Evidence for heterogeneous reactions in the Antarctic autumn stratosphere

Abstract

REACTIVE chlorine compounds are known to cause ozone depletion in the Antarctic stratosphere, but they can be bound into an inactive form through reactions with nitrogen dioxide. In the spring, NO₂ can be converted to a long-lived reservoir species, HNO₃, on the surface of polar stratospheric clouds^1,2. This removes NO₂ from the stratosphere and allows chlorine-catalysed ozone destruction to proceed. It has been suggested that similar reactions may take place on background sulphate aerosols in the Antarctic stratosphere³, but as yet there has been no unambiguous evidence for these reactions in the absence of polar stratospheric clouds (although there have been observations of ozone loss attributed to volcanic aerosols^4,5). Here we present measurements of Antarctic stratospheric NO₂ and HNO₃ concentrations taken in 1991. Our results demonstrate that reactive nitrogen was converted to HNO₃ in autumn, before temperatures were low enough for polar stratospheric clouds to form. We conclude that heterogeneous chemistry on background aerosols was responsible for this conversion, which brought with it the potential for additional ozone loss in the autumn.