The effect of volcanoes on ozone has been wldely debated since evidencewas produced that ozone abundance decreased following the eruption in 1982 of El Chichon in Mexico. The argument presented in the early 1980's was that volcanoes release chlorine into the atmosphere (mainly in the form of hydrogen chloride) and that chlorine has the potential to destroy ozone efficiently in the stratosphere. This natural source, however, affects the stratosphere much less than the anthropogenic source provided by industrially manufactured chlorofluorocarbons (CFCs). In fact, hydrogen chloride is rapidly removed from the troposphere by washout processes in clouds, whereas the CFCs are not. Thus, CFCs easily penetrate into the stratosphere where they are photolysed by solar ultraviolet radiation and release reactive chlorine atoms.
The gas-phase reactions involved in the destruction of stratosphericozone by chlorine account only for a small fraction of the ozone depletion recorded since 1980. In addition, the largest ozone trends (approximately 10-30 per cent per decade), which have been observed between the tropopause and 20 Km altitude, are not reprodeuced by models based on gas-phase chemistry alone. the discrepancy is now commonly attributed to the role of solid particles (such as ice crystals in polar stratospheric clouds) or small liquid droplets (such as the sulphate aerosol particles) observed in the lower stratosphere. as a result of heterogeneous reactions occurring on the surface of these particles. nitrogen oxides are transformed into nitric acid and, especially in the coldest regions of the lower stratosphere, the unreactive and most abundant forms of chlorine molecules (for example, ClONO2) are partly converted into reactive radicals. Thus, under these circumstances, the atmosphere shifts from a situation where the ozone loss is controlled primarily by nitrogen oxides to a situation where it becomes more readily determined by the abundance of chlorine.
During non-volcanic periods, the aerosol load in the atmosphere remains relatively limited but after major volcanic eruptions, such as those of El Chichon in April 1982 and Mount Pinatubo in June 1991, it can be raised by one or two orders of magnitude, so that heterogeneous reactions occur at a much faster rate. Thus, as shown by models based on ourcurrent knowledge of the chemical kinetics involved, the ozone molecules of the lower stratosphere should become more vulnerable to atmospheric chlorine and so to man-made CFCs. The analysis by Grant et al. of ozone profiles measured at Brazzaville, Congo (4 degrees S), Ascension Island (8 degrees S), and Natal, Brazil (6 gedrees N) after Pinatubo suggests that the ozone concentration in the 3-6 months following the eruption was reduced by as much as 15-20 percent in the altituderange (approximately 24-25 km) where the largest volcanic aerosol loading was observed.
These significant reductions in ozone could potentially result from heterogenous conversion of NOx on the surface of sulphate aerosols. Alternatively, heating in the aerosol layer may have drawn ozone-poor, tropospheric air into the lower stratosphere so that the change was associated with transport. Or perturbations of solar radiation fields by the aerosol layer may have reduced the rate of photochemical production of ozone.
Although heterogeneous chemical mechanisms on sulphate aerosols are expected to be most efficient near the polar vortex, no convincing evidence of significant ozone depletion, at these high latitudes, associated with the eruption of Mount Pinatubo has yet been produced. The paper in this issue by Hofmann et al. reports for the first time unusually high ozone reduction in Antarctica during September 1991 at altitudes where polar stratospheric clouds are usually not observed. Ozone reduction approaching 50 per cent was detected both in the 11-13 km and 25-30 km regions, resulting in an ozone column abundance 10-15 per cent lower than in previous years.
Different potential explanations are invoked by the authors, including meridional and vertical transport, and homogeneous chemical loss which is found to account for at most a 10 per cent reduction in ozone at 30 km compared with 1980. After careful analysis, Hofmann et al. conclude that the depletion below 12 km results from the presence of large quantities of sulphate aerosols produced from the sulphur injected during the eruption of Mount Hudson in Chile (46 degrees S) in August 1991. The ozone depletion observed above 25 km altitude appears to be associated with transport of air from the Wedell Sea and the Palmer Peninsula area where polar stratospheric clouds seem to have been produced at relatively high altitude by mountain lee waves.
Guy Brasseur is at the National Center for Atmospheric Research, Boulder. Colorado 80307, U.S.A
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