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Concept 05 / 05Environmental Chemistry

Stratospheric Pollution: Formation and Depletion of the Ozone Layer

Natural formation and breakdown of stratospheric ozone and its role in absorbing ultraviolet radiation, destruction by chlorofluorocarbons through a chlorine radical chain, the Antarctic ozone hole and polar stratospheric clouds, nitrogen oxides, effects of ozone depletion and the Montreal Protocol.

Figure 1
Formation of ozone. In the stratosphere, UV radiation splits oxygen molecules, and the atoms combine with O2\mathrm{O_{2}}: O2→UVO+O\mathrm{O_{2} \xrightarrow{UV} O + O}; O+O2⇌O3\mathrm{O + O_{2} \rightleftharpoons O_{3}}. Ozone itself absorbs UV and breaks down again (O3→UVO2+O\mathrm{O_{3} \xrightarrow{UV} O_{2} + O}). This steady state keeps a layer of ozone that absorbs about 99%99\% of the harmful UV radiation (UV-B and UV-C) from the Sun. Ozone thickness is measured in Dobson units; below about 220 DU220\,\mathrm{DU} is called an ozone hole. Depletion by CFCs. Chlorofluorocarbons (freons such as CF2Cl2\mathrm{CF_{2}Cl_{2}}), used in refrigerators, air conditioners, aerosol sprays and foam, are very stable and non-toxic, so they survive long enough to diffuse into the stratosphere. There, strong UV breaks them (see figure): CF2Cl2→UV∙CF2Cl+Cl∙\mathrm{CF_{2}Cl_{2} \xrightarrow{UV} {}^{\bullet}CF_{2}Cl + Cl^{\bullet}} Cl∙+O3→ClO∙+O2\mathrm{Cl^{\bullet} + O_{3} \rightarrow ClO^{\bullet} + O_{2}} ClO∙+O→Cl∙+O2\mathrm{ClO^{\bullet} + O \rightarrow Cl^{\bullet} + O_{2}} The chlorine radical is regenerated, so it acts as a catalyst: a single Cl atom can destroy on the order of 10510^{5} ozone molecules before being removed. The Antarctic ozone hole. Discovered in 1985. In summer, chlorine is stored in inactive reservoirs: ClO∙+NO2→ClONO2\mathrm{ClO^{\bullet} + NO_{2} \rightarrow ClONO_{2}} and Cl∙+CH4→CH3∙+HCl\mathrm{Cl^{\bullet} + CH_{4} \rightarrow CH_{3}^{\bullet} + HCl}. In the extremely cold polar winter, polar stratospheric clouds form, and on their surfaces the reservoirs react: ClONO2+H2O→HOCl+HNO3\mathrm{ClONO_{2} + H_{2}O \rightarrow HOCl + HNO_{3}}; ClONO2+HCl→Cl2+HNO3\mathrm{ClONO_{2} + HCl \rightarrow Cl_{2} + HNO_{3}}. When the Sun returns in spring (September to October), sunlight splits HOCl and Cl2\mathrm{Cl_{2}} into chlorine radicals, which destroy ozone rapidly, creating the ozone hole. Other ozone-depleting substances. Nitric oxide from supersonic aircraft (NO+O3→NO2+O2\mathrm{NO + O_{3} \rightarrow NO_{2} + O_{2}}), halons (bromine compounds used in fire extinguishers; Br is even more effective than Cl), carbon tetrachloride, methyl chloroform. Effects. More UV reaching the Earth causes skin cancer (melanoma), cataracts and sunburn, suppresses the immune system, kills phytoplankton (disrupting the food chain), damages plant proteins and leads to harmful mutations, increases evaporation and reduces soil moisture, and degrades paints and fibres. Control. The Montreal Protocol (1987) phased out CFCs and other ozone-depleting substances; replacements include HCFCs and HFCs (which do not destroy ozone but are greenhouse gases). The ozone layer is slowly recovering. Key Takeaways: - Ozone forms from O+O2\mathrm{O + O_{2}} and absorbs harmful UV. - CFCs release Cl∙\mathrm{Cl^{\bullet}}, which catalytically destroys O3\mathrm{O_{3}} via ClO. - Polar stratospheric clouds convert reservoirs (ClONO2\mathrm{ClONO_{2}}, HCl) into photolysable Cl2\mathrm{Cl_{2}} and HOCl. - The ozone hole appears over Antarctica in spring. - Pitfall: CFCs are harmful precisely because they are so unreactive in the troposphere, allowing them to reach the stratosphere. Brain-Teaser 1: Write the net reaction of the chlorine-catalysed ozone destruction cycle and identify the catalyst and the intermediate. Solution: Adding Cl∙+O3→ClO∙+O2\mathrm{Cl^{\bullet} + O_{3} \rightarrow ClO^{\bullet} + O_{2}} and ClO∙+O→Cl∙+O2\mathrm{ClO^{\bullet} + O \rightarrow Cl^{\bullet} + O_{2}} gives O3+O→2O2\mathrm{O_{3} + O \rightarrow 2O_{2}}. Cl∙\mathrm{Cl^{\bullet}} is the catalyst (consumed and regenerated); ClO∙\mathrm{ClO^{\bullet}} is the intermediate. Brain-Teaser 2: Why does the ozone hole form over Antarctica in spring rather than in winter? Solution: In winter the polar vortex isolates very cold air in which polar stratospheric clouds form and convert inactive chlorine reservoirs into Cl2\mathrm{Cl_{2}} and HOCl. But breaking these into radicals needs sunlight, which is absent during the polar night. When sunlight returns in spring, the stored chlorine is released as radicals all at once, destroying ozone rapidly. Brain-Teaser 3: HFCs like CF3CH2F\mathrm{CF_{3}CH_{2}F} are used instead of CFCs. Why do they not damage ozone, and what is their drawback? Solution: They contain no chlorine or bromine, so UV cleavage releases no radical that catalyses ozone destruction, and their C-H bonds let them be broken down in the troposphere. However, they are strong greenhouse gases, so they contribute to global warming.

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