The term refers principally to the chemical processes that determine the concentration of ozone in the stratosphere. It also refers to the reactions that oxidise and decompose trace substances released into the atmosphere from natural and anthropogenic sources. If these are stable enough to escape oxidation in the troposphere, they find their way across the tropopause into the stratosphere and are oxidised there. See also Tropospheric Chemistry.

The stratosphere
Here we are concerned with the lower part of the stratosphere, from a height of about 15 km at the tropopause to the stratopause, about 50 km above the earth's surface. In the lower stratosphere the temperature increases with height from ca. -50 ºC at the tropopause to ca. 0 ºC at the stratopause. From a meteorological viewpoint, the region is very stable with little vertical mixing. The pressure falls from ca. 0.2 atm at the tropopause to ca. 10-3 atm at the stratopause.

Ozone in the stratosphere
The lower stratosphere is relatively rich in ozone with a maximum volume ratio of ca. 8000 ppb at a height of ca. 40 km. The total column density of ozone is ca. 300 Dobson Units, which corresponds to a layer of ozone which would be 3 mm thick, if it was at one atmosphere pressure and 0 ºC.

The principal concern in stratospheric chemistry is with the reactions that have previously served to maintain the average concentration of ozone at a roughly constant level. The great interest is why there is presently a general decrease of ozone in the stratosphere of about 1% per year, as well as a startling reduction over Antarctica in the spring.

The basic mechanism for ozone formation and removal was proposed by Chapman some 60 years ago. Oxygen is photolysed by short wavelength ultraviolet light to form oxygen atoms that further react to give ozone:

When measurements of stratospheric ozone concentrations were made, it was found that the Chapman scheme over-estimated ozone concentrations by a factor of about five. It became clear that there are further reactions that remove ozone. The concentrations of other trace substances in the stratosphere are too low (< ca. 1 ppb) to remove ozone in single step reactions, so the additional the processes have to be catalytic chain reactions.

The nitrogen oxides, NO and NO2, provide an example. These are formed from nitrous oxide, N2O, that is produced at the earth's surface and diffuses slowly up through the troposphere and across the tropopause into the stratosphere. Nitric oxide (NO) reacts with ozone forming NO2 that in turn can react with atomic oxygen to re-form NO.

Two further important examples of catalytic pairs of reactions that remove odd oxygen, are the OH/HO2 cycle:

When all the reactions are taken into account, the predicted concentrations of ozone agree well with those found experimentally.

The reduction in stratospheric ozone; the effect of CFCs and stratospheric aircraft.
The current decrease in the concentration of stratospheric ozone is attributed to the increased concentration of active chlorine (Cl and ClO) released into the stratosphere by the photolysis of chlorofluorocarbons, CFCs. The physical properties of the CFCs, combined with their chemical inertness, make them ideal for use in refrigerators and air conditioners, and also as drivers for making polymer foams. However they are also inert to reaction with the hydroxyl radical in the troposphere and thus are not oxidised, as most pollutants are, but simply accumulate in the air. They gradually make their way up through the tropopause into the stratosphere where they are photolysed with short wavelength ultraviolet light, producing chlorine atoms. With trichlorofluoromethane, CFC-11, for example:

The concentration of stratospheric ozone is likely to continue to decrease. Despite the reductions in the manufacture of CFCs, most of the CFCs produced to date are still in the troposphere. It will take decades for them to diffuse into the stratosphere where they will ultimately be decomposed, releasing chlorine in the process and enhancing the destruction of ozone. Only when the atmospheric concentration of CFCs decreases can one expect the ozone concentration in the stratosphere to recover.

The hydrochlorofluorocarbons, HCFCs, such as CF3CH2F (HCFC-134a), now being introduced as substitutes for the CFCs, have hydrogen atoms in the molecule that open them to attack by OH and degradation in the troposphere. They should not therefore affect the level of ozone in the stratosphere so severely, but they and their products may pose problems as greenhouse gases in the atmosphere.

The concern about removal of ozone in the stratosphere originated from plans in the late nineteen sixties for large fleets of supersonic aircraft that would normally fly in the stratosphere. These aircraft would have emitted large quantities of nitrogen oxides and water vapour at a height where their stratospheric concentrations are very low. The two ozone removal cycles involving NO/NO2 and OH/HO2 would have been further activated and large amounts of ozone would have been destroyed. The aircraft numbers at that time were over-estimated and no problem arose. Recently the idea has been revived and once again there is concern that damage to the ozone layer will result.

The Antarctic ozone hole: complications with surface chemistry
The large decrease in the stratospheric ozone concentration discovered over the Antarctic continent, which occurs as the light returns in the spring, was surprising in many ways. It demonstrated particularly that the models of stratospheric chemistry, which are presented in a much simplified form above, were incomplete; there is no way that the reactions so far described could account for the sudden decrease observed in the Antarctic spring.

The probable explanation of the phenomenon involves two particular meteorological factors. First, a large circumpolar vortex of westerly winds forms over Antarctica in the winter and largely isolates the stratosphere over the pole from that over the rest of the earth. The isolation allows time for the air to be processed and prevents the ingress of warmer ozone containing air from the north. Second, the extremely low stratospheric temperatures allow polar stratospheric clouds (PSCs) to form during winter. Normally there are no clouds in the stratosphere because water vapour concentrations are too low. PSCs are thought to be formed from crystals of hydrated nitric acid which does condense at the very low prevailing temperatures.

It is conjectured that during the winter, reservoir compounds such as ClONO2 and HCl can be absorbed on the surface of the crystals in the PSCs. There they react together and form molecular chlorine.

As the season progresses, the temperatures rise, the formation of the dimer is inhibited and the vortex breaks up allowing an influx of air richer in ozone; the concentration of stratospheric ozone is gradually restored.

The sharp decrease in ozone concentrations each spring over Antarctica can be expected to intensify with the years as the CFCs, presently in the troposphere, diffuse into the stratosphere. There is now some evidence that a similar decrease may occur over the Arctic in the northern spring.

The discovery of the ozone hole demonstrated to the scientific community that, despite the advances in understanding and the sophistication of experiments and models, the atmosphere still has the capacity to spring surprises. To those responsible for environmental policy, it was a warning that the uncontrolled release of pollutants into the atmosphere, combined with the inherent complexity of the chemistry, can lead to unforeseen effects that are extremely difficult to reverse.

For an excellent account of stratospheric chemistry see: Wayne, R. P. (1991) Chemistry of Atmospheres , Clarendon Press, Oxford, chapter 4.

P.B.