Aerosol-Cloud Interactions in the Lower Free Troposphere as Measured at the High Alpine Research Station Jungfraujoch in Switzerland

Within the WMO Global Atmosphere Watch (GAW) program continuous measurements of aerosol parameters have been performed at the Jungfraujoch high Alpine site since 1988. This measurement site is located on an exposed mountain col in the Bernese Alps, Switzerland, at 3580 meters altitude. Throughout the year the station is within clouds about 40% of the time. In warm months, the site is influenced by injection of planetary boundary layer air into the free troposphere during sunny afternoons due to thermal convection, while in winter it is usually in the undisturbed free troposphere. Several intensive field studies, named CLACE (Cloud and Aerosol Characterization Experiment), have been performed in both summer and winter within international collaborations. State-of-the-art instrumentation is employed to characterize the aerosol size distribution (Scanning Mobility Particle Sizer, SMPS; Optical Particle Counter, OPC). The University of Manchester and the Max Planck Institute in Mainz operated two Aerodyne Aerosol Mass Spectrometers (AMS) to measure the size segregated chemical composition. Other measured parameters were the hygroscopic properties of the particles (Hygroscopicity Tandem Differential Mobility Analyzer, H-TDMA), cloud microphysics (Particulate Volume Monitor, PVM; Forward Scattering Spectrometer Probe, FSSP; Cloud Particle Imager, CPI), and particle morphology (Environmental Scanning Electron Microscope, ESEM). An Air Ion Spectrometer (AIS) and an outdoor SMPS were also employed. The latter two instruments are especially well suited to measure nanometer sized particles (charged particles and sum of neutral and charged particles, respectively) in order to elucidate their formation mechanisms and rates. Different inlets are used for these instruments: A heated inlet (25°C) designed to prevent ice build-up and to evaporate cloud particles at an early stage, ensuring that the cloud condensation nuclei and/or ice nuclei are also sampled. This is called the total inlet. An interstitial inlet operated with a PM2 cyclone impactor removed all cloud droplets and ice crystals from the ambient air. Within a cloud the sampled air thus represents the interstitial (or unactivated) aerosol fraction. In addition, the Institute for Tropospheric Research (IFT) operated a Counterflow Virtual Impactor (CVI, Mertes et al., 2005). The CVI was part of a new prototype sampling system (Ice-CVI) that allows for the separation of small ice particles from large ice crystals, cloud droplets and interstitial aerosol particles. The extracted ice particles are dried airborne in the system and the remaining residual particles which correspond to the former ice nuclei were analyzed with a variety of different instruments. Differencing the response downstream of the different inlets provides insight in the fractionation of aerosol particles between the cloud phase and the interstitial phase