Ice nuclei properties within a Saharan dust event at the Jungfraujoch in the Swiss Alps

The new portable ice nucleation chamber (PINC) developed by the Institute for Atmospheric and Climate Sciences of ETH Zurich was operated during two measurement campaigns at the high alpine research station Jungfraujoch situated at 3580m a.s.l, in March and June 2009. During this time of the year, a high probability of Saharan dust events (SDE) at the Jungfraujoch has been observed. We used an impactor with a cutoff size of 1 mu m aerodynamic diameter and operated the system at -31 degrees C and relative humidities of 127% and 91% with respect to ice and water, respectively. Investigation of the ambient number concentration of ice nuclei (IN) in the deposition nucleation mode and during a SDE in the free troposphere is reported. The results discussed in this paper are the first continuous IN measurements over a period of several days at the Jungfraujoch. The average IN concentration found during the campaign in March was 8 particles per liter whereas during the campaign in June, the average number concentration was higher up to 14 particles per liter. Two SDEs were detected on 15 and 16 June 2009. Our measurements show that the SDEs had IN number concentration up to several hundreds per liter. We found the best correlation between the number concentration of the larger particle fraction measured by an optical particle counter and the IN number concentration during a Saharan dust event. This correlation factor is higher for particles larger than 0.5 mu m meaning that a higher concentration of larger particles induced higher IN number concentration. No correlation could be found between the black carbon mass concentration and the variations in IN number concentration.