Role of sulphur photochemistry in tropical ozone changes after the eruption of Mount Pinatubo.

RECENT observations suggest that the eruption of Mount Pinatubo in June 1991 has had a considerable effect on ozone concentrations in the tropical stratosphere (refs 1, 2, and J. W. Waters, personal communication). Although stratospheric ozone losses following volcanic eruptions are generally attributed to the presence of sulphate aerosol, we present model calculations which demonstrate that gas-phase sulphur chemistry may have played a part in the tropical ozone perturbations that followed the Pinatubo eruption. We find that in the first month or so after the eruption, the large amount of SO injected into the tropical atmosphere catalyses mid-stratospheric ozone production. On the other hand, the SO cloud absorbs solar radiation, thereby reducing the rate of O photolysis (and hence of ozone production) below it. These two effects cancel each other out at an altitude of about 25 kilometres. After one or two months, most of the SO has been oxidized to sulphate; the efficiency of these two mechanisms then becomes negligible (although ozone remains perturbed in the lower stratosphere because of its long photochemical lifetime in this region). The model features show good agreement with initial ozone measurements following the eruption, including both the mid-altitude switch from ozone loss to ozone gain, and the increase and subsequent decrease in the total ozone column.