Back together again

stratospheric clouds, but, especially in the years following a large volcanic eruption such as that of El Chichon in 1982, the effect could be n o t i ~ e a b l e ~ ~ ~. The efficiency of these processes depends on the chlorine load of the atmosphere, and so is gradually increasing as a result of the use of chlorofluorocarbons. If the amount of sulphur injected into the stratosphere by the eruption of Mount Pina-tubo in the Philippines in June is 2 or 3 times larger than that emitted by El Chichon, as is indicated by satellite observations, a reduction in the ozone column could, according to our model calculations, reach significant values during next winter at mid-and high latitudes , after the volcanic material has spread out over the entire Northern Hemisphere. It has also been suggested" that substantial amounts of ozone could have been destroyed as a result of increased production of nitrogen oxides by energetic protons in the solar wind following the large solar flares of 1989. Nitrogen oxides are known to destroy ozone catalytically at altitudes of 20-50 km. Large reductions in ozone concentrations in the upper stratosphere were reported" after a similar event in August 1972. Although TOMS data seem to indicate that a limited amount of ozone was destro ed in the vicinity of the South Pole" following the two proton events in March 1989, model calculations12 show that the largest percentage reduction in the ozone concentration (about 20 per cent) following the intense flares of the second half of 1989 should have taken place near 40 km, leading to a reduction in the ozone column much smaller than the one derived by the recent trend analysis. Evidently, the indications are that ozone in the middle atmosphere is more sensitive to anthropogenic chlorine than previously predicted by numerical models. Heterogeneous chemistry on cloud and aerosol particles both at high and l m CNS mid-latitudes is probably playing a key role that is not yet fully understood. The importance of dilution processes from the polar regions in both hemispheres also needs to be evaluated. The Arctic campaign organized by European, American and Soviet scientific groups this coming winter will provide vital information to address these questions. Systems Maps But the polar aspects of the mission might be somewhat overshadowed by the chemical and radiative perturbations associated with the recent eruption of Mount …