Cloud condensation nuclei (CCNs), also known as cloud seeds, are small

particle In the physical sciences, a particle (or corpuscle in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass. They vary greatly in size or quantity, from s ...

s typically 0.2 μm, or one hundredth the size of a

cloud In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles, suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may ...

droplet. CCNs are a unique subset of aerosols in the atmosphere on which

water vapour Water vapor, water vapour, or aqueous vapor is the gaseous phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Water vapor ...

condenses. This can affect the radiative properties of clouds and the overall atmosphere.

Water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

vapour requires a non- gaseous surface to make the transition to a

liquid Liquid is a state of matter with a definite volume but no fixed shape. Liquids adapt to the shape of their container and are nearly incompressible, maintaining their volume even under pressure. The density of a liquid is usually close to th ...

; this process is called

condensation Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor ...

. In the

atmosphere of Earth The atmosphere of Earth is composed of a layer of gas mixture that surrounds the Earth's planetary surface (both lands and oceans), known collectively as air, with variable quantities of suspended aerosols and particulates (which create weather ...

, this surface presents itself as tiny

solid Solid is a state of matter where molecules are closely packed and can not slide past each other. Solids resist compression, expansion, or external forces that would alter its shape, with the degree to which they are resisted dependent upon the ...

or liquid particles called CCNs. When no CCNs are present, water vapour can be

supercooled Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid below its freezing point without it becoming a solid. Per the established international definition, supercooling means ''‘cooling a substance be ...

at about for 5–6 hours before droplets spontaneously form. This is the basis of the

cloud chamber A cloud chamber, also known as a Wilson chamber, is a particle detector used for visualizing the passage of ionizing radiation. A cloud chamber consists of a sealed environment containing a supersaturated vapor of water or alcohol. An energetic ...

for detecting subatomic particles. The concept of CCN (must associate to a supersaturation ratio) is used in

cloud seeding Cloud seeding is a type of weather modification that aims to change the amount or type of precipitation, mitigate hail, or disperse fog. The usual objective is to increase rain or snow, either for its own sake or to prevent precipitation from ...

, 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 An aerosol is a suspension (chemistry), suspension of fine solid particles or liquid Drop (liquid), droplets in air or another gas. Aerosols can be generated from natural or Human impact on the environment, human causes. The term ''aerosol'' co ...

) is on the order of 0.0001 mm or 0.1 μm or greater in diameter. The number of cloud condensation nuclei in the air can be measured at ranges between around 100 to 1000 per cm³. The size distribution and chemical composition of CCNs over oceans usually have obvious differences compared to those over land. The total mass of CCNs injected into the atmosphere has been estimated at over a year's time.

Composition

There are many different types of atmospheric

particulates Particulate matter (PM) or particulates are microscopic particles of solid or liquid matter suspension (chemistry), suspended in the atmosphere of Earth, air. An ''aerosol'' is a mixture of particulates and air, as opposed to the particulate ...

that can act as CCN. The particles may be composed of dust or

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...

soot Soot ( ) is a mass of impure carbon particles resulting from the incomplete combustion of hydrocarbons. Soot is considered a hazardous substance with carcinogenic properties. Most broadly, the term includes all the particulate matter produced b ...

black carbon Black carbon (BC) is the light-absorbing refractory form of Chemical_element, elemental carbon remaining after pyrolysis (e.g., charcoal) or produced by incomplete combustion (e.g., soot). Tihomir Novakov originated the term black carbon in ...

from grassland or forest fires, sea salt from ocean wave spray, soot from factory smokestacks or internal combustion engines,

sulfate The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...

from

volcanic A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often fo ...

activity,

phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater Aquatic ecosystem, ecosystems. The name comes from the Greek language, Greek words (), meaning 'plant', and (), mea ...

or the oxidation of

sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...

and secondary organic matter formed by the oxidation of

volatile organic compounds Volatile organic compounds (VOCs) are organic compounds that have a high vapor pressure at room temperature. They are common and exist in a variety of settings and products, not limited to house mold, upholstered furniture, arts and crafts sup ...

. 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 Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption (chemistry), absorption or adsorption from the surrounding Natural environment, environment, which is usually at normal or room temperature. If water mol ...

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. This is made even more complicated by the fact that many of the chemical species may be mixed within the particles (in particular the sulfate and organic carbon). Additionally, while some particles (such as soot and minerals) do not make very good CCN, they do act as ice nuclei in colder parts of the atmosphere.

Abundance

The number and type of CCNs can affect the precipitation amount, lifetimes, and radiative properties of

s and their lifetimes. Ultimately, this has an influence on

climate change Present-day climate change includes both global warming—the ongoing increase in Global surface temperature, global average temperature—and its wider effects on Earth's climate system. Climate variability and change, Climate change in ...

. Modeling research led by Marcia Baker revealed that sources and sinks are balanced by

coagulation Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a thrombus, blood clot. It results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The process of co ...

and coalescence which leads to stable levels of CCNs in the atmosphere. 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 that can form CCN at SGP site were performed using a research aircraft
CCN study by Kulkarni et al 2023 describes the complexity in modeling CCN concentrations

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 Albedo ( ; ) is the fraction of sunlight that is Diffuse reflection, diffusely reflected by a body. It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects ...

. 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 Radiative forcing (or climate forcing) is a concept used to quantify a change to the balance of energy flowing through a planetary atmosphere. Various factors contribute to this change in energy balance, such as concentrations of greenhouse gases ...

. 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.

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 Dimethyl sulfide (DMS) or methylthiomethane is an organosulfur compound with the formula . It is the simplest thioether and has a characteristic disagreeable odor. It is a flammable liquid that boils at . It is a component of the smell produc ...

(DMS) produced by algae found in seawater. Large

algal blooms Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular microalgae, suc ...

, observed to have increased in areas such as the South China Sea, can contribute a substantial amount of DMS into their surrounding atmospheres, leading to increased cloud formation. As the activity of phytoplankton is temperature reliant, this negative-feedback loop can act as a form of climate regulation. '' The Revenge of Gaia'', written by James Lovelock, an author of the 1987 study, proposes an alternative relationship between ocean temperatures and phytoplankton population size. This has been named the anti-CLAW hypothesis In this scenario, the stratification of oceans causes nutrient-rich cold water to become trapped under warmer water, where sunlight for photosynthesis is most abundant. 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

through decreasing CCN-induced cloud formations and increases the solar radiation allowed to reach ocean surfaces, resulting in a positive-feedback loop.

From volcanoes

Volcanoes emit a significant amount of microscopic gas and ash particles into the atmosphere when they erupt, which become atmospheric aerosols. 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 Global cooling was a conjecture, especially during the 1970s, of imminent cooling of the Earth culminating in a period of extensive glaciation, due to the cooling effects of aerosols or orbital forcing. Some press reports in the 1970s specu ...

. Almost 9.2 Tg of sulfur dioxide () is emitted from volcanoes annually. This sulphur dioxide undergoes a transformation into

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...

, which quickly condenses in the stratosphere to produce fine sulphate aerosols. The Earth's lower atmosphere, or troposphere, cools as a result of the aerosols' increased capability to reflect solar radiation back into space.

Effect on air pollution

References

External links

www.grida.no

An easy experiment to do at home (in French)
{{Authority control Cloud and fog physics Particulates