Saharan dust events at the Jungfraujoch: detection by wavelength dependence of the single scattering albedo and analysis of the events during the years 2001 and 2002
1MeteoSwiss, Aerological Station, Les Invuardes, CH-1530 Payerne, Switzerland
2Laboratory of Atmospheric Chemistry, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
3Swiss Federal Laboratories for Materials Testing and Research, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland
4Laboratory of Radio- and Environmental Chemistry, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
Abstract. Scattering and absorption coefficients have been measured continuously at several wavelengths since March 2001 at the high altitude site Jungfraujoch (3580 m a.s.l.). From these data, the wavelength dependences of the Ångström exponent and particularly of the single scattering albedo are determined. While the exponent of the single scattering albedo is usually positive, it becomes negative during Saharan dust events (SDE) due to the greater size of the mineral aerosols and to their different chemical composition. This change in the sign of the single scattering exponent turns out to be a simple means for detecting Saharan dust events. The occurrence of SDE detected by this new method was largely confirmed by visual inspection of filter colors and by studying long-range back-trajectories. An examination of SDE over a 22 months period shows that SDE are more frequent during the March–June period as well as during October and November. The trajectory analysis indicated a mean traveling time of 96.5 h with the most important source countries situated in the northern and north-western part of the Saharan desert. Most of the SDE do not lead to a detectable increase of the 48 h total suspended particulate matter (TSP) at the Jungfraujoch. During Saharan dust events, the average contribution of this dust to hourly TSP at the JFJ is 16 μg/m3, which corresponds to an annual mean of 0.8 μg/m3 or 24% of TSP.