Urs Baltensperger
Laboratory of Atmospheric Chemistry
Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
11.11.2005, 13.00 c.t.
Room S3120
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 Earth’s radiation
budget. Within the Global Atmosphere Watch (GAW) program of the World
Meteorological Organization (WMO) the participating stations aim at contributing
to a greater understanding of both the direct and indirect effect of aerosols on
climate. Within this 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 metres altitude. In addition to the long-term program,
several intensive field studies, named CLACE (Cloud and Aerosol Characterization
Experiment), were performed in both summer and winter within international
collaborations. The goals of these intensive campaigns were:
• A full physical, chemical, and optical characterisation of the aerosol at the
Jungfraujoch in order to better quantify the direct aerosol effect.
• An investigation of the interaction of aerosol with clouds, for a better
quantification of the indirect effect.
One finding is that in mixed phase clouds the activated fraction of aerosol
particles is strongly dependent on the relative fraction of ice in the cloud.
This is explained by the Bergeron-Findeisen process, which describes the effect
of a water vapour flux from liquid droplets to ice crystals. The lower the
ambient temperature, the more liquid droplets evaporate and a higher fraction of
CCN is released back to the interstitial aerosol phase.