Aerosol-cloud interaction at the high-Alpine site Jungfraujoch

Urs Baltensperger

Laboratory for Atmospheric Chemistry, Paul Scherrer Institute
CH-5232 Villigen PSI, Switzerland

Aerosols influence the atmospheric energy budget through direct and indirect effects. Direct effects refer to the scattering and absorption of radiation and their subsequent influence on the planetary albedo and the climate system. Indirect effects refer to the increase in available cloud condensation nuclei (CCN) due to an increase in anthropogenic aerosol concentration. This is believed to change the cloud droplet number concentration for a constant cloud liquid water content (LWC), and the resulting increase in cloud albedo influences the Earthís radiation budget. Within the Global Atmosphere Watch (GAW) program of the World Meteorological Organization a number of stations aim at contributing to a greater understanding of both the direct and indirect effect of aerosols on climate.

This talk presents results from the high alpine research station Jungfraujoch (JFJ, 3580 m asl, Switzerland). Here, a number of aerosol parameters have been measured since 1995. Besides this continuous program, a series of Cloud and Aerosol Characterization Experiments (CLACE 1 to 3) were performed in 2000, 2002, and 2004. These experiments were designed to investigate the chemical composition of aerosol particles, their hygroscopic properties, their light scattering properties, and their interaction with both liquid and ice phase clouds. Concerning the latter, the investigation of the consequences of the Bergeron-Findeisen process was a major focus.

The latest experiment in 2004 was conducted with the participation of the Institute for Tropospheric Research in Leipzig, UMIST Manchester, the Max Planck Institute for Chemistry in Mainz, the University of Copenhagen, the DLR in Wessling, the Technical University of Darmstadt, and ETH Z¸rich. Different inlet systems were used to sample ambient air: A heated inlet was used, designed to evaporate all activated cloud droplets and ice crystals at an early stage of the sampling process. A second inlet was operated with a 2.5 μm cut-off diameter impactor to remove cloud droplets, which resulted in only the interstitial aerosol being sampled. As a third inlet system, an ice-phase Counterflow Virtual Impactor (CVI) was operated. This Ice-CVI system separates small ice particles from large ice crystals, cloud droplets and interstitial aerosol particles. The extracted ice particles were dried airborne in the system and the remaining residual particles which correspond to the former ice nuclei were analyzed. The sampled air from the three different inlet systems was then brought into the laboratory where aerosol measurements were performed at low relative humidity (RH < 20%). The physics and chemistry of the aerosol and cloud particles was measured by a wide range of instruments. The combination of employing different sophisticated inlet systems and a wide range of physico-chemical instruments allowed a thorough investigation of the interaction between aerosol and (liquid and ice-phase) cloud particles. By duplicating and/or triplicating identical measurements for the total aerosol, the interstitial aerosol and the ice nuclei, cloud activation and heterogeneous ice nucleation properties were derived by differencing. Furthermore, properties of liquid cloud droplets and ice crystals were characterized by a number of in situ instruments.