Polar Stratospheric Cloud
Background These clouds exist at very high
altitude (~70,000 ft) within Earth's stratosphere. Clouds do not
normally form in the stratosphere due to its extreme dryness. During winter at high latitudes, however,
stratospheric temperature sometimes becomes low enough to promote
formation of clouds. The colorful appearance of these clouds is
caused becasue they contain similar sized, small particles each of which
defracts sunlight in a similar manner.
The clouds often remain fully illuminated for about 20 minutes following
sunset at the ground because of their high altitude, resulting in a
spectacular appearance of the twilight sky. A good gauge of the
altitude of these clouds is their illumination pattern as the
sun rises or sets at the ground.
Called "Mother of Pearl" by Scandinavians due to their iridescent appearance, these clouds are composed of mixtures of naturally occurring water and nitric acid. Chemical reactions that occur on the surface of these clouds result in a remarkable transformation of stratospheric composition. Chlorine that is supplied to the stratosphere mainly from industrial sources is converted from relatively unreactive forms to other forms that are highly reactive with ozone, leading to substantial ozone depletion. It is a great irony that these beautiful clouds participate in a chain of events that results in the depletion of stratospheric ozone by man-made chlorine.
A compelling account of the relationship between polar stratospheric clouds and ozone depletion is provided in an article entitled "The Hole Story", written by Gabrielle Walker, that appeared in the 25 March 2000 issue of New Scientist. A scientific review of these issues is given in an article entitled "Stratospheric Ozone Depletion: A Review of Concepts and History" written by Susan Solomon, that appeared in Volume 37 of the journal Reviews of Geophysics.
JPL Research Activities
An image of a polar stratospheric cloud has been chosen for our front page because many atmospheric chemistry research projects at JPL are focused on understanding the properties of these clouds and the consequences of their existence. For example:
• laboratory kineticists measure the rate of reactions on the surface of these clouds that lead ultimately to production of highly reactive ClO (chlorine monoxide) and they also measure the rate at which ClO depletes ozone.
• laboratory spectroscopists measure fundamental physical properties of ozone and various other molecules that enable the concentration of these gases to be measured in polar regions by in situ and remote instruments such as ALIAS, ALIAS II, ATMOS, MkIV, MLS, and SLS.
• numerous field instruments observe the chemical transformations that take place (e.g., chlorine transformations, denitrification and dehydration, and ozone depletion) when air is exposed to polar stratospheric clouds and investigate the physical properties of the clouds.
• the modeling and analysis program quantifies the impact on ozone of enhanced concentrations of ClO that result from exposure of air to polar stratospheric clouds.
It must be noted that atmospheric chemists at JPL also study changes to ozone in other regions of the stratosphere, and in the troposphere, that do not involve polar stratospheric clouds.
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Author: Ross J. Salawitch
Page Design: Aaron B. Milam