Aerosols generally range in size up to 10µm. In clouds, when water vapor condenses, it generally condenses upon aerosols that act as cloud condensation nuclei (CCN). Aerosols that range in size from .1µm to 10µm in diameter are effective as CCN. Cloud droplets range in diameter from about 10µm to 100µm. When droplets grow to 100µm, they are considered rain drops. Rain drops can be as big as 6 millimeters in diameter.

CCN are abundant in the atmosphere and range from a few 10s per cubic centimeter over oceans and rural areas to tens of thousands per cubic centimeter in heavily urbanized areas. CCN are comprised of many natural and human made particles. They include dust, pollen, silicates (from soil), smoke particles and sea salt which is distributed into the atmosphere by oceanic wave action. Ice nuclei, those particles on which, ice deposition, rather than condensation occurs are much rarer than CCN. Ice nuclei become active as the temperature gets colder. It is estimated that the concentration of ice nuclei is only about one per liter at –20ºC, and less at temperatures warmer than that. A major type of natural ice nuclei is kaolin, or tiny particle of clay. Artificial ice nuclei include silver iodide (AgI) which works at a temperature of –4ºC and colder.

Man-made aerosols result from carbon being emitted into the air by combustion processes. These combustion processes also emit gases such as sulfur dioxide and oxides of nitrogen (SO₂ and NO_x) . These substances further react in the air resulting in liquid and solid aerosols that can change the character of clouds. With more condensation nuclei, the moisture available in a cloud is spread over more droplets. As a result, the droplets are smaller, the reflectivity of the cloud is increased, and precipitation may be delayed or reduced. Areas downstream from where the aerosols are produced can experience acid rain. See http://www.epa.gov/airmarkets/acidrain/#what.

Increased aerosols also affect the visibility in clouds. In the past we have seen how human produced aerosols affected visibility in fog. (Fog is simply cloud attached to Earth’s surface.) Before clean air acts were passed in the 1960s and 1970s, there were many occasions with fog that was characterized by very low visibility. In London, a smoke-enhanced fog persisted from 5 through 9 December, 1952. Chimney smoke from residences was identified as a prime cause for the severity of this event. Many people died because of the hazardous aerosols contained in the fog. Things are better today. Although conditions still have much room for improvement, we have seen visibility greatly improve in the industrialized world over the past few decades.

For more on the Great London Fog see:
http://www.met-office.gov.uk/education/historic/smog.html

URLs: Aerosols and Clouds

Why cloud and aerosol research is important.
http://www.etl.noaa.gov/et2/research/clouds/cloud.html
http://climate.gsfc.nasa.gov/tropo_aerosols.html

Background on clouds, chemistry and climate. Comparison to other planets is included.
http://astrobiology.arc.nasa.gov/palebluedot/abstracts/toon.html

Atmospheric research from Seinfeld Research Group.
http://www.che.caltech.edu/faculty/jhs/research.html

Background on volcanic aerosols, nice diagram.
http://www-sage3.larc.nasa.gov/solar/learning-aerosol.html

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