High-time-resolution investigation of cloud condensation nuclei in mid-level cloud systems on Mt. Rax (1450 m amsl)

Project number: Research project P 13143 Project title: CCN in Low-Level Cloud Systems ABSTRACT OF THE PROJECT In the current debate on global change, the effect of both natural and anthropogenic aerosols is studied intensively. Aerosols interact with solar radiation both directly as particles and indirectly because of their cloud droplet forming potential. Cloud droplets form by condensation of water vapor on pre-xisting aerosol particles (the cloud condensation nuclei, CCN) under superaturated conditions. If the optical properties of a cloud change because of changes of CCN, the global albedo might be affected and consequently the global radiative balance might be shifted. For the low supersaturations found in the atmosphere, these CCN particles should be soluble. Until several years ago, only inorganic salt particles were considered as possible CCN, but new evidence was then found that the organic fraction of the aerosol might contribute significantly to CCN. In an earlier project, the contribution of organic material to CCN and its role in the scavenging process was measured on Mt. Sonnblick (at 3106 m amsl). The results of this previous study are the basis for the proposed project. CCN concentrations were found to be very variable with time, while the usual sampling methods for chemical analysis suffer from sampling times of several hours. The scavenging ratios of the organic ions were about a factor of 4 lower than those of the inorganic substances. The project "high time resolution investigation of cloud condensation nuclei in mid-level cloud systems on Mt. Rax (1640 m amsl)" is aimed at a better understanding of the role of organic aerosel material in the cloud nucleation and scavenging process. During this study, the physical and chemical properties of aerosol particles and cloud droplets will be studied with a variety of methods on a low-level mountain during of 4 intensive campaigns of up to three weeks at times when both cloudy and cloud-free periods are likely. During no-cloud conditions (i. e. in the RAX- 1 experiment), CCN concentrations and the chernical composition (inorganic ions, selected organic compounds, black carbon) of the total aerosol will be obtained with a time resolution of 30 minutes. Long-term irnpactor samples will yield the time-averaged size distribution of these compounds and additionally for black, total and organic carbon. The total aerosol number concentration will be measured with a condensation nuclei counter in real time to investigate the fraction of aerosol particles that can act as CCN. As CCN data are so scarce globally, an extension of the CCN data base alone would already be a significant contribution to current knowledge on atmospheric aerosols. For the first time, however, CCN measurements are coupled here to high time resolution chemical analyses of the aerosol. Changes of the aerosol composition and their effect on the fraction of CCN in the total aerosol (measured with a condensation nuclei counter) can thus be followed. When the measurement station is within a cloud (i.e. in the RAX-2 experiment), cloud water will be collected and the chemical composition of the interstitial aerosol will again be measured with the high time resolution. The collected cloud water will be analyzed at once for black carbon and later for a full set of chemical compounds (inorganic ions, selected organic compounds, total carbon). Scavenging ratios of the compounds will be obtained and used to obtain new insights into the wet scavenging process for organic aerosol material.