Examples of large efficient ice nucleating particles in clouds above Switzerland

Extremely rare aerosol particles nucleate ice in clouds when they cool slightly below 0 °C. Although indirect evidence points to a biological origin of the most efficient ice nucleating particles (INPs), direct observation of ice nucleation, morphology, and elemental composition on the same single particles obtained from clouds is lacking for activation temperatures warmer than −10 °C. We developed an approach by which larger such particles (> 5 μm) were isolated from others, activated as INPs, and subsequently investigated by scanning electron microscopy and energy dispersive x-ray (SEM/EDX) analysis. Most of the 30 particles identified that way in clouds sampled at Jungfraujoch (3580 m a.s.l.) were agglomerates of mineral and organic material. The agglomerates had nothing in common except that they were holding an efficient ice nucleating molecule, which became apparent through the formation of an ice crystal but remained unidentified by SEM/EDX.