Ozone's Problem
with Polar Stratospheric Clouds

Using a GIF animation, this page describes in detail the processes by which ozone is destroyed over Antarctica by polar stratospheric clouds during the austral spring.

Introduction

The Antarctic ozone hole is produced every spring over the earth's south pole because of the special conditions that are present in the stratosphere over Antarctica and the presence of completely anthropogenic chemicals spilled into the environment called chlorofluorocarbons (CFCs).

Unique Meteorology and Polar Stratospheric Clouds

One of the major factors producing the special chemistry that occurs in the stratosphere there is the cold temperatures present during the Antarctic winter. The air in the stratosphere is completely in the dark during the austral winter. Antarctica is also totally surrounded by water (as contrasted with the north pole which has landed geographic features on many sides around it) and therefore meteorologically isolated from air at higher latitudes during the winter. This physical condition forms an isolated air mass swirling over Antarctica called the south polar vortex. In the air of the polar vortex, the temperatures drop to very low levels in the stratosphere, below 80 degrees below zero C. At these temperatures, chemicals present in the stratosphere freeze out and form polar stratospheric clouds (PSCs). It is the chemical reactions that occur on the PSCs that result in the large decrease in ozone during each austral spring over Antarctica that we called the Antarctic ozone hole.

Chlorofluorocarbons

The relative chemical inertness of CFCs is one of the major reasons for they were used in air conditioners, refrigerators, foam blowing agents, and as solvents for cleaning circuit boards. Medical inhalers for asthmatics still use chlorofluorocarbons as an inert carrier for the method of aerosol dispersion necessary for the quick dispensing of medicine to people who need the drugs quickly and effectively sprayed into their lungs. This is one of the few allowed exemptions for CFCs.

CFCs TROPOSPHERIC inertness is based on the fact that there are few gas phase tropospheric components that will destructively react with them in the layer of the atmosphere closest to the earth. Furthermore the wavelengths of light-that pass through the gases in the upper atmosphere and get down to the troposphere-are not short enough (read energetic enough) to photolytically decompose chlorofluorocarbons.

This chemical inertness means that chlorofluorocarbons have long atmospheric lifetimes and become well-mixed throughout the troposphere. James Lovelock, the inventor of the most sensitive analytical detector for CFCs-the electron capture detector sailed into the southern Pacific ocean (1971-72) on the RRS Shackleton-far away from the majority of CFC polluting sources in the northern hemisphere (mainly the United States and Western Europe). In his travels through the marine troposphere he detected CFCs all along the route using his new detector, thereby proving that CFCs that had been spilled/leaked/released in the northern hemisphere had evaporated and diffused throughout the well-mixed troposphere. In 1989 the tropospheric organic chlorine concentration was about 3.9 ppbv throughout the troposphere. Methyl chloride (monochloromethane), the only major naturally (biologically) produce organochlorine molecule made up about 0.6 ppbv of that total; the rest is almost all anthropogenic.

My Radio Told Me that CFCs Can't Be an Atmospheric Problem!

CFCs not only spread throughout the troposphere, they also diffuse into the stratosphere. The evidence for the presence of CFCs in the stratosphere is only argued against by people who have a vested interest

a) in suggesting that industrial chemicals are not near as damaging as the peer-reviewed scientific evidence suggests because "industry is the engine for the free enterprise system and what THAT produces is inherently good"; or
b) in suggesting that scientists are dishonest leaches sucking on the government teat and they have fabricated the evidence for the presence of CFCs in the atmosphere so that they can increase the amount of government grant money that would subsequently be applied to a "mythical problem"; or
c) in maintaining the ear of people who trumpet criticisms of the scientific process, the scientific process that has proven that CFCs are a very important component in the anthropogenic changes to the atmosphere.

This final reason is particularly insidious because it is based not on logical reasoning but instead on efforts to try to increase the profits (of, for instance radio stations and syndication networks) that pay nay-sayers salaries. Scientists should speak out against popular press indictments of well carried out scientific investigations and expose the process by which myth, superstition, and AM radio profits are made.

Arguing against the presence of CFCs in the stratosphere is a waste of time: CFCs have been detected in the stratosphere, their chlorine containing decomposition products have also been detected there, and their fluorine containing decomposition products have been detected in the stratosphere. Period. The data are overwhelming.

Arguing about regulating CFCs, however, though that regulation IS presently under way, may be worth the time.

The Smoking Gun

The presence of CFCs doesn't itself mean that they are responsible for ozone destruction. Instead following the experimental evidence of Rowland and Molina in the early 1970s, the smoking gun was the detection of the photolysis products of CFC destruction in the stratosphere. That smoking gun is chlorine monoxide (and the fluorine containing decomposition products such as COF₂ and COFCl).

Photolysis of Chlorofluorocarbons

CFCs are decomposed by high energy wavelengths of light in the upper stratosphere after they have diffused there from the troposphere. The first chlorine radical-freeing light reaction for CFC-12 (CF₂Cl₂) can be written: CF₂Cl₂ + hv (< about 260 nm) ----> Cl + CF₂Cl

The stratospheric photolysis reactions ultimately lead to the complete destruction of CFCs and the release of all the chlorine atoms the CFC contains. Now the stage is set: Free chlorine atoms are being released into the stratosphere from anthropogenically produced CFCs. Those CFC are ultimately decomposed to release free chlorine atoms. And the free chlorine atoms catalytically destroy ozone in the following way:

Catalytic Destruction of Ozone

Ozone (O₃) reacts with a chlorine atom (which IS a chlorine radical) to produce chlorine monoxide and molecular oxygen, O₂. Since atomic oxygen (O°) is also present from the natural ozone creation cycle called the Chapman Mechanism (QuickTime movie here--923 kb), chlorine monoxide reacts with atomic oxygen to re-produce atomic chlorine and molecule oxygen. In this catalytic cycle two odd-oxygen species (O₃ and O) are removed and the chlorine radical is recycled to attack another ozone molecule. The total ozone destruction cycle looks like this:

O₃ + Cl ----> ClO + O₂
ClO + O ---> Cl + O₂

And here's a QuickTime Movie of that process courtesy of NASA (1 MB).

Active Chlorine and Reservoir Species

Now if that were all there was to the stratospheric chemistry of ozone and chlorine ALL the natural ozone layer would be destroyed. And even though the stratospheric chlorine levels are approximately 6 times their natural levels there are small natural levels of chlorine in the stratosphere (for instance from biological (monochloromethane) and a small amount from volcanic sources); therefore there must be some natural means of removing chlorine atoms from the stratosphere. And, there are. The presence of methane (CH₄) means that chlorine radicals can abstract a hydrogen from methane and form HCl, hydrogen chloride. This a strong (completely dissociate) acid in aqueous solution, but in the dry gas phase of the stratosphere hydrogen chloride exists as a stable, undissociated molecule and is a stable "chlorine radical catcher," or reservoir species.

Two other reactions that can form HCl are hydroxyl radical plus chlorine monoxide or hydroperoxy radical plus chlorine radical:

OH + ClO ---> HCl + O₂
HO₂ + Cl ----> HCl + O₂ Another reservoir species is ClONO₂, somewhat misnamed as chlorine nitrate. This stable molecule is the product of a reaction between nitrogen dioxide and chlorine monoxide, and like hydrogen chloride, acts to sequester active chlorine radicals from the ozone destruction cycle: ClO + NO₂ ---> ClONO₂

Polar Stratospheric Clouds

OK, so here's where polar stratospheric clouds come in:

PSCs are made up of nitric acid and water crystals (most prominent for the clouds forming at about -70 degrees C) mixed with more water containing crystals (which form at temperatures below - 80 degrees). These clouds have two negative effects on ozone:

they sequester oxides of nitrogen (as nitric acid) which could react with chlorine monoxide to form the chlorine nitrate reservoir species (see immediately above), and
they act as an active site on which a very slow gas phase reaction can go very quickly heterogeneously (gases reacting on solid surfaces). This reaction can be viewed as:

HCl + ClONO₂ ---- on ice ----> Cl₂ (gas) + HNO₃(ice)

This reaction releases molecular chlorine into the stratospheric gas phase during the austral winter and thereby helps to remove chlorine reservoir species. When the sun starts to shine on the polar stratosphere at the beginning of austral spring, this chlorine gas is photolyzed back to chlorine radicals which can THEN enter into the catalytic destruction of ozone. The result is that at the beginning of the austral spring (~ October over Antarctica) the ozone in the stratosphere is depleted to less than 20% of its winter levels.

A Humble Attempt at Animating the PSC-Catalyzed Destruction of O₃

The Quicktime movie that describes in detail the processes by which ozone is destroyed over Antarctica by polar stratospheric clouds during the austral spring was created especially for the students of CHM442/ESC440 during the spring of 1997.

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Ozone's Problem with Polar Stratospheric Clouds