A Multi-Channel Expansion Type Condensation Particle Counter

Despite considerable world-wide research efforts the influence of aerosols and clouds still causes the highest uncertainty in our current understanding of climate change. Particle formation processes in the atmosphere are frequently related to freshly nucleated molecular clusters with diameters of the order of 1 – 2 nm. Therefore one focus of the present project was placed on the development of a measurement system capable of detecting, counting and investigating molecular clusters and aerosol particles in the nanometer size range.

In a joint research effort at CERN, Geneva, we studied homogeneous nucleation in multicomponent vapour mixtures under precisely controlled concentrations of water vapour, sulfuric acid vapour and other compounds in the CLOUD chamber. Stable sulfuric acid concentrations were achieved applying an innovative fibre-optic UV system. Results show that atmospheric boundary layer nucleation can be accounted for, if besides sulphuric acid and ammonia in addition certain organic trace compounds are considered as well.

Atmospheric particle formation can be explained by heterogeneous nucleation on freshly nucleated molecular clusters. In this project we have therefore considered some basic aspects of heterogeneous nucleation. This involves careful experiments on contact angles. Furthermore we performed theoretical studies on the influence of electrical charge of the seed particles on the heterogeneous nucleation process.

Recently we discovered strange temperature effects for heterogeneous nucleation. In this project we studied the temperature effect for water vapour. The experimental results reveal unexpected behaviour, which can tentatively be related to strong molecular-scale interactions at the seed particle surface. Furthermore the vapour supersaturations required for activation of seed particles were found to be well below previous theoretical expectation according to the generally accepted Kelvin equation. 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.

A central aspect of this project was the design and construction of a novel condensation particle counter, the versatile Size Analyzing Nuclei Counter (vSANC). Vapour super-saturation in the vSANC is achieved by adiabatic expansion. Software systems for process control and data evaluation have been developed and tested. The vSANC has been carefully characterized. An important feature of the vSANC is its portability. The vSANC has been installed for several months at the background measurement station in Hyytiälä, Finland and applied for experimental observations of atmospheric nanocluster concentrations.