2 Chemistry of the Atmosphere
2.3 O ZONE C HEMISTRY OF THE P ERTURBED P OLAR S TRATOSPHERE
The hypothesis of anthropogenically caused ozone destruction caused worldwide concern, because the processes involved indicated a potentially large ozone depletion. The protection of the biosphere from harmful UV-B radiation by the ozone layer could be seriously reduced by these processes. UV-B radiation is sufficiently energetic to damage DNA and thereby cause skin cancer in humans and the degradation of plants with effects in agriculture and a variety of other eco-systems. In 1995, Crutzen, Molina, and Rowland shared the Noble Prize in Chemistry for their contributions to establishing the link between tropospheric processes, emissions to the atmosphere and stratospheric chemistry.
This honour was most likely catalysed by the discovery of the antarctic "ozone hole" by Farman et al., [1985], which caused a drastic change of view. It became apparent that the concern about the release of CFCs, halons and CH3Br was indeed justified and that a global and vital part of the Earth's atmosphere, the ozone shield, was – beyond previous common belief - vulnerable to anthropogenic activities. This was only possible as a product of different effects. Firstly there was an insufficient understanding of environmental and particularly atmospheric processes, which were being studied by a only small scientific community.
Secondly there existed a far-spread public attitude of general believe in technical feasibility.
This ignored possible negative by-products of the unthinking use of technological and industrial development. So both the knowledge and a readiness required for estimating effects of anthropogenic activities were underdeveloped.
The observation by Farman et al., which was subsequently supported by satellite observations and other field studies, showed that since the 1970s, each spring-time after polar sunrise the ozone concentrations above the station at Halley-Bay, Antarctica dropped. After the rapid losses the ozone concentrations recovered on a time scale of weeks and months. The observed drop increased from year to year, as can be seen in Fig. 2.2 [Finlayson-Pitts and Pitts 2000]. It shows a compilation of data including the Farman et al. data and subsequent measurements by Jones and Shanklin [1995]. The extent and rapidity of the observed loss effect were totally unexpected.
Even though there had been a clear increase in stratospheric chlorine and bromine content, the model calculations done around the early 1980s had predicted only small global ozone losses.
The observations by Farman et al. could not be explained by the chemical models used in these calculations, neither in extent, nor in rapidity, nor in their clear geographical and temporal limitation. The resulting increase in scientific activity led to a much clearer understanding of the different aspects of atmospheric chemistry. The role of dynamics of the atmosphere in the localised occurrence of polar ozone depletion could be identified. The importance of heterogeneous and multiphase processes on liquid and solid surfaces was discovered. These processes are characterised by the so called "perturbed" polar stratosphere during polar winter and spring.
Figure 2.2 From 1957 to 1994 the total column of ozone measured in each year's October above Halley Bay, Antarctica shows a clear and steady decrease of ozone over the years. From [Finlayson-Pitts an Pitts 2000].
During polar winter the stratosphere is pre-conditioned, thereby setting the stage for perturbed ozone depletion chemistry starting at polar sunrise. During polar night radiative cooling of the stratosphere takes place. The resulting temperature gradient causes high atmospheric pressures within the polar region and subsequent strong downward and outward meridional winds leaving the polar region. As in the case of a "normal" atmospheric high pressure area the radial winds are diverted by the Coriolis effect leading to a counter clockwise rotation (southern hemisphere) of the airmasses around the centre near the pole. This large scale circulation of air is commonly referred to as the Polar Vortex [Schoeberl and Hartmann 1991, Schoeberl et al. 1992]. The rotation efficiently separates the inner air masses from the surrounding stratosphere creating a positive feedback effect, as counterproductive warming by mixing with surrounding mid-latitude air masses is suppressed. Under these conditions extreme temperatures can be as low as 185K. At the edges of the vortex wind speeds of up to
≈ 100m/sec ≈ 360km/h can be reached. Inside the vortex the low temperatures enable the formation of crystals in spite of the typically low stratospheric water concentrations of 2-6ppm. A phenomenon observed under such extreme conditions are the polar stratospheric clouds (PSCs). They occur at heights of roughly 20km and can have a quite remarkable appearance of different colours depending on their height and on the presence of other clouds [Sarkissian et al. 1991]. Their formation is still a matter of discussion but the present understanding is that PSC type I are ternary solutions of HNO3, H2SO4, and H2O. PSC type Ia
are HNO3⋅3H2O and PSCs type II are composed of ice crystals. PSCs type I form around 195K in the polar vortex, whereas type II form close to 185K.
Ozone depletion in the vortex depends critically on the existence of PSCs during winter [e.g.
Molina et al. 1987, Molina 1991]. PSCs provide a surface on which heterogeneous – not purely gas phase – chemistry takes place. During polar winter on the surface of the PSC crystals a number of heterogeneous reactions are possible, which slowly transfer chlorine from non-reactive reservoir species like ClONO2, HOCl and HCl back to Cl2, which in contrast to the reservoir species is photolysable:
N2O5 + H2O → 2 HNO3 [1.8⋅10-31cm6/(molec.2⋅sec)] (R2.32) ClONO2 + H2O → HOCl + HNO3 [5.2⋅10-31cm6/(molec.2⋅sec)] (R2.33) ClONO2 + HCl → Cl2 + HNO3 [5.2⋅10-31cm6/(molec.2⋅sec)] (R2.34) HOCl + HCl → Cl2 + H2O [5.2⋅10-31cm6/(molec.2⋅sec)] (R2.35) N2O5 + HCl → ClNO2 + HNO3 [5.2⋅10-31cm6/(molec.2⋅sec)] (R2.36) After polar sunrise the Cl2 molecules are photolysed, releasing chlorine atoms and thereby starting catalytic ozone destruction. Due to the enhanced abundance of chlorine and the low temperatures apart from (M8a) two further mechanisms become important [Molina and Molina 1987]:
ClO + ClO+ M → Cl2O2 + M [2.2⋅10-32cm6/(molec.2⋅sec)] (R2.37)
Cl2O2 + hν → Cl + ClOO (R2.38)
ClOO + M → Cl + O2 +M [6.23⋅10-13cm3/(molec.⋅sec)] (R2.39) 2 x ( Cl + O3 → ClO + O2 ) [1.21⋅10-11cm3/(molec.⋅sec)] (R2.23)
net.: 2 O3 → 3 O2 (M11)
and a mixed BrO-ClO cycle [McElroy et al. 1986]:
ClO + BrO → Br + OClO [6.77⋅10-12cm3/(molec.⋅sec)] (R2.40) ClO + BrO → Br + ClOO [6.07⋅10-12cm3/(molec.⋅sec)] (R2.41) ClOO + M → Cl + O2 + M
ClO + BrO → BrCl + O2 [1.03⋅10-12cm3/(molec.⋅sec)] (R2.42)
BrCl + hν → Br + Cl
Br + O3 → BrO + O2 [1.16⋅10-12cm3/(molec.⋅sec)] (R2.25) Cl + O3 → ClO + O2 [1.21⋅10-11cm3/(molec.⋅sec)] (R2.23)
net.: 2 O3 → 3 O2 (M12)
A by-product of the heterogeneous activation reactions (R2.32) to (R2.36) is the de-nitrification, i.e. the conversion of NOx to HNO3. The general formation of PSCs has a similar effect in that it also removes water and NOx from the gas phase. If the temperatures are sufficiently low to allow the formation of water ice crystals on the surface of "normal" PSC (type I) crystals, another variety of PSCs is formed (PSC type II). PSC type II crystals can grow sufficiently large to allow even sedimentation from the stratosphere thereby de-hydrating and de-nitrificating the stratosphere even further.
De-nitrification is important, because NOx is capable of transferring halogens from its active form, XO to a stable reservoir molecule XONO2:
ClO + NO2 + M → ClONO2 + M [1.8⋅10-31cm6/(molec.2⋅sec)] (R2.43) BrO + NO2 + M → BrONO2 + M [5.2⋅10-31cm6/(molec.2⋅sec)] (R2.44) Cl atoms react with methane, CH4 to form another reservoir species HCl via
Cl + CH4 → HCl + CH3 [1.0⋅10-13cm3/(molec.⋅sec)] (R2.45) In contrast, bromine atoms react slowly or not at all with methane:
Br + CH4 → HBr + CH3 [5.6⋅10-23cm3/(molec.⋅sec)] (R2.46) HOCl and HOBr are also reservoir species formed by reaction of ClO and BrO with HO2:
HO2 + BrO → HOBr + O2 [2⋅10-11cm3/(molec.⋅sec)] (R2.47) HO2 + ClO → HOCl + O2 [4.99⋅10-12cm3/(molec.⋅sec)] (R2.48) As a result of the above reactions and the fact that the bromine reservoirs BrONO2 and HOBr are more photolabile than ClONO2 and HOCl, under non-perturbed stratospheric conditions only a small fraction of chlorine is present as ClO during daytime, but about 50% of bromine as BrO.