Increased chlorine dioxide over Antarctica caused by volcanic aerosols from Mount Pinatubo

THE annual springtime depletion of Antarctic ozone¹ has been shown to be due to the action of chlorine species, activated by reactions occurring on the surfaces of polar stratospheric clouds (PSCs)^1,2. Similar reactions may also take place on the surfaces of liquid sulphuric acid aerosols when the temperature is too high to permit the formation of PSCs ^3-4. Such processes may have been facilitated following the eruption of Mount Pinatubo in June 1991, when unprecedented amounts of sulphur compounds were injected into the stratosphere⁵. Here we present observations of Antarctic chlorine dioxide abundances in the austral autumn and winter of 1991 (when aerosol concentrations were at background levels) and 1992 (greatly enhanced aerosol concentrations). We find that in 1992, unlike 1991, chlorine dioxide levels increased dramatically in the autumn, when PSCs were extremely unlikely to have been present. Model results suggest that this was mainly caused by the direct activation of chlorine nitrate on the aerosol surfaces. The effect of the Pinatubo aerosols probably contributed to the unprecedented depth and areal extent of Antarctic ozone depletion in 1992.