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Cloud Condensation Nuclei Counter
Hygroscopicity Tandem Differential Analyzer


Cloud Condensation Nuclei Counter

The working group Heterogeneous Reactions uses two measuring systems to investigate the water uptake of atmospheric aerosol particles. The hygroscopic growth of the particles is measured with a Hygroscopicity Tandem Differential Analyzer (HTDMA). The particle size and supersaturation at which the aerosol particles (nm size range) become cloud droplets (size: several µm) – the so called CCN activation point – is determined with a Cloud Condensation Nuclei Counter (CCN-C).

In the HTDMA a Differential Mobility Analyzer is used to select a single particle size from the available aerosol size distribution. These particles are humidified and the size change at the set relative humidity is measured with a Scanning Mobility Particle Sizer (SMPS). We are able to measure the hygroscopic growth up to 98% relative humidity with this instrument.

To determine the CCN activity, several different particle sizes are selected with a DMA. For each size the amount of particles which are activated to cloud droplets is measured at several supersaturations with the CCN-C. The activated fraction is obtained by comparing this to the simultaneously measured total aerosol concentration. Thus, it is possible to determine for each set supersaturation the size of the dry particle at which 50% are activated (definition of the activation point).

The interaction of aerosol particles and water vapor, both in the sub- and supersaturated regime, is described by the Köhler equation. It is possible to infer physical-chemical quantities which are part of this equation from combining measurements of hygroscopic growth and the activation point.