Saharan dust events contribute to the aerosol load over Switzerland, mostly during spring and autumn. These events can be detected accurately with continuous measurements at the Jungfraujoch and at the aerological station in Payerne.
Saharan dust events
Mineral dust is a major component of atmospheric aerosol, and the Sahara desert is its greatest source. As a result of strong winds and major turbulence, sand particles from the North African desert region regularly rise up several kilometres into the atmosphere. The largest particles quickly fall back to the ground, but the smaller ones can be transported all the way to the Swiss Alps
Detection of Saharan dust events at the Jungfraujoch
In the past, the contribution of mineral dust to the European aerosol load was gauged based on red rain or by means of the dust particles contained in snow and ice. The optical properties of aerosols at several wavelengths have been measured continuously at the Jungfraujoch high alpine research station since several years. These measurements have enabled the development of a new operational method, which detects the occurrence of Saharan dust events at an hourly resolution. This method can now be used to study the occurrence of Saharan dust events in the Alps.
The single-scattering albedo describes the ratio of light extinction due to scattering. It is an important parameter for obtaining local estimates of the direct radiative forcing by aerosols. The new detection method is based on the inversion of the wavelength dependence of the single-scattering albedo in the presence of mineral dust. Using the Saharan dust events in mid-June 2002, this effect is clearly recognizable: The highest single scattering albedo is measured during the event at the highest wavelength (950 nm), while the single scattering albedo in normal cases is greatest at the smallest wavelength.
Tracking Saharan dust using laser beams
Saharan dust events can be detected with LIDAR instruments. Such equipment is used at the aerological station of MeteoSchweiz in Payerne. It sends laser beams into the atmosphere, where light reacts with particles. Part of the radiation is reflected back to the surface of the Earth, where it can be collected and analysed. Aerosol profiles can be compiled and possible Saharan dust events detected.
This type of measurement is used to illustrate the stratification on 10 April 2011. At 06:00 am, a layer of high aerosol concentration was measured above Payerne at an altitude of 6000 m above sea level. Within nine hours, the aerosol layer subsided to an altitude of 2000 m above sea level, with a simultaneous increase in the concentration. A second aerosol layer was registered at 10:00 pm at an altitude of 4000 m above sea level; it subsided to 2000 m above sea level within 15 hours. In-situ measurements conducted at the Jungfraujoch during the same period likewise reveal high aerosol concentrations, and the particles were identified as Saharan dust.
Climatology of Saharan dust events
The Saharan dust events have been recorded since 2001. In general, they contribute significantly to the aerosol load above the Alps, both during spring (March to June) as well as in October and November. Only few events occur during summer, and very short ones during winter. The majority of Saharan dust events (48%) only last several hours, but 24% of events extend beyond one day. Trajectory analyses reveal a typical travel time of mineral dust between two days and one week. Between 10 and 34 events are registered each year. This corresponds to 200-650 hours of Saharan dust at the Jungfraujoch. The average hourly contribution of Saharan dust during a Saharan dust event is 0.8 µg/m³. On an annual average, this corresponds to 24% of the total aerosol mass concentration.