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

Abstract

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 nm <particle diameter (Dp)<800 nm] were determined with a time resolution of 6 min. A forward scattering spectrometer probe (FSSP-100) measured the cloud droplet size distribution, and a particle volume monitor (PVM-100) was used to measure liquid water content (LWC). In addition, the aerosol chemical composition (major soluble ions) was determined in two size classes (total and sub-micron particles). Agreement within the range of measurement uncertainties was observed between the droplet number concentrations derived from the aerosol size distribution measurements (total minus interstitial) and those measured by the FSSP. The observed particle diameter at 50% activation (D₅₀) was typically around 100 nm for LWC > 0.15 g m⁻³. Below this value, D₅₀ increased with decreasing LWC. A dependence of D₅₀ on the accumulation mode (Dp>100 nm) number concentration (N_{tot,Dp>100}) was only found for concentrations less than 100 cm⁻³. For higher values of N_{tot,Dp>100} the D50 remained constant. Furthermore, a decrease of the effective radius of cloud droplets (R_eff) with increasing Ntot,Dp>100 was observed, providing experimental evidence for the microphysical relation predicted by the Twomey effect. A modified Köhler 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.