 |
Stratospheric Processes And their Role in Climate
|
||||||||
| Home | Initiatives | Organisation | Publications | Meetings | Acronyms and Abbreviations | Useful Links |
|
Stratospheric Processes And their Role in Climate
|
||||||||
| Home | Initiatives | Organisation | Publications | Meetings | Acronyms and Abbreviations | Useful Links |
Between November 1999 and April 2000, two major field experiments, the European Commission-sponsored Third European Stratospheric Experiment on Ozone (THESEO 2000) and the NASA-sponsored SAGE III Ozone Loss and Validation Experiment (SOLVE), collaborated to form the largest field campaign yet mounted to study Arctic ozone loss. Briefly, the campaign involved research aircraft, balloons, ozonesondes, ground-based and satellite instruments, which were greatly augmented by meteorological and chemical models. In all more than 400 scientists from the European Union, Canada, Iceland, Japan, Norway, Poland, Russia, Switzerland and USA were involved. A description of the SOLVE-THESEO 2000 activities and the early findings was published in SPARC newsletter No. 15 (Newman, 2000).
The main aims of SOLVE-THESEO 2000 were to study the processes leading to ozone loss in the Arctic vortex and how the ozone amounts over northern mid-latitudes are affected. These processes occurring in the Arctic vortex have been closely studied over the past decade or so and, qualitatively, are fairly well understood. The Arctic stratosphere cools during polar night and at sufficiently low temperatures, polar stratospheric clouds (PSCs) form. Heterogeneous reactions occur on the surface of these clouds which convert relatively inactive chlorine compounds such as HCl and ClONO2 into forms such as ClO which can lead to rapid chemical destruction of ozone in the presence of sunlight. This ClO then gradually revert back to the inactive forms and the ozone destruction slows down. These processes largely take place in the Arctic vortex. As the vortex breaks down, ozone-depleted air is mixed with mid-latitude air.
Given the planning involved, the campaign was fortunate, from a scientific perspective, to take place during a cold winter in the Arctic stratosphere in which there was large ozone loss. The processes involved were therefore extremely well observed, and since the end of the campaign scientists have been poring over the measurements trying to understand what happened.
The results were discussed at the joint SOLVE-THESEO science meeting held in Palermo, Italy from September 25-29 2000. Generous support for the meeting was provided by CNR, ENEA, Regione Siciliana and the city of Palermo. Over 250 scientists participated with over 50 talks and just under 200 posters. Many of these are being written up for submission for publication in two special issues of JGR or in other journals. In this article we report on some of the key findings presented at the meeting and on the summaries of the discussions of outstanding issues prepared by groups of rapporteurs. The findings presented below are only a small part of those presented at the meeting, and we apologise in advance to those whose results are not included.
As discussed in SPARC newsletter No. 15, the Arctic stratosphere was cold during the 1999/2000 winter and the vortex was stable into mid-March at which time it was split with one substantial remnant present well into April. The temperatures were below PSC existence temperatures from late November until mid-March. A number of studies of the long-lived tracer data collected during SOLVE-THESEO 2000 found that there was a relatively small amount of mixing in of mid-latitude air into the vortex, probably less than 10% of the vortex volume at around 18 km. The amount of mixing in of mid-latitude air masses is an important issue for at least three reasons. First, the chemical reaction rates governing the activation and deactivation processes and the ozone loss itself are affected if chemically distinct air is mixed in, particularly for nitrogen rich air. Second, improved quantification of the in-flow across the vortex edge is inherently valuable for understanding the mass balance of the vortex. And third, the mixing in of air across the vortex edge can cause errors in the ozone losses estimated using tracer/ozone correlations.
The long-term context of the 1999/2000 winter and the possibility of long-term changes in the Arctic vortex were discussed at some length (see also several articles in SPARC Newsletter No. 15). The past decade has certainly seen, on average, a colder, longer-lived vortex, but it is hard to define the significance of these changes. The most robust change is the longer duration of the vortex which during the 1990s broke up, on average, in March, compared to February during the 1980s and earlier. The fundamental reason for this is unclear, but it does seem related to the decreased wave driving during the January-February period.
One of the scientific foci of SOLVE-THESEO 2000 was to improve understanding of the formation, chemical impact and importance of mountain waves. A number of beautiful case studies were presented which showed that the meteorological mesoscale modelling of these events was good and, further, that the new ECMWF analyses with their improved resolution also described mountain wave events well. However in such a cold winter where PSCs were ubiquitous in the vortex, the chemical impact was probably not great although this might not be true in warmer winters where temperatures were close to the PSC existence temperatures.
Finally there was a lot of discussion of the quality of the trajectories derived from analysed wind fields. The largest errors were observed at lower pressures (the uncertainties in the analyses increased at pressures below about 30 hPa). More work is needed to quantify these uncertainties at all altitudes as trajectories are such a basic tool used in Lagrangian photochemical box models, identification of the origin of air masses and in ozone loss studies.
The long, cold winter combined with advances in instrumentation, particularly for the in situ particle measurements, has provided a wealth of data. There were two particularly notable findings: (a) the first measurements of with a chemical composition of nitric acid trihydrate (NAT) (Voigt et al., Science, 290, 1756, 2000) and (b) the discovery of PSC particles with radii of 15 micron or so, much larger than previously thought (Fahey et al., Science, in press).
The chemical composition measurements showed the presence of layers of NAT particles and layers of sulphate ternary solution particles. Some of the NAT particles were present near or even above their equilibrium temperature. These balloon-borne observations were made over northern Scandinavia in the presence of mountain waves. They underline the fact that the evolution of particles in rapidly changing temperature fields such as mountain waves is complex and at the same time, they show that such complex processes can be unravelled.
The large particle observations (‘rocks’) were made more generally within the Arctic vortex from the ER-2 aircraft. Their size, and low concentration, meant that they had previously eluded discovery as most available instruments lose sensitivity above a few microns. The particle size also causes them to fall quickly through the stratosphere, and, as they contain HNO3 and water, their sedimentation can denitrify and, to a lesser extent, dehydrate the Arctic vortex. This was borne out by measurements showing extensive denitrification and some dehydration in the coldest regions and enhanced nitric acid and water vapour amounts lower at lower, warmer altitudes. However it is still not clear what the nucleation mechanisms are for these PSCs, and there was a great deal of debate on this subject.
An unprecedented set of instruments (on aircraft, balloons, satellites and the ground) observed the chemical evolution of the Arctic stratosphere from November 1999 through April 2000. In conjunction with measurements made in other winters over the past decade or so, they have allowed studies of chemistry on a whole range of temporal (minutes to years) and spatial scales (metres to 1000’s of kilometres).
In situ measurements of long-lived tracers during SOLVE-THESEO 2000 showed that the total organic chlorine (that in the form of the source gases ? CFCs, CH3CCl3, CCl4, etc.) has now peaked in the stratosphere, some 6-7 years after it peaked in the troposphere. As in the troposphere, the early response to the Montreal Protocol is driven mainly by the steep decline in the relatively short-lived (5 years) CH3CCl3.
For the first time, simultaneous in situ measurements of all major chlorine species were made on the ER-2. Preliminary results from these were presented which showed progress toward understanding whether the chlorine budget is balanced within the estimated uncertainties. Since nearly all the major chlorine species were measured by more than one technique (in situ and remote), it was agreed to make sure that proper comparisons are made to achieve a consistent data set, as is already being done for the long-lived tracer data. Currently the main features of the chlorine activation and deactivation in 1999/2000 are clear, with a long activated period and slow recovery. However more detailed studies showed that a number of quantitative issues remain. For example, there were reports that comparisons of ozone loss calculated from the observed ClO and BrO amounts were less than the empirically determined ozone losses. However the ozone loss calculated by the SLIMCAT 3D chemical transport model were in good agreement with the empirically derived ozone losses in 1999-2000 (Sinnhuber et al., GRL, 27, 3473, 2000), unlike some previous winters.
Bromine compounds are significant contributors to Arctic and mid-latitude ozone loss. Bromine activation is now much better understood and there is good evidence that 50-80% of inorganic bromine is in the form of BrO in most conditions (Arctic, mid-latitudes, tropics, all seasons). The overall understanding of the stratospheric bromine budget has also improved (i.e. putting the uncertainties associated with the tropospheric sinks and sources to one side). A decadal increase in BrO consistent with tropospheric increases seems to have occurred, and there is strong evidence for a source of bromine in the very low stratosphere from short-lived compounds.
Ozone loss in the 1999/2000 winter was large and extensive. At altitudes around 18km, 60-70% of the ozone was destroyed between January and mid-March, slightly larger that the previous record loss which occurred in 1995/96. The column ozone loss inside the column was 20-25% in 1999/2000, less than the previous record years largely because the vertical extent of the ozone loss was less than in those years. There was reassuringly good agreement between the estimates of ozone loss found by several techniques using measurements from a number of instruments. A number of technical issues about the various techniques used to estimate ozone were discussed. However, once the differences resulting from the different time periods considered were taken into account, it seemed as though these were not that important in the 1999/2000 winter because the ozone loss was large and because the vortex was strong and well isolated. As mentioned above, there was good agreement with the loss calculated by the SLIMCAT 3D CTM, there was poorer agreement with other models. Further work is needed to understand the causes of these differences.
A number of modelling studies investigated the ozone loss at mid-latitudes, including the impact of the polar ozone loss. The interannual variability in dynamics causes the export of air from the vortex to mid-latitude to vary greatly in amount and timing, so that even for a given chemical ozone loss inside the vortex, the effect on mid-latitudes will also vary. Modelling studies are now realistically quantifying this effect. Chemical transport model calculations also showed how polar processes vary from year to year, and they additionally showed how the importance of changes in the contributions of the different chemical loss cycles. All models showed that a strong, long-lived vortex has little effect on mid-latitudes until it starts to break up. This was true dynamically (no export of ozone-poor air) and chemically (no chemical perturbation at mid-latitudes without mixing out or activation outside the vortex).
A number of related issues came up several times during the meeting. Chief among these was one of the main driving forces of SOLVE-THESEO 2000 ? how will climate change affect the future stratosphere? Or, more dramatically, will there be an Arctic ozone hole? The interaction of climate change with the stratosphere and with the ozone depletion processes is complex and will be a major area of research in the future. As one example, there is a clear need for a greater understanding of stratospheric water vapour for dynamic, radiative and chemical reasons. In the polar vortices, the water vapour concentration will be one of the factors which determines the temperatures at which PSCs can form, with higher water vapour levels raising the PSC existence temperatures. It may well be the radiative effect of the increased water vapour which is causing the Arctic vortex to cool. If these conditions also lead to enhanced denitrification and to more stable vortices, how much will the effect of the declining halogen levels be offset by the increased water vapour and cooling stratosphere? There are many uncertainties in such trains of thought ? equally they are plausible and need to be investigated further.
Further details on the SOLVE-THESEO 2000 campaign can be found at:
http://www.nilu.no:80/projects/theseo2000/
http://cloud1.arc.nasa.gov/solve/
http://www.ozone-sec.ch.cam.ac.uk/
and on the SOLVE-THESEO 2000 science meeting, including the rapporteurs’ reports at:
http://hyperion.gsfc.nasa.gov/Personnel/people/Newman,_Paul_A./speaker.html
We thank all scientists and technical support staff involved in SOLVE-THESEO 2000 for their magnificent efforts during the last year. We also thank all those who helped in organising the meeting at Palermo, including Dr Adriani’s team at CNR-IFA, Rebecca Penkett, Kathy Wolfe and her team, and Georgios Amanatidis (EC Research DG) and Mike Kurylo (NASA HQ).