thumb|Aerosol pollution over [[North India|northern India and Bangladesh (Satellite image by NASA)]]
Cloud condensation nuclei (CCNs), also known as cloud seeds, are small particles typically 0.2 μm, or one hundredth the size of a cloud droplet. CCNs are a unique subset of aerosols in the atmosphere on which water vapour condenses. This can affect the radiative properties of clouds and the overall atmosphere. Water vapour requires a non-gaseous surface to make the transition to a liquid; this process is called condensation.
In the atmosphere of Earth, this surface presents itself as tiny solid or liquid particles called CCNs. When no CCNs are present, water vapour can be supercooled at about for 5–6 hours before droplets spontaneously form. This is the basis of the cloud chamber for detecting subatomic particles.
The concept of CCN (must associate to a supersaturation ratio) is used in cloud seeding, which tries to encourage rainfall by seeding the air with condensation nuclei (CN, which does not associate to supersaturation ratio). It has further been suggested that creating such nuclei could be used for marine cloud brightening, a climate engineering technique. Some natural environmental phenomena, such as the one proposed in the CLAW hypothesis also arise from the interaction between naturally produced CCNs and cloud formation.
Properties
Size
A typical raindrop is about 2 mm in diameter, a typical cloud droplet is on the order of 0.02 mm, and a typical cloud condensation nucleus (aerosol) is on the order of 0.0001 mm or 0.1 μm or greater in diameter. lifetimes, and radiative properties of clouds and their lifetimes. Ultimately, this has an influence on climate change. There is also speculation that solar variation may affect cloud properties via CCNs, and hence affect climate.
Airborne measurements
The airborne measurements of these individual mixed aerosols can form CCN at a SGP site when performed using a research aircraft.
Applications
Cloud seeding
Cloud seeding is a process by which small particulates are added to the atmosphere to induce cloud formation and precipitation. This has been done by dispersing salts using aerial or ground-based methods. Other methods have been researched, like using laser pulses to excite molecules in the atmosphere, and more recently, in 2021, electric charge emission using drones. The effectiveness of these methods is not consistent. Many studies did not notice a statistically significant difference in precipitation while others have. Cloud seeding may also occur from natural processes such as forest fires, which release small particles into the atmosphere that can act as nuclei.
Marine cloud brightening
Marine cloud brightening is a climate engineering technique which involves the injection of small particles into clouds to enhance their reflectivity, or albedo. The motive behind this technique is to control the amount of sunlight allowed to reach ocean surfaces in hopes of lowering surface temperatures through radiative forcing. Many methods involve the creation of small droplets of seawater to deliver sea salt particles into overlying clouds.
Complications may arise when reactive chlorine and bromine from sea salt react with existing molecules in the atmosphere. They have been shown to reduce ozone in the atmosphere; the same effect reduces hydroxide which correlates to the increased longevity of methane, a greenhouse gas.thumb|Phytoplankton bloom in the North Sea and the Skagerrak – NASA
Relation with phytoplankton and climate
A 1987 article in Nature found that global climate may occur in a feedback loop due to the relationship between CCNs, the temperature regulating behaviors of clouds, and oceanic phytoplankton. This phenomenon has since been referred to as the CLAW hypothesis, after the authors of the original study. A common CCN over oceans is sulphate aerosols. These aerosols are formed from the dimethyl sulfide (DMS) produced by algae found in seawater. This inhibits the growth of phytoplankton, resulting in the decrease in their population, and the sulfate CCNs they produce, with increasing temperature. This interaction thus lowers cloud albedo through decreasing CCN-induced cloud formations and increases the solar radiation allowed to reach ocean surfaces, resulting in a positive-feedback loop. By increasing the number of aerosol particles through gas-to-particle conversion processes, the contents of these eruptions can then affect the concentrations of potential cloud condensation nuclei (CCN) and ice nucleating particles (INP), which in turn affects cloud properties and leads to changes in local or regional climate.
Of these gases, sulfur dioxide, carbon dioxide, and water vapour are most commonly found in volcanic eruptions. While water vapour and carbon dioxide CCNs are naturally abundant in the atmosphere, the increase of sulfur dioxide CCNs can impact the climate by causing global cooling. Almost 9.2 Tg of sulfur dioxide () is emitted from volcanoes annually.