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E. Linacre and B. Geerts |
4/'00 |
Ozone has four effects - in high concentrations it is harmful to animal and plant life, it helps destroy other pollutants by reacting with them, it acts as a greenhouse gas, and (last but not least) it helps shield life on Earth from the Sun’s ultra-violet radiation.
stratospheric ozone
The creation of stratospheric ozone is chiefly due to what is called the ‘Chapman cycle’ (Section 1.4), which represents the production and subsequent annihilation of ozone by solar UV radiation (1). Destruction of ozone also results from catalytic reactions with hydrogen atoms, hydroxyl radicals (symbolised by HO-, derived from water H2O), nitric oxide (NO, from the exhausts of high-altitude aircraft), and so-called ‘halogens’, notably chlorine and bromine compounds (Note 1.E). A ‘catalytic’ reaction allows a single molecule of the triggering chemical to repeatedly react with the source chemical until its reservoir becomes depleted. However, there are complications, such as the inhibition of the destructive effects of both ClO (a compound of chlorine and oxygen) and NO2 (from NO) by their forming chlorine nitrate (ClONO2) instead. Chlorine nitrate does not destroy ozone, but it has the potential to destroy ozone since it is a reservoir of ClO.
There is still much to be learnt about the atmospheric chemistry of high-level ozone. The process is complicated by the presence of non-gaseous particles such as ice crystals, especially in areas where the temperature drops below -80°C, as occurs in Antarctica’s lower stratosphere in winter. Chemical reactions in an inhomogenous environment (i.e. at the surface of the ice) behave very differently. On an ice lattice chlorine readily combines with ozone to form oxygen. It is now widely accepted that the springtime Antarctic ozone hole is a byproduct of man-made emissions of halogens such as CFCs. When in ‘91-‘92 Mount Pinatubo produced a veil of dust in the tropical lower stratosphere, ozone concentrations were reduced there.
Why does the Antarctic ozone hole occur only in spring? The catalytic reactions with halogens involve an initial photochemical breakdown of compounds such as CFCs, which is not possible in winter when polar sunshine is inadequate. On the other hand, the minute ice crystals of very thin stratospheric clouds disappear in the relative warmth of summertime. Both conditions for ozone breakdown occur at the same time only in spring, before the ice crystals have gone, but when solar radiation is already enough for photochemistry.
The production of chlorine-based chemicals now is decreasing as an outcome of the 1988 Montreal Protocol, and lower stratospheric concentrations of these (generally long-lived) chemicals should peak around the turn of the century, so that it is expected that the ozone hole over the South Pole will no longer occur after the middle of the next century. Chlorine levels are already falling in the lower atmosphere.
Since ozone absorbs both upwelling terrestrial radiation at 10 mm (the main atmospheric window) and infrared solar radiation (through the Chapman cycle), the ozone layer is heated, mainly at lower latitudes and in the summer hemisphere. This generates a very slow, thermally direct circulation, known as the Brewer circulation within the stratosphere. This consist of the ascent of lower-stratosphere air over the equator, followed by flows poleward, and then subsidence. At the same time, there is a single circulation cell from the summer to the winter hemisphere, and clearly this circulation reverses direction every six months. Strong circumpolar winds especially around Antarctica in winter prevent this meridional circulation to extend towards the South Pole.
Ozone amounts over Antarctica can now be measured by a Total Ozone Mapping Spectrometer (TOMS), which currently is aboard two polar-orbiting sattelites (2). The first TOMS on a satellite was on NASA’s Nimbus - 7, from 1978 - 1993.
The thickness of the ozone is expressed in ‘Dobson units’. 100 Dobson units (DU) is equivalent to a gaseous layer of pure ozone with a thickness of 1 mm at sea level. The global average column ozone concentration is about 300 DU, and within the ozone hole the concentration frequently dips below 100 DU. The average size of the Antarctic hole in 1996 was 21 million km2, as in the previous four years, i.e. about the area of North America. Over the Arctic, a much weaker hole has recently been noted in March/April. The hole is asymmetric and it extends over N. Canada and Siberia, in accordance with the circumpolar vortex. The lowest concentrations ever observed are near 200 DU.
Stratospheric ozone has decreased globally by about 3% over the past 20 years, in Antarctica by 50% over 25 years, and in Melbourne (Australia) by 8% over 40 years or so (3). It is not clear why stratospheric ozone concentrations are declining globally. The concentrations vary from day to day, and they reflect the movement and evolution of frontal systems.
An increase in atmospheric greenhouse gas concentrations not only explains surface warming but also cooling in the lower stratosphere. Such theoretical cooling has been verified using radiosonde and satellite data. Greenhouse gases inhibit longwave radiation upwelling from the ground from maintaining temperatures there. This extra coldness may prolong the duration of stratospheric clouds. These remove nitrogen oxides that normally deactivate chlorine monoxide molecules, which are implicated in catalytic reactions destroying ozone at those levels (4). In other words, the lower stratospheric cooling due to climate change fosters ozone destruction, perpetuating the springtime ozone hole over the poles.
tropospheric ozone
There is insufficient UV near the surface for the Chapman cycle to operate, so tropospheric ozone arises in different ways:
Lightning-induced ozone is produced in the same way as in the stratosphere: the high-energy radiation from a lightning stroke dissociates oxygen (and water vapor), and the resulting radicals (O and HO-) quickly combine with oxygen to produce ozone.
Pollution-related ozone results from reactions involving oxides of nitrogen (NO and NO2), carbon monoxide, organic compounds of the kind found in car exhausts, water vapour and hydroxyl radicals (HO-). Especially important is the photochemical decomposition of NO2. Then ozone itself photodissociates to produce energetic oxygen atoms that in turn produce HO- radicals following reaction with water vapour. If organic molecules are present, these radicals react to form HO2 and other organic-peroxy radicals, which are able to react with NO to generate NO2 for further production of ozone (1). The nitrogen oxides and the organic compounds (or carbon monoxide) are called ‘precursors’ of ozone formation. Some organic precursors, such as isoprene, arise naturally, notably over forests. The amount of tropospheric ozone is also influenced by reactions within clouds. Concentrations tend to be higher in summer, when there is more solar energy, and within or downwind of large cities, where more precurors are emitted. In areas experiencing frequent summertime thunderstorms, the manmade contribution (industrial/transport activities) can not be readily discriminated from the natural ozone production. Tropospheric ozone measurements at Cape Grim at the northwest tip of Tasmania in 1990 showed a maximum of 28 parts per billion in July and a minimum 16 parts in January. By contrast, measurements of light quality in 1860 at Hobart (also in Tasmania) suggest little variation of ozone during that year (5).
References
(1) Hales, J. 1996. Scientific background for AMS policy statement on atmospheric ozone. Bull. Amer. Meteor. Soc., 77, 1249-53.
(2) Anon 1996. 1996 Antarctic ozone hole below record average size. Bull. Amer. Meteor. Soc., 77, 2979-80.
(3) Randel, W. 1997. Comment at a public forum in Melbourne, 16/5/97.
(4) Hecht, J. 1999. Polar alert. New Scientist, 162, 12 July, p.6.
(5) Smith, I. 1997. Melbourne Centre forum on ozone. Aust. Meteor. Ocean. Soc. Bulletin, 9, 49-50.
