The state of the ozone layer: A 1994 science update
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- CICERO Reports 
Both the ground based and satellite observation data bases have been extended since the previous international ozone assessment. There are several satellite systems in operation that provide total ozone data, which have been extensively compared with ground based data. One problem with the satellite observations is that the widely used satellite date from the TOMS instrument on the NIMBUS 7 satellite have not been available since the beginning of May 1993 due to satellite failure. A new TOMS instrument was launched on a Russian spacecraft in August 1991 and have been in continuous operation since then. Although this gives continuous TOMS observations, there are still questions about how good the trend estimates after May 1993 are using the new TOMS data. We should also bear in mind that, although, we have extended data sets by including the last years of observations, the deduced trends are strongly affected by the low values of total ozone obtained during 1992 and 1993, and there are uncertainties how representative these low values are in connection with long term ozone depletion studies. Ozone research has to a large extent focused on ozone reductions at high northern latitudes during the last three to four years. Of particular interest has been the low ozone column densities observed at high and middle northern latitudes during the winters of 1992 and 1993. A natural question which has been raised is weather or not we here see a result of enhanced chlorine and bromine levels which in combination special atmospheric conditions lead to enhanced ozone depletion. The June 1991 volcanic eruption of Mt. Pinatubo lead to high particle concentrations in the lower stratosphere the following two years, in addition the extreme low temperatures at high northern latitudes during the winter of 1993 resulted in Polar Stratospheric Cloud (PSC) formation that was substantial higher than during a "normal year". Both these conditions led to more efficient ozone loss than we have experienced in previous rears, and in 1994. Several of the source gases of importance for the ozone depletion problem have shown reduced growth rate during the beginning of the 1990s. These changes in observed growth rate are difficult to explain for most gases. For the CFCS, however, it seems that reduced growth is a result of reduced emissions Which is as a result of the control measures that have been implemented to protect the ozone layer. A further consequence of this is that we within the next few years can expect stratosphere inorganic chlorine levels to peak, Thereafter a gradual decrease will occur over the next century. This shows that the Montreal agreement has been a success, Which has led to substantial reductions in the emission of CFCS. Another consequence of the Montreal agreement is a significant increase in the HCFCs over the last couple of years. However, the absolute levels of the newly introduced HCFCS, like HCFC-141b and HCFC-142b, are very low. The significant reduction in the growth rate in other source gases like CH4, N2O and CO2 during the beginning of the 1990s is somewhat puzzling, as there are no good explanation why this has occurred. There are, however, indications that for some gases (e.g. CHJ the reduced growth rate could be partly due to the large column ozone reductions. Methyl bromide and its potential role in the ozone depleting process have drawn increased attention since the previous ozone assessment. One of the largest uncertainties in the estimates of future man made impact on the ozone layer is connected to limited understanding of the role of methyl bromide. Methyl bromide is the main contributor to reactive bromine (BrO and Br) in the stratosphere. Measurements show that its concentration in the troposphere is about 10 to 15 pptv. Important uncertainties are connected to how large the man made contribution is to its sources, and to its tropospheric lifetime. Emissions occur both from anthropogenic and biogenic sources. An important antropogenic source is from the use of pesticides which showed marked increase during the 1980s. Other anthropogenic sources are biomass burning and automobile emissions. Large oceanic emissions have been suggested. The main sink is believed to be atmospheric oxidation by OH, but there may also be important oceanic sinks. The overall lifetime is in the range 1 to 2 years (loss through OH alone gives a lifetime of approximately 1.7 years). There have also been made advances in modelling of ozone and ozone chances in the stratosphere. The estimates of ozone distribution and ozone changes rely heavily on 2-D model calculations. These model studies do now include the impact of heterogeneous processes occurring on aerosol particles and to some ex-tent on PSCS. New laboratory studies have given us a better basis for estimating the impact of heterogeneous processes, and it is clear that heterogeneous chemistry in the lower stratosphere are the main cause of total ozone loss which take place as a result of enhanced ozone loss from chlorine and bromine species. Model calculations indicate that probably 2/3 or more of ozone depletion by chlorine and bromine compounds occur in the lower stratosphere as a consequence of heterogeneous reactions on particles. Inclusion of heterogeneous chemistry in the atmospheric models have made bromine reactions more important for the ozone depleting process. As a global average one bromine atom is estimated to be approximately 40 more efficient in destroying ozone than one chlorine atom. Approximately 20 0f the ozone destruction in the present day stratosphere is estimated to be due to chemical reactions involving BrO. Estimates of Ozone Depletion Potentials (ODPS) are in agreements with previous estimates, although more emphasis is given to the short-lived compounds (HCFCS, methyl bromide). Such compounds have transient ODPS, over limited time horizons (20 to 100 years) which can be much larger than the long term steady state ODP. A typical example is the ODP for methyl bromide which has a long term ODP value of 0.71, while the value over a time horizon of 20 years is 2.3. Increasing attention is given to the possible interaction between the ozone and the climate issues through chemical changes initiated by ozone changes in the atmosphere which affect climate gases (including ozone itself). This issue has been treated in the ozone assessment in close collaboration with the treatment of indirect chemical effects in the ongoing IPCC (Intergovernmental Panel for Climate Change) assessment of climate change (IPCC, 1994). Although this is a controversial issue, where the uncertainties are large in the estimates, it is nevertheless clear that chemical changes involving ozone will be of importance for estimates of GWPs (Global Warming Potentials) for important greenhouse gases like CH4 and the CFCS. The indirect effect of CFC, through ozone reductions in the stratosphere, seems to give a negative effect which is as large as the direct effect but with opposite sign. New model studies of the impact of a projected future fleet of supersonic transport on ozone through the emission of NOx show that it is probably small. This is a result of strong coupling between the nitrogen and chlorine compounds where reduced efficiency of chlorine reactions compensate for enhanced ozone loss through increased nitrogen oxides. New observations of UV-B fluxes in the polar regions confirm observations that were referred to in the previous international ozone assessment that the observed ozone reductions during the last years have led to significant increases in harmful UV-B radiation during the periods of low ozone values. Model studies indicate that the increases in tropospheric UV-B radiation during the last years with low column densities significantly have affected tropospheric chemical processes involving key gases like ozone and OH.