Aerosol pollution over 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.[1] 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.[2] Water requires a non-gaseous surface to make the transition from a vapour 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 −13 °C (9 °F) for 5–6 hours before droplets spontaneously form. This is the basis of the cloud chamber for detecting subatomic particles.[3]

The concept of CCN is used in cloud seeding, which tries to encourage rainfall by seeding the air with condensation nuclei. It has further been suggested that creating such nuclei could be used for marine cloud brightening, a climate engineering technique.[4] 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.



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.[1] The number of cloud condensation nuclei in the air can be measured at ranges between around 100 to 1000 per cm3.[1] The total mass of CCNs injected into the atmosphere has been estimated at 2×1012 kg over a year's time.[1]


There are many different types of atmospheric particulates that can act as CCN. The particles may be composed of dust or clay, soot or black carbon from grassland or forest fires, sea salt from ocean wave spray, soot from factory smokestacks or internal combustion engines, sulfate from volcanic activity, phytoplankton or the oxidation of sulfur dioxide and secondary organic matter formed by the oxidation of volatile organic compounds.[1] 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. 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.[2]


The number and type of CCNs can affect the precipitation amount,[5] lifetimes, and radiative properties of clouds and their lifetimes. Ultimately, this has an influence on climate change.[4] Modeling research led by Marcia Baker revealed that sources and sinks are balanced by coagulation and coalescence which leads to stable levels of CCNs in the atmosphere.[6][7] There is also speculation that solar variation may affect cloud properties via CCNs, and hence affect climate.[8]

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.


Cloud seeding

Main article: 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.[9] Other methods have been researched, like using laser pulses to excite molecules in the atmosphere,[10] and more recently, in 2021, electric charge emission using drones.[11] The effectiveness of these methods is not consistent. Many studies did not notice a statistically significant difference in precipitation while others have.[citation needed] Cloud seeding may also occur from natural processes such as forest fires, which release small particles into the atmosphere that can act as nuclei.[12]

Marine cloud brightening

Main article: 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.[13] 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.[14] Many methods involve the creation of small droplets of seawater to deliver sea salt particles into overlying clouds.[15][16]

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[17].

Phytoplankton bloom in the North Sea and the Skagerrak – NASA

Relation with phytoplankton and climate

Main article: CLAW hypothesis

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.[18] 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.[18] Large algal blooms, 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.[19][18] 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.[20] 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.[20]

Volcanic ash and gas emissions from Alaska's Pavlof Volcano —NASA

From volcanoes

Volcanoes emit a significant amount of microscopic gas and ash particles into the atmosphere when they erupt, which become atmospheric aerosols.[21] 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.[22]

Of these gases, sulfur dioxide, carbon dioxide, and water vapour are most commonly found in volcanic eruptions.[23] 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.[24] Almost 9.2 Tg of sulfur dioxide (SO2) is emitted from volcanoes annually.[22] This sulphur dioxide undergoes a transformation into sulfuric acid, which quickly condenses in the stratosphere to produce fine sulphate aerosols.[24] The Earth's lower atmosphere, or troposphere, cools as a result of the aerosols' increased capability to reflect solar radiation back into space.

See also


  1. ^ a b c d e "Formation of Haze, Fog, and Clouds: Condensation Nuclei". Retrieved 25 November 2014.
  2. ^ a b Hudson, James G. (1993-04-01). "Cloud Condensation Nuclei". Journal of Applied Meteorology and Climatology. 32 (4): 596–607. Bibcode:1993JApMe..32..596H. doi:10.1175/1520-0450(1993)032<0596:CCN>2.0.CO;2. ISSN 1520-0450.
  3. ^ National Research Council. Division on Earth and Life Studies, National Research Council. Board on Atmospheric Sciences and Climate, National Research Council. Ocean Studies Board (2015). Climate Intervention : reflecting sunlight to cool Earth. Washington, D.C. ISBN 978-0-309-31483-1. OCLC 914166140.((cite book)): CS1 maint: location missing publisher (link)
  4. ^ a b T., Houghton, J. (2001). Climate change 2001 the scientific basis. Cambridge University Press. ISBN 0-521-80767-0. OCLC 1295485860.((cite book)): CS1 maint: multiple names: authors list (link)
  5. ^ Khain, A. P.; BenMoshe, N.; Pokrovsky, A. (1 June 2008). "Factors Determining the Impact of Aerosols on Surface Precipitation from Clouds: An Attempt at Classification". Journal of the Atmospheric Sciences. 65 (6): 1721–1748. Bibcode:2008JAtS...65.1721K. doi:10.1175/2007jas2515.1. S2CID 53991050.
  6. ^ Baker, Marcia B.; Charlson, Robert J. (1990). "Bistability of CCN concentrations and thermodynamics in the cloud-topped boundary layer". Nature. 345 (6271): 142–145. Bibcode:1990Natur.345..142B. doi:10.1038/345142a0. ISSN 1476-4687. S2CID 34623897.
  7. ^ Baker, M. B. (1 January 1993). "Variability in concentrations of cloud condensation nuclei in the marine cloud—topped boundary layer". Tellus B: Chemical and Physical Meteorology. 45 (5): 458–472. Bibcode:1993TellB..45..458B. doi:10.3402/tellusb.v45i5.15742.
  8. ^ Yu, Fangqun; Luo, Gan (2014-04-01). "Effect of solar variations on particle formation and cloud condensation nuclei". Environmental Research Letters. 9 (4): 045004. Bibcode:2014ERL.....9d5004Y. doi:10.1088/1748-9326/9/4/045004. ISSN 1748-9326. S2CID 121900557.
  9. ^ Vonnegut, B.; Chessin, Henry (1971-11-26). "Ice Nucleation by Coprecipitated Silver Iodide and Silver Bromide". Science. 174 (4012): 945–946. Bibcode:1971Sci...174..945V. doi:10.1126/science.174.4012.945. ISSN 0036-8075. PMID 17773193. S2CID 37459080.
  10. ^ "Laser creates clouds over Germany". New Scientist. 2010-05-02. Archived from the original on 2010-12-05. Retrieved 2022-12-05.
  11. ^ "UAE to test cloud-busting drones to boost rainfall". BBC News. 2021-03-17. Retrieved 2022-12-05.
  12. ^ Barry, Kevin R.; Hill, Thomas C. J.; et al. (2021-02-16). "Observations of Ice Nucleating Particles in the Free Troposphere From Western US Wildfires". Journal of Geophysical Research: Atmospheres. 126 (3). Bibcode:2021JGRD..12633752B. doi:10.1029/2020JD033752. ISSN 2169-897X. S2CID 233962401.
  13. ^ Ahlm, Lars; Jones, Andy; Stjern, Camilla W.; Muri, Helene; Kravitz, Ben; Kristjánsson, Jón Egill (2017-11-06). "Marine cloud brightening – as effective without clouds". Atmospheric Chemistry and Physics. 17 (21): 13071–13087. Bibcode:2017ACP....1713071A. doi:10.5194/acp-17-13071-2017. hdl:10852/59113. ISSN 1680-7324.
  14. ^ Intergovernmental Panel on Climate Change (ed.), "Anthropogenic and Natural Radiative Forcing pages 705 to 740", Climate Change 2013 - The Physical Science Basis, Cambridge: Cambridge University Press, pp. 705–740, doi:10.1017/cbo9781107415324.019, retrieved 2022-12-05
  15. ^ Evans, J. R. G.; Stride, E. P. J.; Edirisinghe, M. J.; Andrews, D. J.; Simons, R. R. (2010-07-06). "Can oceanic foams limit global warming?". Climate Research. 42 (2): 155–160. Bibcode:2010ClRes..42..155E. doi:10.3354/cr00885. ISSN 0936-577X.
  16. ^ Barreras, F.; Amaveda, H.; Lozano, A. (June 2002). "Transient high-frequency ultrasonic water atomization". Experiments in Fluids. 33 (3): 405–413. Bibcode:2002ExFl...33..405B. doi:10.1007/s00348-002-0456-1. ISSN 0723-4864. S2CID 122323760.
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  18. ^ a b c Charlson, Robert J.; Lovelock, James E.; Andreae, Meinrat O.; Warren, Stephen G. (1987). "Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate". Nature. 326 (6114): 655–661. Bibcode:1987Natur.326..655C. doi:10.1038/326655a0. ISSN 1476-4687. S2CID 4321239.
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  20. ^ a b Lovelock, James (2007). The revenge of Gaia : why the Earth is fighting back - and how we can still save humanity. London: Penguin. ISBN 978-0-14-102597-1. OCLC 72867986.
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  22. ^ a b Arghavani, Somayeh; Rose, Clémence; Banson, Sandra; Planche, Céline; Sellegri, Karine (2021-03-04). "The Effect of using a New Parameterization of Nucleation in the WRF-Chem model on the Cluster Formation Rate and Particle Number Concentration in a Passive Volcanic Plume". Egu General Assembly Conference Abstracts. Bibcode:2021EGUGA..2312058A. doi:10.5194/egusphere-egu21-12058. S2CID 236762909.
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Further reading