EVIDENCE FOR HETEROGENEOUS REACTIONS IN THE ANTARCTIC AUTUMN STRATOSPHERE

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 clouds1,2. 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 aerosols4,5). 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.