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 The Ozone Hole Tour!

Centre for
Atmospheric Science


University of Cambridge

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Polar Stratospheric Clouds

Formation of Chlorine Radicals

Polar stratospheric clouds (PSCs) are the medium on which reservoir chlorine compounds are converted into ozone-destroying chlorine radicals.

Although the precise chemistry and details of PSCs are not fully understood, there are essentially two types of PSC: known as type 1 and type 2.

Type 1 PSCs are believed to be nitric acid and water mixtures which form just above the frost point (about 195 K). They can be eiter solid or liquid, depending on the conditions.

Type 2 PSCs are less common and are formed of water-ice crystals at lower temperatures (e.g. 188 K at 25 km altitude).

The normally inert reservoir chlorine compounds (ClONO2 and HCl) can react on the surfaces of the PSC ice particles:

HCl + ClONO2 -> HNO3 + Cl2
ClONO2 + H2O -> HNO3 + HOCl
The Nitric Acid (HNO3) remains in the PSC particles.

Denoxification

In addition, denoxification (removal of gaseous Nitrogen Oxides NOx) occurs on PSCs. NO2 is in gas-phase equilibrium with N2O5:
2 N2O5 <-> 4 NO2 + O2
N2O5 is removed from the gas-phase by the following reactions, catalysed by PSC particles:
N2O5 + H20 -> 2 HNO3
N2O5 + HCl -> ClNO2 + HNO3
The overall effect is a net removal of NO2. This is significant because ClO is an important catalyst in the destruction of Ozone, but is itself removed by the reaction:
ClO + NO2 + M -> ClONO2 + M
(where M is any air molecule)
Thus a decrease in the levels of NO2 helps maintain large levels of ozone-destroying ClO.

As the clouds grow, they begin to settle out of the stratosphere, taking the Nitric Acid with them. This removal of nitrogen compunds is termed ''denitrification''. It further encourages denoxification.

Formation of PSCs

We do not yet fully understand the mechanism for PSC freezing, and this remains one of the largest uncertainties in stratospheric ozone modelling.

The following figure shows results from a model of PSC growth, assuming that type 1 particles are frozen when they form. As the temperature decreases there is a sudden burst of type 1 PSC formation, followed by rapid growth as nitric acid and water vapours condense on the particles. This appears as a sharp rise and right turn in the figure at around 195 K. When the temperature falls to about 187 K, type 2 particles form on the larger of the type 1 particles. This appears as a sharp left turn in the figure, at around 187 K. Type 2 particles reach sufficient sizes to fall out of the stratosphere, remove nitric acid and water, and so prevent the deactivation of chlorine radicals back into reservoir compounds.

Model of growth of type 1 and 2 PSCs

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© 1998. Centre for Atmospheric Science, Cambridge University, UK. No text or graphics can be used or reproduced without explicit written permission. This version designed and maintained by Dr. Glenn Carver. Original concept and design Owen Garrett.