Lab and field studies of the ability of aerosols to act as cloud condensation nuclei (CCN)

Aerosols are precursors to cloud droplets, and their size, concentration, and affinity to water vapor directly influences cloud microphysical and radiative properties (see movie). The subset of the aerosol particles that activates to form cloud droplets at a given water vapor supersaturation are called CCN.

Courtesy of Zoran Ristovski.

There are many different types of atmospheric particles that can act as CCN. The particles may be composed of dust, soot, or black carbon from grassland or forest fires, sea salt from ocean wave spray, soot from combustion engines, sulfate from volcanic activity, phytoplankton or the oxidation of sulfur dioxide and secondary organic matter formed by the oxidation of VOCs. The ability of these different types of particles to form cloud droplets varies according to their size and also their exact composition, as the hygroscopic properties of these different constituents are very different. Sulfate and sea salt, for instance, readily absorb water whereas soot, organic carbon and mineral particles do not. The water uptake and their ability to act as CCN is more complicated by the fact that many of the chemical species may be mixed within the particles (in particular the sulfate and organic carbon).

The experimental group at IAC-ETH uses a Cloud Condensation Nuclei Counter (CCNC) that is a commercially available instrument from Droplet Measurement Technologies, to study the ability of aerosols to act as CCN. This version of a CCNC provides in situ measurements of CCN and operates between 0.1 and 2% supersaturation. The working principle of the CCNC involves a diffusion chamber in which a controllable, quasi-uniform supersaturation with respect to water vapor is established. Known concentrations of aerosol particles that are fed into the chamber are activated in the supersaturated environment, i.e. they grow by water vapor condensation to droplet sizes. The number of activated particles is counted by an optical particle counter, thereby determining the subset of particles that have the potential to form clouds under the given environmental conditions.

The CCNC was deployed during multiple laboratory and field studies to investigate CCN properties of, e.g., different types of mineral dust particles, wood burning and diesel soot particles, and those of atmospheric aerosols in urban and remote regions. In an ongoing PhD-project, we explore cloud formation in the high Arctic during summertime (Arctic Summer Cloud Ocean Study ASCOS) which takes place under certain meteorological but extreme pristine atmospheric conditions.