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Stratospheric Processes And their Role in Climate
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| Home | Initiatives | Organisation | Publications | Meetings | Acronyms and Abbreviations | Useful Links |
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Stratospheric Processes And their Role in Climate
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| Home | Initiatives | Organisation | Publications | Meetings | Acronyms and Abbreviations | Useful Links |
In an overview lecture, A.R. Ravishankara (Aeronomy Lab., NOAA, Boulder, USA) discussed the role of gas phase
and heterogeneous/multiphase reactions in the atmosphere, with particular emphasis on the interplay between the reactions
in these media. Following talks dealt with the atmospheric processing (or ageing) of aerosols in the atmosphere as observed
in the field, or simulated in the laboratory or in numerical models.
J. Abbatt (Univ. of Toronto) presented new studies of tropospheric ice chemistry in which the interactions of a number
of trace gases with laboratory ice surfaces have been observed. It appears that sulphur dioxide and hydrogen peroxide interact
poorly but react together on ice to form sulphuric acid. These reactions may be non-negligible in the presence of moderately
thick ice clouds. P. Behr et al. (Univ. of Essen and Wisconsin) also showed molecular beam studies directly
useful for a better understanding of the aerosol-mediated decomposition of HCl and HBr on sulphuric acid aerosols.
Such studies have the potential to bring the capture of many molecules. Ion induced aerosol formation in the upper troposphere
(UT) was reviewed by F. Arnold et al. (MPI Heidelberg) with an emphasis placed upon findings obtained
in aircraft based and laboratory measurements. They measured the mass distribution and composition of ions in different
UT environments including cloud free air masses, clouds and exhaust plumes of jet aircraft. These measurements clearly
show cases of rapid ion growth leading to large ions, which represent already stable aerosol particles.
C. O'Dowd (Univ. of Ireland and Helsinki) discussed recent advances in elucidating new particle formation in the marine
and coastal boundary layers. It is speculated that iodine oxide over the open ocean may also provide the material for the
production of new particles of detectable sizes. T. Koop et al. (ETH Zurich) also discussed the microphysics of
sea-salt aerosols at low temperature suggesting that they are most likely liquid most of the time under polar marine boundary
conditions. R. Sander et al. (MPI Mainz) then discussed the existing published and unpublished data sets
showing widespread halogen activation. They compiled measurements of bromine in aerosols particles and in the gas phase
at marine locations at low and mid-latitudes.
These presentations were followed by a short but lively poster session.
It was decided to hold a similar session at the EGS assembly next year with more time devoted to posters.
The session was supported by NERC (UK). Conveners were: Th. Peter,
Institute for Atmospheric and Climate Science, ETH Zürich, Switzerland, (
Thomas.Peter@ethz.ch), http://www.iac.ethz,
K. Law, Department of Chemistry, University of Cambridge, U.K., (
Kathy.Law@atm.ch.cam.ac.uk),, http://www.atm.ch.cam.ac.uk/, A. Bregman, IMAU, Utrecht University, The Netherlands, (A.Bregman@phys.uu.nl), http://www.fys.ruu.nl/~wwwimau/
The session included three sub-sessions: 1. Photochemical processes, 2. Chemical observations and mixing processes, 3. Heterogeneous chemistry, aerosol and cloud processes.
The first sub-session, focussing on processes studied by laboratory measurements and by chemistry-transport modelling,
was started by R. Zellner, who stressed the general importance of acetone photolysis for the HOx budget in the tropopause
region. Besides the commonly assumed products CH3CO and CH3, a second pathway is possible from
the excited CH3CO, leading to 2 CH3 radicals and one CO molecule. Although the quantum yield of this
process is almost unity below about 230 nm, it almost drops to zero beyond 270 nm, so that this photolysis pathway gives only
a minor contribution. N.M. Donahue presented laboratory studies on the reaction NO2 + OH and showed that
the formation of several metastable products is possible, with HONOO as the most favourable. However, no evidence of these
products was found in the experiments, basically corroborating the JPL recommendations. Balloonborne observations of IO at
Kiruna and Gap in 1999 were presented by H. Bösch. Taking proper account of solar centre-limb darkening
corrections reduces the fraction attributed to IO absorption significantly. They deduced an upper limit in the lower stratosphere
of 0.06-0.99 pptv at Kiruna and 0.14 pptv at Gap. Th. Röckmann investigated N2O isentope
fractionation. Photochemical processes lead to altitude dependent isentopic enrichment of heavy isotopes that provides
information on the relative contribution of photochemical and transport processes in the stratosphere. Comparisons between
balloonborne observations and results from the Mainz global 2D chemistry-transport model showed an underestimation of the
isotopic enrichment by the model possibly caused by model transport deficiencies. M. Dameris presented model results
of the chemical effect of decreasing the cruising altitude of the subsonic air traffic by 1 km. In the tropopause region this
resulted in a decrease by 30 % in NOx , and in the free troposphere in a 10 % increase.
The second sub-session was devoted to measurements in the tropopause region. J. Baehr showed NO, NOy,
O3, and CO measurements both in the northern and southern hemisphere (SH and NH) as part of the INCA campaign.
She showed concentrations in the SH to be on average a factor of 3-5 lower than in the NH. Enhanced levels were observed
in the free troposphere in the ITCZ. D.E. Oram discussed canister samples in the free troposphere of a variety of halons
from aircraft flights between Frankfurt and Johannesburg as part of the CARIBIC program. He demonstrated that several
CFC substitutes increased between 1998 and 2000. The total bromine budget was estimated as 22-24 pptv. Evidence was
further given of biomass burning from the correlation between CO and CH3I. M.O. Koehler presented
an ozone climatology based on MOZAIC between 1994 and 2000. Especially in the relatively warm winters 1997/98 and
1998/99 there was more ozone in the mid-latitudes at the 340 K isentropic level. A comparison with SLIMCAT revealed
the model to underestimate ozone during these winters due to ozone loss calculated at higher latitudes.
W.J. Collins et al. showed result from a Lagrangian model experiment including 105 air parcels and 70
chemical species. Their approach allows to distinguish shallow stratosphere-troposphere exchange processes from
deep exchange. S.A. Penkett gave an overview of the results from the ACTO and EXPORT aircraft measurement
campaigns. The observations included a large set of different species and particles, yielding evidence of anthropogenic
combustion products in the free troposphere, transported upwards by convection. The discussion of ACTO was deepened
by J. Methven illustrating how these observations can be used to identify the origin of the air masses.
A. Zahn introduced the chemopause as a measure of the tropopause, derived from the O3-CO relation
observed on the CARIBIC flights. He showed that the chemopause exhibited a significant seasonality with altitudes
varying between 80-120 ppbv. E. Pavelin showed results from LIDAR and airborne tracer measurements
in the tropopause region above Wales. They traced a significant variation in ozone correlating with wind gusts.
From the observations they could deduce an eddy diffusivity of 1.6 m2 s-1 in gravity
waves and 0.05-1.1 m2 s-1 in the jet stream.
The final sub-session covered heterogeneous chemistry in observations and modelling. R. Salawitch highlighted the
conflicting interpretation of lower stratospheric HCl measurements during the last decade. The observations seem to indicate
a recovery of HCl after mount Pinatubo eruption in 1991, while another interpretation of the same data set concluded no change
in HCl. He also demonstrated an imbalance in inorganic chlorine (Cly), based on recent airborne campaigns (SPADE, SOLVE).
J. Hendricks showed box model results of heterogeneous chemistry on cirrus cloud particles in the upper troposphere.
By introducing periodic cirrus events he concluded that the fate of HNO3 taken up on the ice particles is central
for the calculated ozone loss. G. Pitari presented model intercomparisons of the effect of heterogeneous chemistry
on ozone by assuming a double aerosol density produced by increasing subsonic air traffic. A substantial decrease of the
current radiative forcing estimate must be expected, resulting from heterogeneous chemistry. H. Ziereis discussed
NOy uptake on cirrus cloud particles, based on the INCA observations. For surface areas of 100-1000
mm2/cm3 more than 95% of the NOy resided in
the gas phase, whereas with larger surface areas (> 5000 m
m2/cm3) 20-30 % uptake was observed. The estimated fractional coverage was less then
one monolayer. Finally, A. Thomas showed particle profiles in the tropical region, observed during APE-THESEO,
February 1999. Ultrathin high level clouds were detected at 17-18 km altitude, close to the tropical tropopause, with
particle diameters around 10 mm. The effects of these clouds, e.g. on
dehydration, are not yet well understood.
The three sub-sessions were corroborated by a common poster session featuring 14 contributions, spanning the full
spectrum of investigations in the UTLS, from 3D modelling work, via lab studies to field observations.
The 1,5-day session began with several talks on the issues surrounding the Arctic Oscillation: is it a 'physical' mode,
in the sense of having an intrinsic dynamical identity, or is it just a statistical construction? Does the downward phase
propagation seen in several papers represent downward influence or is it purely an artefact of differing response times?
Studies using a variety of approaches, from simple 1D models through to comprehensive statistical studies of NCEP and
ECMWF data showed many new insights, but a comprehensive consensus remains elusive. Two presentations showed that
the downward propagating signal was associated with strongly disturbed winters. Another study showed the characteristic
downward phase propagation to be associated with a coherent 4-month cycle in EOF's 1 and 2 of the zonal mean zonal wind.
Studies of transport emphasised the importance of chemistry-dynamics feedbacks in the dispersion of volcanic aerosols and
the importance of regional distribution of troposphere to stratosphere exchange in determining the water vapour budget of
the stratosphere.
Work on the influences of solar variability emphasised the importance of resolving the differences in variability in different
spectral bands. One study predicted that the Antarctic ozone hole would persist beyond 2050.
A mechanistic study of polar stratospheric warmings found surprisingly strong sensitivity to the zonal wind in the upper
(40-50km) tropical stratosphere. The modulation of stratospheric mixing associated with interannual variability in the
troposphere was described.
In the sub-session on gravity waves (GW) new results suggesting the existence of a significant source of GW in the southern
winter polar stratosphere were presented. The observed peak in gravity wave energy in the tropics was discussed.
Another study showed that GW can force planetary waves in the mesosphere. Measurements by the Egrett aircraft over
Wales gave very high-resolution information over the structure of mountain waves and the turbulence caused by their breaking.
A parameterisation of GW which reproduces the k-3 saturated spectrum was described.
The poster session described sensitivity of the Brewer-Dobson circulation to CO2 and sea surface temperature changes,
analysed discrepancies between ground based and space based water vapour measurements, described a modelling study
of the QBO and gave an outline of the resources provided by the British Atmospheric Data Centre.
A more detailed report is available at http://www.atm.ox.ac.uk/user/juckes/egs01_st201_rep.html.
Owing to the nature of the session, a wide variety of topics were covered. T. Neubert et al. presented results on the first European sprite campaign; there were also presentations on the importance of CH3CN and HCN in the middle atmosphere. R. Salawitch (and colleagues) from JPL discussed the importance of HNO4 for HOx chemistry. Two presentations on both theoretical analysis and on measurements of isotopic enrichment of stratospheric N2O were made by a group of scientists from the Max-Planck Institute for Chemistry in Mainz. Several satellite measurements were discussed: M. Riese et al. discussed the modelling of nitrogen species from CRISTA, S. Tilmes et al. showed an analysis of chemical ozone loss in the Arctic in 1996-97 from ILAS and HALOE, and very recent results from the winter 2000-2001 from GOME were presented by M. Weber of the GOME team in Bremen. GCM results on the coupling between climate and ozone chemistry (C. Brühl et al.) and CTM results on the interaction on mixing and chemistry were presented (Y. Orsolini et al.). Further foci were PSC and H2O observations as well as several papers on ozone measurements from mid-latitudes, and from Norway (ALOMAR, Andenes).
This year sub-session on 'Global Ozone' attracted 13 oral contributions and 15 poster presentations. The audience of
about 50 scientists listened to an interesting program with lively discussions.
Five papers were given on model calculations to investigate the feedback of dynamical, chemical and radiative processes.
A. Jrrar (Cambridge Univ., UK) studied the dynamical contribution of the downward trend in northern mid-latitudinal
ozone with the SLIMCAT model and by a statistical trend analysis. M. Dameris (DLR, Germany) presented an
interactive coupled chemistry-climate model to estimate the development of dynamical and chemical key parameters.
Another Cambridge University study of P. Braesicke and J. Pyle was assessed to how the tropospheric
forcing is related to the total ozone winter increase in high latitudes. G. Pitari (Univ. of L'Aquila, Italy) investigated
the impact of stratospheric sulphate aerosols in relation to the ozone recovery rate. C. Brühl
(MPCH Mainz, Germany) gave an overview on 20 years 'time-slice' experiments with a fully coupled CCM to investigate
the future of the Arctic ozone behaviour.
Two papers showed measurements of total ozone and its interannual variations in high latitudes (C. Basdevant and
H. Teitelbaum, LMD-IPSL, Paris, France) and over Tibet (Zou et al., Chinese Academy of Science,
Beijing, China).
Two papers dealing with GOME total ozone observations were presented. H. Shets (Royal Met. Inst., Belgium)
discussed a comparison with ground-based observations, and A. Ortenzi (Univ. of Tor Vergata Rome, Italy) analysed
the November 1999 ozone hole over Europe. Another November 1999 mini hole investigation based on POAM III data was
given by L. Hood (Univ. of Arizona, USA).
A statistical approach to ozone was introduced by R. Toumi (Imperial College, London, UK), who discussed in his
presentation the non-Gaussian or Gaussian statistics of time series of ozone and temperature.
The last two presentations showed measurements of tropospheric ozone. R. Zbinden (Observatoire
Midi-Pyrénées, France) gave an interesting overview on the measurements made on 15,000 flights
of normal A340 aircraft between 1994-2000 in the frame of MOZAIC. G. Laneve (Univ. of Rome, Italy)
discussed the importance of ozone data from the African continent which can contribute to our knowledge of the tropical
atmosphere.
The poster session was very well attended. 15 posters were presented both on ground based measurements and on model
based studies.
This session aimed at the presentation of first consolidated results from the APE-GAIA (Airborne Polar
Experiment-Geophysica Aircraft In Antarctica) field campaign carried out over the Antarctic peninsula, to investigate
lower stratospheric chemistry and transport at the boundary of the southern polar vortex. The campaign was based in
Ushuaia (Argentina) and performed in September-October 1999.
An overview was given by U. Cortesi (IROE-CNR, Italy). He recalled the scientific objectives and the main components
of the mission funded by the Italian National Programme for Antarctic Research (PNRA) with contributions from nine
countries involved in aircraft measurements, meteorological and modelling support and other co-ordinated activities.
A synopsis of the five Antarctic flights was provided, along with a summary of results from both airborne observations
and ground-based or balloon-borne correlative measurements.
The subsequent presentations focussed on the findings from individual instruments and on specific events.
M. Hoepfner (ForschungZentrum Karlsruhe, Germany) reported about limb-sounding emission measurements
of ClONO2 and HNO3 performed by the Mid-infrared spectrometer MIPAS-STR during
three flights at the boundary and inside the Antarctic stratospheric vortex. During the 990923 (23rd September
1999) and 991012 flight the inner part of the vortex was reached, with clear evidence of denitrification. On 991002,
the aircraft entered a region with very low temperatures due to mountain wave activity over the Antarctic peninsula
at the vortex edge where dense PSC were observed. By correlation of MIPAS HNO3 and ClONO2
data with tracers, the amount of de-nitrification and its vertical extent below the aircraft could be estimated. On 990923
the denitrification was observed above 15km and on 991012 above 14km.
An important case study, aiming at cross-validation of lower stratospheric composition measurements performed by
the two limb sounders onboard the M-55, was presented by C. Blom (ForschungZentrum Karlsruhe, Germany).
Intercomparison of 2D cross sections for ozone measured by SAFIRE-A and MIPAS-STR during the flight of 990923
highlighted, within a good overall agreement, specific discrepancies that require further analysis.
Results obtained by the GASCOD-A4p UV/Vis spectroradiometer were discussed
by I. Kostadinov (ISAO-CNR, Italy). The instrument, operating in the 280-850 nm spectral range, performed
remote-sensing measurements of O3, NO2 and BrO slant column by applying the DOAS methodology.
Quasi in-situ values for the same gases were calculated by using a new retrieval code developed at ISAO.
GASCOD-A4p observed also 2p
up-welling and down-welling solar irradiation, thus measuring the actinic flux and calculating the photolysis rate of NO2
along the flight.
G. Redaelli (Aquila, Italy) discussed the modelling tools used to identify transport related tracer variation during
the flights. Based on such products, a filament of vortex air had been correctly forecasted for the 991008 and then sampled
with great detail at different altitudes, by in situ and remote sensing instruments onboard the M-55. He presented
also a work by M. Volk (J.W. Goethe-Univ. Frankfurt, Germany), on the ozone loss and mixing, as derived from
M-55 in-situ observations of tracers and ozone. While nearly complete ozone loss was found only inside the vortex, ozone
loss up to 60% was found routinely even outside the region delineated by the sharp tracer gradients. F. Cairo
(IFA-CNR, Italy) discussed the principal results on PSC observed by the M-55 aerosol payload, in comparison with synoptic
and mesoscale modelling.
Posters addressed both technical and scientific APE-GAIA issues. One poster focussed on the stabilisation of the line of sight
implemented for the MIPAS-STR and on its validation from atmospheric spectra measured during the 990923 flight.
An estimate of the total inorganic chlorine budget and partitioning, based on simultaneous measurements of CLONO2 by
MIPAS-STR and of HCl and ClO by SAFIRE-A, was shown. Finally, in situ measurements of ozone by the fast
chemiluminescence sensor FOZAN, and water vapour by the fluorescent Hygrometer FLASH, were described.
These data were used for the detection of the chemical boundary of the ozone-depleted region and a filament out
of the vortex edge.