Size-dependent aerosol activation at the high-alpine site Jungfraujoch (3580 m asl)

Microphysical and chemical aerosol properties and their influence on cloud formation were studied in a field campaign at the high-alpine site Jungfraujoch (JFJ, 3580 m asl ). Due to its altitude, this site is suitable for ground-based in-cloud measurements, with a high cloud frequency of 40%. Dry total and interstitial aerosol size distributions [18 nm100 nm) number concentration (Ntot,Dp>100) was only found for concentrations less than 100 cm−3. For higher values of Ntot,Dp>100 the D50 remained constant. Furthermore, a decrease of the effective radius of cloud droplets (Reff) with increasing Ntot,Dp>100 was observed, providing experimental evidence for the microphysical relation predicted by the Twomey effect. A modified Ko¨ hler model was used to quantify the critical supersaturation for the aerosol observed at the JFJ. Ambient supersaturations were determined from the derived supersaturation curve and the calculated D50. As an example, a critical supersaturation of 0.2% was found for 100 nm particles.