Nitric acid cloud formation in the cold Antarctic stratosphere: a major cause for the springtime ‘ozone hole’

Abstract

Large depletions in stratospheric ozone were first reported by Farman et al.¹ at Halley Bay (76°S), and confirmed by satellite observations². Chubachi³ gives a detailed account of ozone decreases and temperatures in the lower stratosphere during the spring of 1982 at 69°S. There is now evidence² for annual declines in total ozone by ∼6 and 3% in regions of total ozone minima and maxima, respectively, from September to mid-October since the late 1970s. We propose here a chemical mechanism for the formation of the ozone hole. It involves removal of gaseous odd nitrogen by ion- and/or aerosol-catalysed conversion of N₂O₅ and ClONO₂ to HNO₃ vapour, followed by heteromolecular HNO₃–H₂O condensation, leading to HNO₃–H₂O aerosols. At an altitude of 17km, these processes start at temperatures below 205±5 K, well above the condensation temperature of pure water vapour. We propose that the absence of gaseous odd nitrogen and catalytic methane oxidation reactions driven by sunlight in early spring lead to large OH concentrations which rapidly convert HCl to ClOX. Catalytic reactions of ClOX and BrOX cause drastic ozone destructions and can account for the springtime ‘ozone hole’ first observed by Farman et al.¹. By our model the depletion would be mainly due to emissions of industrial organic chlorine compounds. Arctic regions may also become affected. The depletion lasts while HNO₃, but not HCl, is incorporated in the particles in the temperature range 205±5 K to 192 K.