulfur dioxide at the 3 km level over central Europe (Sonnblick observatory, Austria) and statistical trajectory source analysis

Continuous sulfur dioxide (SO2) measurements were performed at the Sonnblick Observatory (SBO), Austria (3106 m a.s.l.) for several weeks during summer, autumn and spring 1995/96. The average SO2 concentration was 0.30 μg m-3 standard temperature and pressure (STP; 273 K, 1013 hPa) (105 pptv), the median 0.10 μg m-3 STP (35 pptv). For 80% of the time sulfur dioxide concentrations were at “clean levels” (<0.30 μg m-3) interrupted by “episodes” lasting up to two days. During the episodes SO2 levels rose generally above 0.5 μg m-3 and reached up to 8.0 μg m-3 in spring. Sulfur dioxide concentrations showed a strong seasonal variation with very low concentrations during summer and fall (means 0.11 and 0.17 μg m-3) and elevated concentrations during spring (mean 0.70 μg m-3). An analysis of the diurnal variation indicated that upslope winds are not responsible for the elevated SO2 levels observed during SO2 episodes; rather cloud transport appears to be the predominant process for lifting SO2 from the boundary layer to the mid troposphere. For air and cloud transport across Western European countries hydrogen peroxide (H2O2) is apparently in excess and most SO2 is removed by oxidation leading to “clean conditions”. For the less frequent air and cloud transport from the East and Southeast SO2 appears to be in excess leading to the “SO2 episodes” at the 3 km level. These results are substantiated by in-cloud observations of elevated levels of SO2 at SBO (Brantner et al., 1994) and trajectory statistics for the SO2 data set from SBO and analysis of the meteorological conditions during transport.