The atmosphere modifies solar energy as it passes through by acting as a filter, reducing certain types of incoming radiation by absorption. One of the most important filtering effects for life on earth is the reduction of ultraviolet radiation (UV): this is radiation of shorter wavelength than the visible spectrum. Low levels of UV are useful to life: it is used in Vitamin D synthesis, for example. However, high levels of UV are harmful, causing skin cancers, genetic damage and climate change.
UV radiation can be subdivided into three bands:
UVC is entirely filtered out by the atmosphere, and of the UV that reaches sea-level, organisms are most sensitive to UVB.
The most important component of the atmosphere for filtering UV is ozone (O3). Ozone is bluish in colour, and has strong, pungent smell (its name comes from the Greek word ozein: to smell). We have already encountered ozone as a component of photochemical smog. 90% of naturally occurring atmospheric ozone occurs in the stratosphere between 10-50 km, with maximum concentrations at 20-25 km. - it is this part of the atmosphere that is referred to as the Ozone layer. Stratospheric ozone is very important: it removes almost all incoming UVC, and 70% of the incoming UVB at equator, 90% at poles. It is processes of UV absorption by ozone that gives the stratosphere its characteristic thermal structure: in the stratosphere, temperature increases upward (due to energy acquired during UV absorption), unlike the lower part of the atmosphere (troposphere) in which temperature usually decreases upwards. The thermal structure of the stratosphere is very stable, and it acts as a lid on the lower atmosphee, limiting the size of thunderstorms and other weather systems. The ozone layer is very diffuse. Atmospheric ozone is measured in Dobson Units (DU). 100 DU is equivalent to a 1mm thick layer of pure ozone at surface pressure (1000 mbar). The average amount of ozone in the atmosphere is about 300 DU.
Ozone levels fluctuate on a daily basis (30%) and multi-annually (10%). In general though, a natural dynamic equilibrium exists between processes of creation and destruction. In recent years, this equilibrium has been disrupted by human activities, resulting in widespread destruction of ozone.
Ozone is formed by the impact of UV on the common form of oxygen (diatomic oxygen: O2). The energy of the radiation causes O2 to split into single O atoms. These free oxygen atoms then combine with O2 to form O3. This reaction is reversable: O3 + O = 2 x O2; or O3 + UV = O + O2, each reaction being associated with the absorption of UV radiation of a specific wavelength. The ozone levels in the stratosphere reflect the relative rates of creation and destruction. In nature, the two rates are linked: high rates of O3 production also encourage high rates of destruction. The turnover is highest in equatorial areas, where the received solar radiation flux is at its maximum. Reactions involving ozone are accelerated by the presence of catalysts: substances that aid a reaction, but are themselves left unchanged. The same catalyst can cause very many reactions before being removed from the system by other processes. The main natural catalysts for ozone destruction are:
OH (known as the hydroxyl radical), derived from H2O (water), CH4 (methane) and H2 in the stratosphere. OH has aimportant effect, especially at altitudes greater than 40km, and accounts for 11% of natural ozone destruction.
NO (nitric oxide) which accounts for 50-70% of natural ozone destruction. Some nitrogen oxides are formed by the action of cosmic rays. Supernovas (expoloding stars) may have caused brief spikes of NO, reducing O3 by c. 90% for decades. Nitrogen dioxide (NO2) in lower stratosphere can encourage survival of ozone, by combining with chlorine monoxide (ClO), but higher in the stratosphere it is a precursor of nitric oxide, and contributes to ozone destruction.
C Chlorine is a very efficient catalyst. Each free Chlorine atom can destroy up to 100,000 ozone molecules before it is removed. At has been argued that significant amounts of chlorine monoxide (ClO) may enter the stratosphere in large volcanic eruptions. For example, an eruption of Mt Hudson, Chile, in August 1991 temporarily worsened the Antarctic ozone hole, and the eruption of Mt. Pinatubo, Phillipines, in June 1991, injected aerosols into lower stratosphere, had a temporary impact on ozone. However, most eruptions are not big enough to inject chlorine into the stratosphere. The chemistry of ozone destruction and creation involves very complex interactions between molecules: 100-200 interactions are included in some recent models.
Br Bromine is about 10 times more effective that chrorine in depleting ozone. Fortunately it is also less common. Natural sources are mainly in the form of methyl bromide (CH3Br), a by-product of biologic productivity in the oceans. Methyl bromide is also widely used as a fumigating agent for soil pests, nearly 15% of which is used in California for controlling agricultural pests.
The single most important point about catalysts is that minute amounts of trace gases can vastly accelerate ozone destruction. In nature, catalysts are in long-term balance with ozone cycle. Human modification has severely disrupted this balance by the input of anthropogenic catalysts, especially chlorine compounds.
Three potential sources of man made ozone destroyong compounds:
Nuclear explosions
The rapid release of energy in a nuclear explosion forms NOx from O2 and Nitrogen. The gases are carried into the stratosphere by the intense convection associated with the explosion. NOx destroys ozone, so atmospheric bomb tests should have had an effect. Most atmospheric testing was conducted 1945-1963, but some nations (e.g. France, China) continued until 1980s. The calculated effect is a temporary 3% reduction of ozone, but this value is within natural variations, and monitoring methods were crude at that time. The sunspot cycle should have resulted in an ozone peak in early 1960s, but this did not happen, suggesting that testing did have an effect.
Supersonic transport
All aircraft produce water, CO2, CO, NOx in exhaust gases. Most commercial flights are confined to the troposphere, but high-level supersonic aircraft inject pollutants into the stratosphere (c. 20km altitude). Of these nitrogen oxides are believed to be especially harmful. Theoretical fears about ozone destruction were used to argue against widespread adoption of plans to build fleets of supersonic passnager aircraft, and were partly responsible for such plans being adopted in the USA. Europe (Concorde) and USSR (Concordski) went ahead anyway, but the fleet remained small for commercial reasons. The Space Shuttle is also believed to harm stratospheric ozone, but is not used heavily enough to pose an important threat.
Halocarbons
These are artificial molecules made up of chlorine, fluorine, bromine and carbon, made by alteration of hydrocarbons by replacement of some atoms. They are called halocarbons because Chlorine, Fluorine and Bromine are all halogens, occupying the same column in the Periodic Table of elements. Examples of halocarbons are: CFC-11: C F Cl 3; CFC-12: C F2 Cl 2. These particular compounds are also known as CFCs, or Chlorofluorocarbons, composed of chlorine, fluorine, and carbon. CFCs were invented in the1920s by Thomas Midgely (the same scientist who first suggested lead as an additive to petrol). At sea-level, they are non-poisonous and non-flammable (Midgely inhaled gas and blew out candle as demonstration of this), and were thus an ideal repleacment for toxic compounds then in common use (SO2 was used as a refridgerant). Production rose as result of lifestyle changes in late 20th Century, with CFCs used in refridgeration and air conditioning; propellants for spray cans (paint to hair spray); Foaming agents for polystyrene foam (fast food containers, furniture, insulation).
The gases were released into the atmosphere during the manufacturing processes (foam), during use (spray cans), or at end of life of the product (e.g. fridge). Whereas CFCs are Non-toxic and Inert at sea-level temperatures and pressures, they remain in the atmosphere, and gradually rise into the stratosphere, where conditions of higher energy exist. Under heavy UV bombardment, the gases are no longer inert. CFCs are very susceptible to breakdown under UV radiation, releasing chlorine and bromine, which is a powerful catalyst in ozone destruction. Chlorine molecules are only neutralised when bonded with hydrogen (from hydrogen oxides or CH4).
World production of CFCs rose by 9% pa in 1960s and reached 700,000 tonnes in 1973 and 1.26 million tonnes in 1986. These gases will remain in the atmosphere for 40-150 yr, so their effects will be felt long after reductions in production.
In the early 1970s, several workers predicted that there would be a severe reduction of stratospheric ozone. The first concerns were voiced over supersonic aircraft, but in 1974 Mario Molina and Sherwood Rowland (Univ. California) first drew attention to the Chlorine connection. They estimated that the destruction was equal to that caused by all natural processes. Others (e.g. Crutzen) predicted lesser, but still serious effects. The issue caught media attention, which focused on aerosol sprays, which accounted for >70% of CFC production in 1975. Chemical companies (eg. DuPont in US) and spray-can manufacturers contested results, and claimed the burden of proof rested with scientists. They argued that a major industry should not be jeopardized on theoretical grounds alone (jobs and economy were held up as hostages). Industry thus advocated a wait and see approach, and an innocent until proven guilty principle. Environmentalists argued that the opposite principle should apply: do not use until proven safe. In the US, debate raged in the media, and in hearings of State and Federal legislature. A National Academy of Sciences report (1976) supported the case against CFCs. This was very significant: at that time, the US acounted for 50% of production. The level of concern and scientific reasoning was enough to ensure action in US (1978) and Canada (1980), but not in Europe. (UK scientists reached similar conclusions, but here, debate was more restrained.) No legislation was enacted, but voluntary reductions were put in place, partly due to consumer pressure (product boycotts). Concern had faded by the early 1980s, as there had been no noticeable effects. The situation changed dramatically in 1985 with the announcement of the discovery of the Antarctic ozone hole.
The major turning point in the ozone issue was the announcement of an Ôozone holeÕ over Antarctica by Joe Farman (British Antarctic Survey):
Farman, J., Gardiner, B., and Shanklin, J. 1985. Large losses of total Ozone in Antarctica reveal seasonal Clox/NOx interaction. Nature 315, 207.
Farmans results based on measurements from balloons, and were later given added force by NASA satellite observations using specially designed instruments, eg. TOMS: Total Ozone Mapping Spectrometer, on NASA Nimbus 7). The link between the ozone hole and CFCs was established in 1987, when a US-led project based at Punta Arenas in southern Chile found ClO in the lower stratosphere over Antarctica.
Antarctic ozone fluctuates naturally throughout the year: low in winter, high in summer. During the southern polar winter, the continent becomes very cold, and air sinks. The surrounding stratospheric air rotates clockwise in the Polar Vortex, isolating the cold air over Antarctica from the rest of the stratosphere. This prevents the flow of ozone from lower latitudes, where most is produced, so catalytic reactions deplete ozone. In spring (October-November) the vortex weakens, allowing the transfer of ozone from lower latitudes. The depth of the ozone hole reflects the timing of the breakdown of the vortex: Late breakdown equates with a deep hole, whereas early breakdown allows levels to recover earlier.
In recent years, the annual cycle has changed, with a ÔdeeperÕ hole, persisting for longer, and minimum levels have been lower. Severe ozone losses begin in August, persisting until December (cf. November in natural cycle). Ozone depletion is thus most intense in spring. The reason for this lies in polar stratospheric chemistry and physics. In mid winter, the polar core is cold enough (-80¡C) to form polar stratospheric clouds of frozen nitric acid (Type 1 PSCs) and water ice (Type 2 PSCs). Chemical reactions on the surface of ice crystals (heterogeneous reactions) release chlorine in the reactive form of Cl2 from non-ozone destroying molecules (Cl O N O2, H Cl). This free chlorine becomes activated when sunlight returns to the polar skies in the spring. The zone of ozone depletion extends over whole continent and beyond, and typically swirls around on a weekly basis, thus affecting different regions at different times. Depletion of ozone is most pronounced in the lower stratosphere (12-24km altitude).
Ozone depletion is not expected to be so obvious over the arctic, because the north polar regions are neither as cold nor as isolated as the Antarctic. Polar stratospheric clouds are less abundant, and the circumpolar vortex much weaker. This means that less chlorine should be released in the winter, and ozone transfer from lower latitudes can occur. Nevertheless, an ozone hole has been present since late 1980s, although it is weaker than that in the south. Examples: March 1992: 10-20% decreases in Arctic ozone. Losses are also recorded in the Northern hemisphere mid-latitudes of 2-7%. In March 1993, the ozone layer over Toronto (43¡) and Edmonton (53¡) was the thinnest on record. Recently, it has been realised that ice clouds formed in the lee of mountain ranges (e.g. Greenland, Scandinavia, northern Russia), can provide reaction surfaces allowing the activation of chlorine. Northern hemisphere ozone losses are more serious than those in the south due to much higher population densities.
Reference: Carslaw: 1998: Increased stratospheric ozone depletion due to mountain-induced atmospheric waves. Nature 391, 675-678.
Biological: High intensities of UVB are harmful to all life. Small fluctuations can be coped with, but large, and long-term increases are potentially very dangerous. High UVB causes cell damage (altering proteins) and genetic damage (DNA). Humans (especially pale skinned varieties) can suffer immediate effects (sunburn and eye damage); and long term effects (skin cancer). Most cancers are non-malignant and curable, but melanomas are malignant and usually fatal. The link between UVB and skin cancer is well established: studies have demonstrated latitudinal variation in cancers of Caucasians in the USA, and Australia has10 times more skin cancers than Europe. Recent increases are mainly due to increased holidaying in sunny areas, and current incidences are the result of cell damage caused 10-20 years ago. Average UVB receipts have increased 8% since 1980 in S. Australia: this cannot explain present cancers, but bodes ill for the future. The Australian and NZ governments have campaigns to discourage exposure, urging people to use sunscreen, hats, and to change their habits. The US EPA predicts 800,000 additional cancer deaths in next century. Eye damage and Cataracts are also potential problems. Land plants: About 200 plant species have been studied for UVB sensitivity, and half show significant adverse effects, including decreased photosynthesis, reduced leaf area, and restricted growth.
Marine ecosystems: Increased UVB may reduce photosynthesis in phytoplankton, with knock-on effects for food chains. Studies are underway in Antarctic waters.
Climatological: absorption of UVB by ozone reduces incoming shortwave (tending to reduce surface temps), but it also has an effect on the outgoing longwave (tending to increase surface temps). In balance, ozone depletion is thought to result in slight cooling, but is probably insignificant. CFCs, however, are significant greenhouse gases, as are their replacements.
Montreal Protocol: this was major legislation introduced in1987 under the UN Environmental Program. 31 countries signed an agreement to protect the stratospheric ozone layer. The US EPA proposed a 95% reduction by 2000 AD, but participants agreed to 50%. Also, 3rd World countries were allowed to increase CFC use to allow increased use of refridgerators, etc. Overall, the protocol amounted to only a 35% reduction by 2000. This was a historic agreement, but scientific evidence indicated that it was not enough. Helsinki (1989): 80 signatories agreed to complete CFC elimination by 2000. Production bans were the brought forward in 1992 (Copenhagen) to 1996 (CFCs) and 1994 (Halons). Manufacturers responded positively, reducing production and searching for alternatives. The substitutes were designed to break down rapidly in the troposphere: they still have a negative effect, but much less so than CFCs. The rapidity of agreements related to new, powerful scientific data, but also to new economic conditions created by consumer awareness and government funding. Funding from US, Europe and Japan (the main CFC users) was provided to pay for 3rd World uptake of alternative technologies, guaranteeing the participation of India and China.
On September 17, 1997, at the 9th Meeting of Montreal Protocol Parties participants agreed to an accelerated phase out of methyl bromide and proposed further amendments to the Montreal Protocol. Industrialized countries will now be subject to a phase out of methyl bromide by the year 2005, with exemptions for "critical uses," and interim deductions of 25% by the year 1999, 50% by the year 2001, and 70% by the year 2003. Developing countries that are covered by Article 5 of the treaty, have agreed to a 20% reduction in consumption by the year 2005 and phase out by the year 2015.
Internet Resources:
An excellent and well-illustrated site about ozone destruction:
http://www.atm.ch.cam.ac.uk/tour/
An archive of images of ozone levels over the Antarctic and Arctic (updated daily) can be found at:
http://jwocky.gsfc.nasa.gov/index.html
For information about halogens and other elements, check out this great site dedicated to the Periodic Table: