Condensation nucleus

In order for water vapor in the air to condense and become water droplets (cloud particles), small particles are needed to act as nuclei, and aerosols (fine suspended particles, aerosols, also called aerosols) that serve this role are called condensation nuclei. For condensation to occur, the air must be supersaturated with water vapor, and the higher the degree of supersaturation, the smaller the particles can become condensation nuclei. Aerosol concentration measuring devices that apply this principle give a very high degree of supersaturation, so most aerosols, including very small particles, are counted as water droplets. It is customary to call particles measured by this type of measuring device condensation nuclei, but the degree of supersaturation in the actual atmosphere is very low, at only about 1%, so the particles that act as condensation nuclei when natural fog and cloud particles are born are limited to particles with a radius of about 0.1 micrometers or more and hygroscopic. These particles are specifically called cloud nuclei among condensation nuclei. Condensation nuclei with a radius of less than about 0.1 micrometers are called Aitken nuclei. The most important cloud nuclei are sea salt particles and ammonium sulfate particles. Sea salt particles are tiny droplets of seawater that are released into the atmosphere by the bursting of bubbles on the ocean's surface, and then evaporate, leaving only the salt, which then floats in the atmosphere. Ammonium sulfate particles are born from gases that contain sulfur in the atmosphere. Some are naturally occurring, but they are attracting the most attention as a representative type of pollution particle caused by human activities.

[Hiroo Misaki]

"Airborne Particulate Matter," edited by the National Research Council and translated by Wada Osamu et al. (1986, Tokyo Kagaku Dojin)" ▽ "The Physics of Clouds -- From Cloud Droplet Formation to Cloud Movement," written by Takahashi Takashi (1987, Tokyodo Publishing)" ▽ "Measurement and Evaluation of Practical Aerosols," edited by Homma Katsunori (1990, Gihodo Publishing)" ▽ "The Fundamentals of Aerosol Science," edited by the Japanese Society for Aerosol Research and written by Takahashi Kanji (2003, Morikita Publishing)" ▽ "Fine Particles Changing the Climate -- Another Perspective on the Atmospheric Environment," written by Misaki Masaro (Chuko Shinsho)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

空気中の水蒸気が凝結して微水滴（雲粒）となるためには核となる微粒子が必要で、この役割を果たすエーロゾル（浮遊微粒子、煙霧質。エアロゾルともいう）を凝結核という。凝結がおこるためには、また空気が水蒸気の過飽和状態になければならないが、過飽和度が高いほど小さな粒子でも凝結核となりうる。この原理を応用したエーロゾルの濃度測定装置ではきわめて高い過飽和度を与えるので、極微小粒子も含めてほとんどのエーロゾルが微水滴となって計数される。この種の測定装置で計測される粒子を凝結核とよぶ習わしとなっているが、実際の大気中でおこる過飽和度はきわめて低く、たかだか1％程度にすぎないので、自然の霧粒や雲粒が生まれるときに凝結核として働く粒子は、粒子の半径がおよそ0.1マイクロメートル以上で吸湿性のものに限られる。それらの粒子を凝結核のうちでもとくに雲核とよぶ。これに対して半径およそ0.1マイクロメートルより小さい凝結核をエートケン核とよぶ。雲核のなかで重要なのは海塩粒子と硫酸アンモニウム粒子である。海塩粒子は、海面の泡の破裂によって大気中に飛び出した微細な海水滴が蒸発し、塩分のみが残って大気中に浮遊したものである。硫酸アンモニウム粒子は大気中で硫黄(いおう)成分を含むガスから生まれる。自然に発生するものもあるが、人間活動に起因して発生する汚染粒子の代表的なものとしてもっとも注目されている。

［三崎方郎］

『ナショナル・リサーチ・カウンシル編、和田攻ほか訳『気中粒子状物質』（1986・東京化学同人）』▽『高橋劭著『雲の物理――雲粒形成から雲運動まで』（1987・東京堂出版）』▽『本間克典編著『実用エアロゾルの計測と評価』（1990・技報堂出版）』▽『日本エアロゾル学会編、高橋幹二著『エアロゾル学の基礎』（2003・森北出版）』▽『三崎方郎著『微粒子が気候を変える――大気環境へのもう一つの視点』（中公新書）』

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