Sources of Condensation Nuclei

Thornton, D. C., A. R. Bandy, B. W. Blomquist, D. D. Davis and R. W. Talbot (1996) Sulfur dioxide as a source of condensation nuclei in the upper troposphere of the Pacific Ocean. Journal of Geophysical Research 101, 1883-1890... 

Atmospheric water can be present as vapor, hqirid, or ice. The water vapor content of air can be expressed as humidity. Relative humidity, expressed as a percentage, describes the amount of water vapor in the air as a ratio of the maximum amount that the air can hold at that temperature. Air with a given relative humidity can undergo any of several processes to reach the saturation point at which water vapor condenses in the form of rain or snow. For this condensation to happen, air must be cooled below a temperature called the dew point, and condensation nuclei must be present. These nuclei are hygroscopic substances such as salts, sulfriric acid droplets, and some organic materials, including bacterial cells. Air pollution in some forms is an important source of condensation nuclei. 

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