Cloud Condensation and Ice Nuclei

The aerosol particles that seed cloud droplets and ice crystals, controlling how clouds form and precipitate.

Definition

Cloud condensation nuclei and ice nucleating particles are the subsets of atmospheric aerosol that act, respectively, as the sites on which cloud droplets and ice crystals form.

Scope

Covers the sources, composition and size of cloud condensation nuclei and ice nucleating particles; their abundance and spatial variability; the dependence of cloud droplet number and ice crystal concentration on these particles; and the resulting influence of aerosols on cloud microstructure and precipitation.

Core questions

• What makes an aerosol particle effective as a cloud condensation nucleus or ice nucleus?
• How does the abundance of these particles control cloud droplet and ice concentrations?
• How do natural and anthropogenic sources differ in their cloud-nucleating effects?

Key theories

Aerosol control of cloud microstructure

The number and properties of nucleating aerosol set the concentration of cloud droplets and ice crystals, linking aerosol pollution to cloud brightness, lifetime and precipitation.

Mechanisms

Cloud condensation nuclei are typically soluble or hygroscopic particles larger than about 50 nanometres that activate at the modest supersaturations found in clouds; their number largely sets cloud droplet concentration. Ice nucleating particles are far rarer and tend to be insoluble mineral dust, biological particles or other surfaces that catalyse freezing at warmer temperatures than homogeneous nucleation allows. Increasing nucleus concentrations produces more numerous, smaller droplets or crystals, altering cloud reflectivity, lifetime and precipitation efficiency.

Clinical relevance

Because nucleating aerosol determine cloud droplet and ice numbers, they mediate aerosol-cloud interactions that constitute one of the largest uncertainties in estimates of human influence on climate.

History

Recognition that a limited population of aerosol seeds cloud particles dates to early twentieth-century work, but quantifying the global distribution of condensation and especially ice nuclei and their climate effects has been an active research focus since the late twentieth century.

Key figures

• Meinrat Andreae
• Daniel Rosenfeld
• Paul DeMott

Seminal works

• andreae2008
• demott2010

Frequently asked questions

Why are ice nuclei so much rarer than condensation nuclei?

Initiating ice requires a particle surface that matches the ice lattice closely enough to catalyse freezing, a property only a small fraction of aerosol such as certain mineral dusts and biological particles possess.