Chemistry of the Stratosphere

Chemistry of the Stratosphere
There are several techniques used to investigate the chemistry of the stratosphere. The first of these is monitoring which involves analysing the air using spectroscopy. Given that ozone absorbs in the infra-red and ultra-violet regions of the spectrum, the concentration on ozone in a sample can be calculated form the strength of its absorption (figure1). This has to be carried out at different times and in different conditions to ensure any decrease is not due to natural fluctuations. [image] Figure 1 ? Ozone Distribution in the Atmosphere Once the molecules present in the stratosphere are identified, laboratory measurements can be carried out to investigate the reactivity of the molecules concerned and how radiation affects them. Special techniques such as flash photolysis have to be used to work out how fast the reactions are occurring. This technique allowed scientists to work out that the reactions breaking down and making ozone are generally occurring at the same rate and consequently there is a steady concentration of ozone.
The study of meteorology involves learning about the movements of air currents which circle around the lines of latitude and help gas to mix within a layer. Meteorology allows scientists to obtain a better idea of how the reactions occur in the stratosphere as opposed to the very different conditions under which they take place in a laboratory. Information from these different sources is fed into a powerful computer that produces a ?model? of what scientists think happens in the stratosphere. The more data that becomes available the closer to reality the model becomes. These overall simulations can be used to make predictions about future variations in the atmosphere. [image] In 1972, James Lovelock, who was interested in the global spread of gasses in the troposphere, developed a method for detecting CFCs in the troposphere. He detected small concentrations of cfc 11 in rural areas, far away from potential sources. He recognised that such a stable gas would accumulate and move in the atmosphere. Sherwood Rowland and Mario Molina found that when CFCs reached the troposphere they are broken down by the absorption of UV light. Years after these discoveries, a group of scientists flew to the ozone hole and measured concentrations of ClO radicals and ozone. They found that the correlation of ozone fell considerably as that of ClO radicals rose therefore proving that Cl radicals must be involved in ozone depletion (figure 2). [image] Figure 2 ? Measurement of ozone and chlorine monoxide from a flight over the Antarctic Ozone is destroyed in a catalytic cycle, by reacting with radicals present in the atmosphere. If X is a radical the two reactions involved can be written as: [image][image][image]X + O3 XO + O2 XO + O X + O2 Overall reaction: O + O3 O2 + O2 The radical X could be a HO, NO or Cl radicals. The equation shows that ozone is being lost and the X radical involved in the reaction is acting as a catalyst. The formation of the ozone hole over Antarctica is a consequence of the special atmospheric conditions which occur there. In particular, the low stratospheric temperatures, the isolated wind patterns and the disappearance of the sun in winter, for six months. A vortex of winds develops around the pole and isolates the polar stratosphere. This causes the temperature to drop, meaning clouds begin to form in the stratosphere. These clouds are called polar stratospheric clouds (PSCs), and they are made of a mixture of water ice crystals, crystals of water ice mixed with nitric acid, as well as droplets of liquid water mixed with nitric acid and sulphuric acid. These clouds are a catalyst for the chemical reaction between ClONO2 and HCl. [image] Figure 3 When the sun returns in the southern spring the Cl2 molecules are split by the UV radiation into chlorine radicals, which catalyse the destruction on ozone. CFC?s first emerged as refrigerants after other chemicals were found to be less affective. Afters the first commercially successful refrigerator was used in 1855, the increasing population in the UK meant that food distribution became more and more. Refrigeration was needed to maintain cold stores in cities and in ships transporting food. The first really successful domestic refrigerator used sulphur dioxide as a refrigerant because it allowed reliable units to be sold at affordable prices. However, the toxicity of sulphur dioxide became a problem because it leaked slowly out of the imperfect seals. It became necessary to find a replacement as America threatened to put a ban on their use. Compounds of chlorine, fluorine and carbon (CFCs) where recognized to be stable, non flammable and seen combined low toxicity with good performance. These properties made CFC?s and the related HCFC?s (which have the same composition as CFC?s except they also contain hydrogen) good refrigerants. CFC?s and HCFC?s swiftly became the favoured refrigerants for almost all areas. However, aside from being good refrigerants, CFC?s were also found to be useful in other areas. These included, their uses as propellants in aerosols, polyurethane blowing and printed circuit board cleaning. As CFCs became increasingly popular it was suggested that the as they reached the stratosphere they where being broken down by the intense ultra-violet radiation. The possible of Ozone depletion to CFCs led to the signing of ?The Montreal Protocol?, which detailed the phasing out of CFCs and HCFCs. Once again suitable replacements had to be found. tv07_l.gif Figure 4 ? Graph showing global cfc production over time. Hydrofluorocarbons (HFCs) were found to be a suitable replacement for CFCs that did not have the same effect on the ozone layer because they contain fluorine as the only halogen. More energy is needed to release a fluorine atom than is available even in the intense solar radiation. Most HFCs released into he atmosphere are largely destroyed in the lower atmosphere by hydroxyl radicals. However HFCs cause problems when used with refrigerators and air conditioners designed for CFCs and HCFCs. So in order to find HFCs with similar properties to the CFCs, blends of refrigerants were used. However, no economically viable alternative has been found. In northern Europe, the hydrocarbons such a propane, butane and methyl-propane were identified as suitable replacements for HFCs, which would not be damaging to ozone and which would have a lower effect on global warming potentials. However they also contribute to the production of the greenhouse gas, carbon dioxide, and problems such as photochemical smog. It appears that HCFs are the generally preferred replacements. However, in the future cooling technologies not involving CFCs may be introduced if they have technical advantages and if the hazards associated can be controlled.

Chemistry of the Stratosphere 9.5 of 10 on the basis of 2337 Review.