Nucleation on charged and uncharged nanoclusters

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.