Evidence for stratospheric ozone-depleting heterogeneous chemistry on volcanic aerosols from El Chichón

Abstract

STRATOSPHERIC aerosols are thought to promote ozone destruction by reactive halogen compounds in the polar stratosphere. The predominant aerosol usually comprises particles of nitric acid trihydrate; surface-catalysed reactions on these particles produce ozone-destroying halogen compounds and remove reactive nitrogen gases which might otherwise render the halogen compounds inactive^1,2. For periods of up to two years after major volcanic eruptions, atmospheric concentrations of sulphuric acid aerosols in the stratospheric eruption cloud may be comparable to those of nitric acid aerosols, and might therefore also play a part in atmospheric heterogeneous chemistry. Indeed, ozone levels in the Northern Hemisphere were observed to decrease following the large eruption of the Mexican volcano El Chichón in 1982^3,4. Here we report balloon-borne measurements of trace gases made in June 1982 in the stratospheric eruption cloud of El Chichón. We observe an increase in the concentration of nitric acid, which is probably due to the conversion of reactive nitrogen gases on sulphate aerosols. These reactions are therefore likely to have played a part in the destruction of stratospheric ozone.