Cloud condensation nuclei

Cloud condensation nuclei are the subset of atmospheric aerosol particles capable of initiating liquid cloud droplets under suitable atmospheric conditions. Aerosols include tiny solid or liquid particles suspended in air. Sources include sea spray, soil and smoke; particles can also form in the atmosphere from precursor gases and grow. As moist air cools and reaches saturation, water vapour can condense on particles rather than forming cloud droplets spontaneously in otherwise clean air. Condensation and evaporation continue as air parcels mix and conditions change.

Not every aerosol particle becomes a cloud droplet in every cloud. Whether a particle activates depends on its size and composition, and on the supersaturation reached in the rising air. Hygroscopic material takes up water more readily, while particle size also affects the conditions needed for activation. Atmospheric scientists use cloud condensation nuclei concentration measured at a specified supersaturation; a CCN count is therefore tied to the measurement conditions, not a single universal inventory of particles that will seed clouds everywhere.

CCN contribute to the number of droplets in liquid clouds. When more particles activate under comparable conditions, cloud water may be distributed among more numerous, smaller droplets, affecting cloud properties. Aerosols can also interact with radiation directly by scattering or absorbing it. These aerosol–cloud interactions are complex: observed and modeled effects vary with aerosol sources, cloud conditions, location and season. Assessments of global models find substantial uncertainty in simulated aerosol and CCN concentrations, and no single evaluated model performs best across all observed environments.

Researchers measure CCN by exposing sampled aerosol to controlled supersaturation and counting particles that activate into droplets. Reporting the supersaturation is important because a higher value can activate a different and often broader fraction of the aerosol population. CCN observations and models help investigate cloud formation and aerosol effects, but translating particle counts into cloud and climate responses requires accounting for atmospheric dynamics and other cloud processes.