1Institute for Atmospheric and Climate Science, ETH Zurich, Switzerland
2Paul Scherrer Institute, Villigen, Switzerland
3Institute for Atmospheric Physics, University of Mainz, Germany
*now at: University of Tasmania, Tasmania, Australia
Abstract. Mineral dust from the Saharan desert can be transported across the Mediterranean towards the Alpine region several times a year. Occasionally, the dust is deposited with snowfall on Alpine glaciers and appears then as yellow or red layers in ice cores. Two such significant dust events were identified in an ice core drilled at the high-accumulation site Piz Zupó in the Swiss Alps (46°22' N, 9°55' E, 3850 m a.s.l.). From stable oxygen isotopes and major ion concentrations, the events were approximately dated as October and March 2000. In order to link the dust record in the ice core to the meteorological situation that led to the dust events, a novel methodology based on back-trajectory analysis was developed. It allowed for the identification of source regions, the atmospheric transport pathways, and wet deposition periods for both dust events. Furthermore, differences in the chemical signature of the two dust events could be interpreted with respect to contributions from the dust sources and aerosol scavenging during the transport.
The dust deposition during the October event took place during 13–16 October 2000. Mobilisation areas of dust were mainly identified in the Algerian and Libyan deserts. A combination of an upper-level potential vorticity streamer and a midlevel jet across Algeria first brought moist Atlantic air and later mixed air from the tropics and Saharan desert across the Mediterranean towards the Alps. The March event consisted of two different deposition phases which took place during 18–20 and 23–26 March 2000. The first phase was associated with an exceptional transport pattern past Iceland and towards the Alps from northerly directions. The second phase was similar to the October event. A significant peak of methanesulphonic acid associated with the March dust event was most likely caused by incorporation of biogenic aerosol while passing through the marine boundary layer of the western Mediterranean during a local phytoplankton bloom. From this study, we conclude that the whole sequence of mobilisation, transport, and deposition of mineral aerosol should be considered for a detailed understanding of the chemical signal recorded in the ice core at Piz Zupó.