13 Chemistry of Stratospheric Ozone Depletion

Prof. K.S. Gupta

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Contents

  1. Introduction
  2. Why is ozone necessary in stratosphere?
  3. Formation of ozone in stratosphere
  4. Stratospheric ozone depletio
  5. Role of NO, NO2, N2O
  6. Role of chlorine compounds
  7. Role of hydrogen species
  8. Antarctic ozone hole
  9. Stratospheric ozone depletion over the Arctic
  10. Ozone depletion and climate change
  11. Montreal Protocol
  12. References

Introduction

   The ozone layer exists at altitudes between about 10–50 km from Earth surface that is between 100 to 0.1 mb pressure altitude just above the tropopause. The natural seasonal and latitudinal variation in stratospheric ozone is now well established. The ozone distribution in stratosphere is strongly dependent upon stratospheric circulation patterns, which vary according to seasons, short-term meteorological changes and photochemical processes of formation and destruction. Since last several decades, stratospheric ozone as a tracer for atmospheric circulation has been the subject of the study by physicists. In 1984, John Farman and his team belonging to British Antarctic Survey and engaged in measuring routinely the stratospheric ozone over Antarctic stations at Halley Bay (76° S 27°W) and at Argentine Islands (65°S 64°W) since 1956 made a startling statement that value of total ozone had decreased dramatically for about six weeks in September-October (1980-84). The press coined the catchy phrase ozone hole to describe this unusual phenomenon.

 

Why is ozone necessary in stratosphere?

   The presence of ozone in stratosphere is essential because it absorbs the UV-B radiation ( = 280–320 nm). UV-B radiation is absorbed only by O3 and by no other gas. It may be pointed that the UV-A ( 320 –400 nm) radiation are not absorbed by O3. On the other hand UV-C ( 200–280 nm) radiation is absorbed by other gases as well as by O3. Ozone also controls temperature profile of stratosphere. Any loss or depletion of ozone in stratosphere would lead to greater amounts of UV-B radiation reaching the Earth. This would create, among other problems, an increase in melanoma (skin cancer) in humans as these damage surface cells of animals and plants. Under such circumstances, the normal life cycles of animals and plants would be disturbed. The UV radiation may interrupt lower level of food chains also. These possibilities led to a keen public interest in the matter and the stratospheric chemistry and physics became a focal point.

 

Formation of Ozone in Stratosphere

   In stratosphere, O3 is produced by photo dissociation of O2 due to absorption of radiation of < 242 nm(Eq.1) followed by a termolecular reaction of O – atoms with O2 (Eq. 2).

Reactions (1–4) constitute Chapman Mechanism (Chapman, 1930) for the creation/destruction of O3. Reactions (2) and (3) are much faster than (1) and (4), and rapidly interconvert O and O3 (Pilling and Seakins, 1995).

It atmospheric chemistry, a term odd-oxygen, which includes both O and O3, is widely used.

J3 is a complex function of the rate constant of step (3), wavelength dependent solar light intensity and extinction coefficients.

   In the lower stratosphere short wavelength radiation for O2 photolysis are highly diminished having been absorbed higher up, and in upper stratosphere, there is less O2 because of decrease in O2 as the altitude increases. Thus, there is layer type structure of O3 in stratosphere, maximum ozone being around 30 km in the tropics and around 15-20 km over the polar region. Since the level of peak is considerably higher in altitude over the tropics than poles, the ozone is distributed pole-ward through the stratospheric circulation. Ozone accumulates over poles particularly in winter; some of it enters the troposphere over the poles (Bridgman, 1990).

 

 

Stratospheric Ozone Depletion

   The problem of ozone depletion is an area of major concern in stratospheric chemistry and physics. As we would show later, the meteorology plays a big role in ozone depletion over Antarctica. In the Chapman mechanism, there is a serious imbalance in the rates of production and loss of oxygen. Step(18) accounts for only 20% of O3 loss. This necessitated the search of other pathways for O3 loss to restore the equilibrium.

   In equations (19-20), X is a radical which operates the catalytic reaction cycle, destroys ozone and is recovered at the end of reaction cycle. X can belong to the HOx, NOx or ClOx families. Thus, X, which is a radical, can destroy thousands of O3 molecules by chain reaction. The trace gases of importance in stratospheric ozone depletion are hydrogen species (OH, CH4) nitrogen species (NO, N2O, NO2) and chlorine and bromine compounds. Recent studies have indicated that with decreasing use of CFCs and halons, bromine compounds, CH3Br in particular, may become a major cause of ozone depletion in stratosphere (Lal et al., 1994).

 

Role of NO, NO2, N2O

   Paul Crutzen (1970)- the Nobel Laureate in Chemistry for work in atmospheric chemistry(1995) – was the first to suggest the ozone destruction through the involvement of NOx. The latter were produced by the reaction of N2O with O(1D). N2O, which has a long lifetime, is released in the troposphere and passes into the stratosphere. In the NOx cycle, ozone is destroyed by NO, which is formed through reactions (22) and (23). These reactions are the most important source of NO in the stratosphere.

 

Role of Chlorine Compounds

   Although, there was speculation regarding the destruction of ozone through involvement of chlorine, in 1974 M. J. Molina & F. S. Rawland – Nobel prize winner with Crutzen – first suggested that the chorofluorocarbons, CFCs, emitted by manmade activities might be depleting stratospheric ozone, through the removal of odd oxygen by chlorine atom. These compounds are manmade with a very long lifetime leading to increase in their abundance in atmosphere.

 

   CFCs are transported slowly in the stratosphere. Apart from CFC, methyl chloride is released naturally by oceans and by biomass burning. Several other chlorine compounds are also present in stratosphere. The importance of bromine compounds in causing ozone depletion has also been established. The major sources of these compounds are oceans, biomass burning and industrial and other applications.

Chlorine    atoms,    released    from    CFCs/CCl4/other    active    chlorine    compounds                       during

   ClO reacts about six times faster than any NOx species. By far, Cl is the most important destroyer of O3. In the mechanism (26-28), destruction of an ozone molecule involves the consumption of an O atom, whose concentration is not high. To overcome this difficulty, Molina et al., (1987) suggested the following modified mechanism.

   By a chain mechanism, a single Cl atom thus destroys several thousands of O3 molecules. The reactions of Cl atoms cease only, Cl atoms are removed and are placed as a reservoir species by reaction with CH4 or NO2.The ozone depletion mechanism of bromine is similar.

 

Role of Hydrogen Species

   The influence of HO, HO2 and CH4 is less significant except in upper stratosphere. In the HOx cycle, X is generally the hydroxyl radical, OH. It can react with O3 as follows.

   While the H/OH cycle, equation (40), is important in the upper stratosphere and mesosphere, the HO2/OH cycle, equation (42), is important in the lower stratosphere and troposphere.

Paul J. Crutzen, Mario J. Molina and F. Sherwood Rowland shared the 1995 Nobel Prize in chemistry for their work on atmospheric ozone chemistry.

 

Antarctic Ozone Hole

   Farman et al. (1985), whose group has been measuring stratospheric ozone at Antarctica for many years, found that the development of the ozone hole started with the appearance of the Sun over Antarctica after the long winter, and ozone reached a minimum around mid-October. The cause of the ozone depletion was a puzzle to the atmospheric scientists. Scientists propounded several theories to explain this phenomenon. Through measurement of total columnar ozone with the help of Dobson spectrophotometer, Farman et al. (1985) were able to show that the total ozone concentrations over the Antarctic region had been depleting during September-October, with subsequent recovery during the November- December, and this has been going on since 1979.

 

   Although the decrease in O3 varied from year to year, the results of the six – year period firmly established the ozone loss. Soon other workers provided evidence for this ozone loss. Media coined the phrase Antarctic ozone – hole to describe this phenomenon. The results of ozone measurement over Antarctica appeared in a special issue of Geophysical Research Letters, vol. 13, Nov. 1986. Subsequently, the results of a major international effort, the Airborne Antarctic Ozone Experiment (AAOE), on detailed trace gas, aerosol and meteorological measurements over the Antarctic region were published in Geophysical Research Letters, vol. 15, Jan 1988 and Journal of Geophysical Research, vol., 94, 1989.

 

   Subsequently, scientists carried out a detailed analysis of the vertical profiles to establish the extent of ozone loss over South Pole in 1986(Komhyr et al. 1988). The total ozone loss over Antarctica from mid August to early October was nearly 60%, especially in the altitude region of 11-23 km. In 1987, ozone depletion was severe at about 75%, and in a 4 km thick layer centered around 17 km, ozone was almost completely gone, creating ozone hole. The theory, based on the chlorine chemistry occurring under unique meteorological conditions prevailing in stratosphere over Antarctic region, met the universal approval.

 

   The chemical theory requires an overabundance of ClO in the Antarctic stratosphere in the altitude region 12-25 km. AAOE results indicated that the concentrations of the NOx species decreased toward the hole-centre, and ClO concentrations were 100 to 500 times higher than observed outside the hole. In 1987, there was an increase in ClO concentration across a very sharp boundary layer. fluctuating between about 67 and 75°S. The evidence for the involvement of ClO was provided by simultaneously monitoring ClO and O3 during a flight through the hole. Spatial distribution of ClO and ozone showed a marked negative correlation inside the hole. This experiment is a strong evidence for enhanced depletion of ozone by chlorine species.

   Farman et al. (1985) have also pointed out that the Antarctic ozone depletion began when the chlorine loading of the atmosphere exceeded a value of 2 ppbv. At the end of Southern Hemisphere winter, a circumpolar vortex of winds is at its strongest, extremely stable and durable at this time. The vortex blocks any incursion of warmer air from mid latitudes and allows an extensive drop in temperature. The temperature in the stratosphere falls to 193 K or less. The cold core is almost completely isolated from the rest of the atmosphere, and acts as a reaction vessel in which the constituents become chemically conditioned during the long polar night (Pilling and Seakins, 1995).

 

   Although the stratosphere has little water vapor, the low temperature of the stratosphere creates thin polar stratospheric clouds (PSCs), which are located at 10–25 km altitude. The surface of these clouds provides a site for heterogeneous chemical reactions on the surface.

   The chlorine is present as inactive compounds, ClNO2 and HCl. These are known as reservoir species. The reservoir species are formed during winter and in the absence of light by the following reactions.

   When the air flows through, the reservoir species interact with the frozen water. HCl dissolves in the frozen water and ClONO2 remains in the gas form. The following heterogeneous reactions convert the inactive HCl and ClNO2 into active chlorine and HOCl.

Solid HNO3 falls out of stratosphere into the troposphere as ice crystals.

   At the end of Southern Hemisphere winter, as the Sun begins to appear the molecules containing chlorine begins to photodissociate in spring sunlight to produce chlorine atoms. Thus begins the cycle of ozone destruction, equations (26-34). The ClOx – cycle is repeated many times before the reaction between Cl atoms and ClO to generate the reservoir species. The reservoir species hold the destructive chlorine atoms as unreactive species. It is possible to regenerate reactive Cl by reactions (46-47).

 

   Around late October, warming and eventual break down of strong spring vortex occurs. The influx of mid-latitude ozone occurs and ozone hole disappears. Thus, it is the climatic and meteorological conditions coupled with the atmospheric chemistry, which cause ozone hole.

 

Stratospheric Ozone Depletion over the Arctic:

   Significant ozone loss has been observed over Arctic region in recent years, but the extent of depletion is not as large as over Antarctica. This is due to the fact that the circumpolar vortex is neither as strong nor as stable as over Antarctica. Many times during winter, stratosphere warms suddenly due to influx of air from lower latitudes. The Arctic stratosphere is warmer and the prevalence of PSCs is lower than over Antarctica.

 

Control Strategies:

   Alarmed by the possible harmful effects of ozone depletion, at a historic meeting of several countries in Montreal, Canada in Sept. 16, decision was taken to phase out the use of CFCs, and freeze the consumption of halons at 1986 level. Now, the CFCs are being replaced by HCFCs. In troposphereon attack by OH radicals, HCFCs are converted into H2O and radical CFCl2. The latter is oxidized and rained out.

Ozone Depletion and Climate Change

   Ozone depletion and climate change has been the subject of intensive investigation owing to their influence on Earth. Ozone depleting substances such as chlorofluorocarbons (CFCs), hydrofluorocarbons (HCFCs), etc. are all greenhouse gases like ozone itself.

 

   The ozone affects the climate primarily by changing temperature. In stratosphere, ozone generates heat both by absorbing solar ultraviolet radiation and by absorbing infrared radiations coming from troposphere. Obviously, more ozone in stratosphere means its higher temperature. For this reason, a decrease in ozone concentration in stratosphere results in its cooling and its lower temperature.

 

   The strong influence of climate and ozone depletion on each other has been indicated(Shindell et al. 1999). The cooling in stratosphere might be rapid enough that even more ozone depletion takes place possibly due to a feedback mechanism. Stratospheric low winter temperature over poles plays a big role in ozone depletion. When this temperature drops below – 78oC, water, which is in very small quantity, is present as polar stratospheric clouds(PSCs). PSCs are actually ice crystals containing some nitric acid, and H2SO4. The chemical reactions on the surface of these PSCs begin with the beginning of summer in Southern pole, when chlorine radicals, responsible for ozone loss are released. Ozone loss lowers the stratospheric temperature and greater formation of PSCs, which in turn leads to faster loss of ozone. The ozone depletion in stratosphere would allow more UV radiation to reach troposphere and Earth surface because ozone is an absorber of these radiation, particularly of UV-B type, in stratosphere.

 

   The greenhouse effect on global warming is likely to cause changes in circulation pattern in the troposphere and that, in turn, may alter stratospheric circulation patterns. There is suspicion that these changes are aiding the cooling in the stratosphere over the poles.

 

   As on today, our understanding of the link between ozone depletion and climate change is only cursory and needs much work to draw any meaningful conclusion.

 

Montreal Protocol

   It is an international treaty on substances that deplete the ozone layer. It is designed to protect the ozone layer by phasing out the production of numerous substances that are responsible for ozone depletion.

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