Atmospheric ice nuclei at the high-altitude observatory Jungfraujoch, Switzerland

The state of a slightly supercooled ephemeral cloud can be changed by the presence of a few particles capable of catalysing freezing, and potentially result in precipitation. We investigated the atmospheric abundance of particles active as ice nuclei at −8°C (IN₋₈) over the course of a year at the high-alpine station Jungfraujoch (3580 m.a.s.l., Switzerland) through the use of immersion freezing assays of particles collected on quartz micro-fibre filters. In addition, we determined IN₋₈ on a hill in the planetary boundary layer 95 km northwest of Jungfraujoch and in the dust laden Saharan Air Layer reaching Tenerife. Results indicate a strong seasonality of IN₋₈ at Jungfraujoch. Values were largest during summer (between 1 and 10 m⁻³) and about two orders of magnitude smaller during winter. Sahara dust events had a negligible influence on IN₋₈ at Jungfraujoch. Seasonality in the boundary layer was not observed in the upper, but in the lower bound of IN₋₈ values. Values<1 m⁻³ were only found on cold winter days, when IN₋₈ were more likely to have already been activated and deposited than on warmer days. A good correlation between IN₋₈ and maximum daily temperature at Jungfraujoch (R²=0.54) suggests IN₋₈ abundance at Jungfraujoch may be limited most of the year by microphysical processing related to IN activation in approaching air masses.