Nucleation on charged and uncharged nanoclusters

Atmospheric aerosols interact directly by absorption and scattering as well as indirectly as cloud condensation nuclei with solar and terrestrial radiation thereby influencing the earth`s climate. It has been estimated that the influence of aerosols on climate is globally comparable to that of greenhouse gases but opposite in sign. Because of this strong cooling effect, a possible future decrease in aerosol concentration due to better filter technology might lead to an even stronger temperature increase in global climate than was predicted until now. Detailed understanding of the relevant microphysical processes of aerosols and clouds is thus essential for the further development of climate models. Presently, the influence of charged molecular clusters and ionizing radiation on the formation of atmospheric nanoparticles und subsequently on cloud formation and climate change is intensively discussed. Additionally, condensable organic vapors probably responsible for the condensational growth of nanoparticles to cloud condensation nuclei have become subject of intensive studies. In experimental studies in our laboratory we plan to investigate the influence of organic vapors on the heterogeneous nucleation of charged and uncharged particles in the size range of a few nanometers under precisely controlled conditions. Comparisons to numerical models are planned together with Prof. Kulmala and his group at the University of Helsinki. In order to investigate the influence of ionizing radiation on the microphysics of aerosols and clouds, jointly with 19 other research groups we plan to conduct the project CLOUD at CERN, Geneva. Thereby, a precisely adjustable pion beam from a CERN- particle accelerator will be utilized as a model for cosmic radiation. The heart of the CLOUD experiment is a precision expansion cloud chamber that provides controlled and uniform thermodynamic conditions. In this context a specially developed laser light scattering arrangement (Constant-Angle-Mie-Scattering-Methode, CAMS) will be utilized providing non-invasive quantitative measurements of number concentration and growth kinetics of condensing droplets. The proposed project would contribute to a significant progress in the field of climate research and offer the unique chance to conduct presently high priority experiments on nucleation on charged nanoclusters in Vienna as well as to participate as only Austrian research group at the CERN - CLOUD project.

Despite considerable world-wide research efforts the influence of aerosols and clouds still causes the highest uncertainty in our current understanding of climate change. In recent years it has become clear that particle formation due to multicomponent vapor nucleation is a very important mechanism in the atmosphere. It has been pointed out that particle formation processes in the atmosphere may be frequently related to freshly nucleated molecular clusters with diameters of the order of 1 2 nm. The focus of the present project was placed on the formation of aerosol particles by gas-to-liquid phase transitions due to nucleation in supersaturated vapors.In a joint research effort at CERN, Geneva, we studied homogeneous nucleation in binary and ternary vapor mixtures under precisely defined mixtures of water vapor, sulfuric acid and ammonia in the CLOUD chamber. Stable sulfuric acid concentrations were achieved applying a innovative fibre-optic UV system. Results have shown that both mechanisms cannot account for atmospheric boundary layer nucleation and further organic compounds need to be considered.Using the Size-Analyzing Nuclei Counter (SANC), an expansion chamber system developed in our laboratory, we studied heterogeneous nucleation of vapors on pre-existing seed aerosol particles. Thereby we were able to bridge the scale from molecular clusters to nanoparticles. For charged seed particles an enhancement of heterogeneous nucleation and a significant sign preference were observed. The vapor supersaturations required for activation of seed particles were found to be well below previous expectation according to the generally accepted Kelvin equation. From experimental studies of heterogeneous nucleation on single ion molecules we obtained critical cluster diameters strongly exceeding the diameter of the seed molecules, but in good agreement with Kelvin prediction. As a consequence the widely used term "Kelvin diameter" needs to be critically reconsidered and the fundamental detection range of Condensation Particle Counters is extended to particle diameters as small as 1 nm.Our studies of the temperature dependence of heterogeneous nucleation surprisingly revealed unexpected temperature trends for certain combinations of particle and vapor compositions opposite to theoretical expectation. This exciting result seems to be evidence for strong molecular-scale interactions at the seed particle surface.