lCtb /190

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BELGIAN IMPULSE PROGRAMME "GLOBAL CHANGE"

1990-1995

Federal Office for Scientific, Technical and Cultural Affairs

Contracts no. GC/35/002, no. GC/12/003 and no. GC/11/004

SPECTROSCOPIC MEASUREMENTS OF ATMOSPHERIC CHANGES (SMAC)

FINAL SCIENTIFIC REPORT Edited by Paul C. Simon

Institut d' Aeronomie Spatiale de Belgique/Belgisch Instituut voor Ruimte-Aeronomie (IASB/BIRA)

3 av. Circulaire/Ringlaan 3 1180 Bruxelles/1180 Brussel Institut d' Astrophysique Universite de Liege (ULG) 5 av. de Cointe

4000 Liege

KoninKlijK ~elgiscn ln~tituut

~ voor Ruimte-Aeronomie

Laboratoire de Chimie Physique Moleculaire Faculte des Sciences

Universite Libre de Bruxelles (ULB) 50 av. F.D. Roosevelt

1050 Bruxelles

ci

Rlnglaan 3 CC 11 80 Ukkel

iii

www.aeronomle.be

April 1996

.

GEBOEKT IN INVEITTAR!S .... ~ ...

ONDCR NUMMER lCtb

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Without

Global Change Research There is no

Sustainable Development Prof. Harmut Grassl

Chairman ofWCRP

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On account of human activities, the chemical composition of the Earth's atmosphere is changing at an increasing rate since the industrial revolution. The changes in concentration of the minor and trace constituents of natural or anthropogenic origin are modifying the radiative equilibrium are and the stratospheric ozone budget. A major issue for global change is, therefore, the quantification of these changes and their impact on the biosphere.

The evaluation of global atmospheric changes and of their consequences must necessarily rely on precise and accurate measurements of trends and to the comprehension of the processes which determine the complex physical chemistry involved. Without precise long-term measurements, predictions of global changes based on models are impossible. The means to obtain these experimental data are, for a very large part, based on the use of the molecular spectroscopy in the field and in the laboratory.

The principal objectives of the SMAC project are to provide the earliest detection, by spectroscopic field measurements, of changes and trends occurring in the chemical composition of the atmosphere and to determine, in the laboratory, by experimental and computer techniques, the spectroscopic properties of the atmospheric constituents which govern these changes.

The first part was achieved by daily, seasonal and yearly monitoring of temporal variations of several stratospheric constituents for the purpose of understanding the aeronomical processes that govern these variations. A validation of various spectroscopic techniques was necessary in order to reduce the inherent uncertainties of each technique. In addition, such systematic measurements allowed to validate the numerical models used for trend studies and measurements to be carried out in the current decade by satellite (UARS, GOME, ... ). Satellite measurements are adequate to give a global coverage but need high quality ground-based measurements for validation purpose due to the instrument degradations in orbit.

The need for ground-based stations spread over the planet is therefore essential for stratospheric monitoring. The organisation of a world-wide network ("Network for the Detection of Stratospheric Change", NDSC) was initiated in Geneva, in November 1989, during a meeting organised by the World Meteorological Organisation and the International Ozone Commission. It was recognised that the first station of this network namely the Alpine Station was already operational and was a typical model for the new ones. This Alpine station includes the "International Scientific Station at the Jungfraujoch" (ISSJ, Switzerland), the

"Observatoire de Haute Provence" (OHP, France) and the Plateau de Bure (France) which together are responsible for mid-latitude monitoring activities in the northern hemisphere. The Belgian institutions involved in SMAC performed measurements at these sites, but primarily at the ISSJ.

In addition, tropospheric measurements using long-path absorption were carried out in

an urban site at the "U niversite Libre de Bruxelles" in the visible and ultraviolet region. The

results of this technique were compared with the conventional in-situ ozone monitoring

instruments and the various instruments used during SMAC were also intercompared using

the long path set-up.

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2 The quality of the measurements obtained in the field depend directly on the quality of the basic spectroscopic data used in their evaluation. It is therefore important, and in some cases urgent, to measure in the laboratory the spectroscopic parameters such as position, intensity, width and temperature dependencies of the absorption lines used to detect, identify and measure the atmospheric molecules. Such data are also indispensable to evaluate numerical simulations, and the role of these molecules in the potential warming of the planet (greenhouse effect). These aspects were mainly developed by means of Fourier transform spectroscopy and computer simulations of spectra under atmospheric conditions.

The support from the "Global Change Impulse Program" allowed an extension in time, quality and quantity of the contributions of three Belgian groups to the problem of the measurement of the changing chemical composition of the atmosphere. The support allowed the groups to be integrated more readily into the international networks set up at the end of the eighties.

The funds received contributed mainly to maintain or increase the scientific and technical staff in each group and ensure a continuity in the observations and experiments. Part of the funding was also used to refurbish and complete the instrumentation (most of which has been acquired very recently) in the laboratories and at the Jungfraujoch.

The SMAC project was proposed and led by Prof. Paul C. Simon (IASB/BIRA), Prof.

Luc Delbouille (ULG) and Prof. R. Colin (ULB). Many scientists from these three institutions contributed to its achievement. The lists of publications and communications acknowledge their contributions. This report give an overview on the results obtained by atmospheric spectroscopy in the infrared and UV-visible ranges, with the necessary instrument characterisation and validation tasks and the data analysis developments as well.

The Principal Investigators of this project acknowledge the contribution of Drs. R.

Zander (ULG), M. De Maziere, A.-C. Vandaele and M. Van Roozendael (IASB/BIRA), and M. Carleer (ULB) for their valuable contributions in the preparation of this final report.

Part of the budget necessary to acquire new instruments, to maintain the equipments up-to- date and to make new technical developments, came from the "Fonds National de la Recherche Scientique" (FNRS), through grants and contracts of the "Fonds de la Recherche Fondamentale Collective (FRFC), from and the "Loterie Nationale/Nationale Loterij"

(LOTTO). The ULG and ULB participated to the equipment acquisition through their "Fonds Special de la Recherche".

The European Arctic Stratospheric Experiment (EASOE) and the Second European Stratospheric Arctic and Mid-Latitude Experiment (SESAME) campaigns were essentially supported by contracts with the European Commission (EC, DG XII) and partly by the

"Nationaal Fonds voor Wetenschappelijk Onderzoek" (NFWO), including the observations at the ISSJ. "~.

Other pro};cts funded by the EC (the ESMOS and SCUVS series) contributed significantly to

the success of the SMAC project.

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Scientific teams:

IASB/BIRA

P.C. Simon, Co-ordinator M. De Maziere

M. Dymek

C. Fayt (Universite de Mons-Hainaut) 0. Hennen

C. Hermans Y. Kabbadj J.-C. Lambert P. Peeters A.-C. Vandaele M. Van Roozendael ULG

L. Delbouille, promotor P. Demoulin

E. Mahieu F. Melen G. Roland C. Servais R. Zander ULB

R. Colin, promotor M. Carleer

C. Clerbaux

J.-M. Guilmot

S.Fally

M. Herman

D. Hurtmans

J. Vander Auwera

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4 ABSTRACT

This final report summarises the act1v1t1es, both observational and analytical, performed in the frame of the project Spectroscopic Measurements of Atmospheric Changes (SMAC) which itself is part of the Impulse Programme "Global Change" funded by the Office of Scientific, Technical and Cultural Affairs (OSTC). It has been conducted jointly by the Belgian Institute of Space Aeronomy (IASB/BIRA), the "Universite de Liege (ULG) and the "Universite Libre de Bruxelles (ULB) in accordance with tasks outlined in the contracts nr. GC/35/002, GC/12/003 and GC/11/004.

The objective of the SMAC project was to use the absorption spectroscopy to understand and quantify the natural and anthropogenic changes which affect the chemical composition of the Earth's atmosphere.

Within that general frame, the ULG was responsible for the study of the chemical composition of the atmosphere through infrared remote observations carried out at the International Scientific Station of the Jungfraujoch (ISSJ), Switzerland, using the Sun as a source of radiation. These infrared observations have been made using two Fourier transform spectrometers specially tuned for high-resolution operation in the atmospheric "windows"

extending from 2 to 5.5 µm and 8 to 14 µm. Their analysis and interpretation have contributed significantly to the timely expansion and strengthening of similar investigations carried out prior to 1991, allowing to better assess short term changes such as seasonal modulations, to refine and/or appreciate trends primarily caused by human activities and life style of a growing population, and to evaluate the influence of unusual situations such as the heavy aerosol loading in the stratosphere during 1992-93, that resulted from the Mt. Pinatubo volcanic eruption in June 1991.

While the pre-1984 (starting in 1950) observations at ISSJ were primarily performed with high-resolution grating spectrometers, the demand for multiple-species measurements within the same air-masses led to their substitution by state-of-the-art Fourier transform spectrometers able to record at once wide spectral intervals at very high resolution and to achieve high signal-to-noise ratios within reasonable scanning times.

Altogether, 20 telluric constituents have been investigated quite regularly, over more than a decade for many of them. An overview of findings such as species rates of change (in

% per year) over various time intervals, mean vertical column abundances for 1994 (in number molecules/cm ² ⁾ and specific remarks about them is provided in Table I on a molecule-by-molecule basis. This table is not updated for 1995, because the last year's data have not all been validated thoroughly. However, results extending to December 1995 are reported in section 2 of this report, including secular trend estimations, budget evaluations and, when possible, likely explanations of the findings

Since 1989, the ISSJ is acting as the northern mid-latitude NDSC (Network for the

Detection of Stratospheric Change) primary "Alpine Station" for infrared observations, while

the French sites of Haute Provence and Plateau de Bure are more specifically responsible for

Lidar and microwave investigations. In 1990, the ISSJ was further equipped with an UV-

visible instrument (SAOZ) whose operation, under the responsibility of IASB/BIRA, was

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Molecule Period covered 1977-1982 1983-1988 1989-1994

Expon. Trend (%/yr)(+l 4.22 ± 1.42

8.20 ± .65 2.87 ± .42

1994.0 Mean Col.

(molec./cm

²)

3.70 ± .05 EIS

Remarks

Seas. var.± 6% ; max. 04-05; large variability in 0 1-04; tropop. related.

··iiji•Y··· ···i·c/j1·~·i"9·s2··· ···i"o:9··±·jio··· ···i"·.·i"o·;;;··."fii·E·i°s··· ··s~·~;:··~;;·;;;··1··ci;··~·~;·.··o3~04;··i·;~·i~···•·

HF/HCI

co

1983-1988 8.77 ± .82 variability in O 1-04; tropop. related

1989-1994 5.96 ± .50

1951-1986 1985-1989 1990-1994 1950-1987 1984-1994

0.70 ± 0.10 0.73 ± 0.13 0.47±0.11 0.85 ± 0.20 -0.55 ± 0.45

1950-1984 0.23 ± 0.04

1985-1989 0.75±0.17

1990- I 994 0.09 ± 0.21

2.25 ± 0.05 El9

1.22 ± 0.05 EIS 4.00 ± 0.02 EIS

0.19 in 1980; 0.30 in 1994 extreme 0.6 in winter 91-92 Seas. var.± 3-4%; rnax.

09- I 0; tropop. related.

Seas. var.± 20%; max. in March Seas. var.± 3-4%; rnax. 08-09;

tropop. related.

··· ··· ··· ···•··•··· ···

C

₂

H

₆

1951-1988 0.85± 0.30 l.05±0.05El6 Seas.var.±25%;max.03-04;

... I.?..?.?.:.!.?..?.1. ... ~~.-.~.3-.. ^~ . .9:.?.\ ... !.<1!:8~ .. \/.<1r.t~.~i.~i~_Y. . .\t1 .. 9.1.~.Q1. ... .

C

2

H

2

1986-1994 -0.72±2.3 l.84±0.l6El5 Seas.var±40%;rnax.0l-02;

... !.<1!:8~ .. \/.':1!.i~~\.l.\~.Y. .. i.11 .. ~.i.11.t~r. ... . CIONOtl I 986-1994 4.0 ± 0.7 1.19 ± 0.10 E 15 Large variability;

absolute column uncertain± 15%

CHCIF

2

1986-1989 9.60 ± 0.80 1.55 ± 0.03 El5 No seas. var.; highly consistent;

1990-1994 4.60 ± 0.40 absolute column to ± I 0%

··· ··· ··· ··· ···

... c=.q7.f7. ... l.?..8-?.:.1.9-.?.1. ... ... .3-:.3-5. .. ~.:.2-9. ... ... 6-:.?.9 .. ~.9.:9.?. .. ~.1.5-... A~.~.o.!.~~e..~.o.1.~~.i:i .. t.o. .. ~ .. !.?.0.'?. ... .

... <=.~.!Jf ... !.?..8-~.:.!.9-.?..2. ... i11 .. P.ro.g.r~·~·~··· ... A~.s.o..l.~~.e. .. ~J?.~~.~:.O.~~.O.J?.Y. .. ~() .. ~ .. 2.9.% ... .

SF

6

1986-1990 6.90 ± 1.40 ('90) 3.95 ± 0.50 No seas. var.; highly consistent;

El3 absolute column to± 15%

.. °iIN0;<·· 5··· ···i·9·5·i·~·i·9·92··· ···~·;~·~··· ···i·:3·5··±·0:·i·s··E16··· ··s~~~·:·~;~:··±··i"s·%:-·;;:;~;:··~;-~t·~~;···

... ... 1.?..?.~.:}.9-.?.1. ... ... 1.} .. ~ .. ?·.~··· ... !.<1r.8.e .. \/.<1r_..; .. ~.P~.C.~r.().s.c.9_p>:_.~.O. .. t .. l.9.'.!°. ... . NO/l 1984-1994 none 3.50 ± 0.50 E 15 Seas. var up to± 40%; max. 06-07;

... l.<1:.8~ .. \/.<1r.i.~~.i!.i~.Y. ... . H0

2

NOtl 1984-1994 in progress Spectroscopy in progress; weak

feature

···No<•i··· ... i'9.84~··i994··· ···~·;;~··· ···4:40·±·0:3·6·E·is··· ··s~·~;:··;;;:··~~d··~;;;·~b;-i;·ty··;;·g~·;t~~;~t;····

···•··· ... ··· ··· .. ~<1.~: . .9?.~.9.8.;···••···

.. 9..<=.~ ... ... 1.?..8-~.:.!.9-.?.1. ... ~.9.:3-.! ... ~ . .9:.!.:? ... ?:~.~ .. ~ .. 9.:9.5-.. ~.l? ... ?.i.Sr.i.\(!~.a.11.~ .. v.<1:,\<1.?.\.'.\.tY. ... .

... ~.<=.~ ... 1.?..8-1.:.1.9-.?.1. ... 9.:9.~ .. ~ . .9.· .. ~.2. ... .3-:9.? .. ~.9.:.!.~ .. ~.1 .. 5-... ~.9 .. '.!c. .. h.!.8~.e.r .. ~().\11.~.11.s .. i11 .. ~P.r.i.11_g ... .

H

2

CO 1984-1994 in progress 4 ± 2 E 15 Weak features; interferences

··· ... ··· ... S.r.r.\11.8 ... 1.?..~.?. ... ···

... ~;;~97 ... ... 1.?..8-1.:.!.9-.?.1. ... ir.1 .. P.~?g.~~.s.s ... ~J?.~~.~'..0.~~.0.P.Y. .. ~11.C.~.rt_a.i!1.i .. i.11.t~.rre.:.e.11.C.~~ ..

0

3

1984-1994 -0.6±0.2 8.0±0.1El8 Seas.var.±15%;

rnax variab. during O 1-03

··coF~ 1•5··· ···i·9·s4·~·i'9.94··· ···4·.·6·±··0."i··· ···2:9i±·o:4·s°·E14··· ··w~~k·i~·;i~~~;;··i;t~·~r~;~~~~·~···

•·r+Y·f ~~~·d;··~pd;t~d·x~i··i'994 ··· ··· ... .

<*l

Molecules primarily of stratospheric origin

Table I : Summary of Jungfraujoch related trends investigation

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6 accepted soon after as an NDSC entity for NO 2 and 0 3 measurements,.

At the ISSJ, 0 ₃ and NO ₂ total column amounts have been continuously monitored with the SAOZ instrument in operation since June 1990 during the morning and evening twilights by application of the differential absorption method using the sunlight scattered at zenith in the visible range .. The available time-series by now covers 6 years, of which one before the eruption of Mt. Pinatubo. Besides this, OCIO was observed for the first time in January 1995 at mid-latitude with the SAOZ instrument.

SAOZ total ozone measurements have been compared to Dobson and/or Brewer data at seven different sites at mid- and high latitudes. The results show a better agreement for the mid-latitude stations. In average mid-latitude SAOZ measurements agree with other instruments data within I% with a scatter of about 5%. This scatter can be explained by the different observational geometry of the instruments and by uncertainties in the SAOZ AMF.

There might be an additional contribution due to tropospheric multiple scattering in clouds that still need to be quantified. SAOZ/Dobson and SAOZ/Brewer ratios are characterised by seasonal variations with peak-to-peak amplitudes going from about 5% at mid-latitude to a maximum of 15% at Faraday, Antarctica. At mid-latitudes, the amplitude and phase of this seasonality is shown to be consistent with SAOZ AMF uncertainties. For the high latitudes, the same conclusion is likely to apply although the temperature sensitivity of the Dobson and Brewer absorption coefficients needs to be taken into account.

The available time series of NO 2 shows a significant reduction starting in winter 1992, after the eruption of the Mt. Pinatubo volcano. A maximum decrease of about 35% is observed in January 1992 at both stations. The continued series of observations shows the recovery of the NO

2

column until August 1995. These results are compared with 2-D chemical model calculations including the effect of heterogeneous reactions on observed Pinatubo aerosols. In general the modelled NO 2 columns agree qualitatively with the observations although the amplitude of the seasonal variation is underestimated, possibly due to internal limitations of the model which e.g. does not include diurnal changes. The observed and calculated NO 2 percent changes are in good agreement which confirms quantitatively the impact of the heterogeneous chemistry on stratospheric N0 _2.

The ISSJ investigations provided inputs to special campaigns organised by the European Commission, i.e., EASOE (European Arctic Stratospheric Ozone Experiment) and SESAME (Second European Stratospheric Arctic and mid-latitude Experiment)

Measurements of ozone and nitrogen dioxide total amounts were also performed by ground-based visible spectrometry at Keflavik, Iceland (64°N) during EASOE. NO 2 column amounts below l x 10 ¹⁵ molec/cm ² were measured between December 1991 and mid-February 1992, the lowest values being reached inside the polar vortex during the coldest period of the campaign. The usual correlation between sunset NO 2 total columns and 20 hPa temperatures was not observed during the warming of the stratosphere in February. This indicates low N 2 O 5

amounts outside the vortex, possibly related to heterogeneous reactions on Pinatubo aerosol particles. However the low NO

2

contents observed could be attributed partly to the advection of air-masses from the polar night region.

Within the scope of the Second European Stratospheric Arctic and Mid-latitude

Experiment (SESAME), stratospheric OClO, BrO, NO 2 and 0 ₃ were measured by ground-based

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UV-visible spectroscopy in Harestua, Norway (60°N), in addition to the ISSJ observations. The measurements were carried out from the middle of January until the end of March 1994 (SESAME phase I) and from the beginning of November 1994 until the end of March 1995 (SESAME phase III). During January-February 1995, the NO 2 and 0 3 measurements were complemented by direct Sun observations of the same constituents using a Fourier transform UV-visible spectrometer. During both winter periods, significant OClO signatures were observed in January and March. As expected, the abundance of OCIO is correlated to the potential vorticity at 475 K, the largest OCIO columns being seen inside the polar vortex. There is a general anti-correlation between OCIO and NO 2, large OCIO contents appearing to be mainly driven by the stratospheric temperature.

The reliability of the results obtained by infrared and UV-visible absorption spectroscopy can only be achieved by an adequate validation programme covering both quality assurance and quality control for the instrumentation and the retrieval methods dedicated to stratospheric observations. Significant time was further devoted in instrumental characterisation and quality assessments, in particular by means of intercomparison campaigns, and in algorithms consistency evaluations.

The observations performed at the ISSJ also contributed to the validation of space experiments such as HALOE and CLAES on UARS, ATLAS 1, ATMOS and MAPS on Space Shuttle missions and finally, GOME on ERS-2, with also additional measurements made at Harestua after the SESAME campaign.

The measurement of tropospheric constituents was also investigated in the frame of this project. Two different methods were used, namely the Fourier transform spectroscopy in the infrared (FTIR) providing vertical total amounts of tropospheric species, and the Differential Optical Absorption Spectroscopy (DOAS) providing local concentration of tropospheric species for defined optical paths.

Table 1 gives some information regarding the molecules studied in the infrared region at the ISSJ. The target molecules detectable in the UV-visible by the DOAS method are ozone (0 3), sulphur dioxide (SO 2), nitrogen dioxide (NO 2) and trioxide (NO 3).

An optical path of 800 m for tropospheric DOAS was installed on the ULB Campus for urban pollution measurements, and two different instrumentations were developed or adapted for this type of measurement. One is based on a grating spectrometer with different detection systems. The second is a Fourier Transform Spectrometers (FTS) adapted for the first time to DOAS measurements.

The instruments were improved by using a variety of new type of detectors. Careful characterisation was carried out during the time-frame of this subproject. Several performances were improved e.g. in terms of measurement frequency. Dedicated softwares for instrument automation and data analysis were developed and improved, taking into account the specific problems imposed by each type of instrument and its associated detector.

Two intercomparison campaigns for DOAS instruments were organised, the first in

1992 in Brussels (Belgium) and the second in 1994 in Weybourne (UK).

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8 The measurements made during the first campaign by eight different instruments were carefully analysed and yielded an absolute accuracy limit for N0 2, 0 3 and S0 2 measurements made by the DOAS technique using an absorption path of a few hundred meters in a relatively non-polluted urban troposphere.

The second campaign compared seven different instruments through ten days of continuous measurement under field conditions. The comparisons concerned S0 _2, N0 2, 0 _3, N0

_3,

HCHO and HONO. The DOAS were compared with commercial point monitors for N0

2 ,

0

3

and S0

2

and a home-built monitor for HONO. The preliminary results are very encouraging although the full analysis of the data has not yet been completed. Two other comparisons were also made during the campaign. Firstly the DOAS were tested using cells containing known amounts of N0 2 and S0 _2. The agreement was to within about 7%.

Secondly the group's software was tested using synthetic spectra including N0

2 ,

0

3

and HCHO. Agreement was found within about a 5% error. This represents a considerable improvement on the results of the first comparison exercise.

These two campaigns demonstrated the necessity for quality assurance and quality control procedures in order to obtain reliable tropospheric data relevant for high quality scientific studies of the troposphere.

On the other hand, intensive laboratory measurements were conducted to support the

instrument developments and the retrieval softwares. Measurements of absorption cross-

sections of N0 2, S0 2, CS 2, and a series of CFCs, HCFCs, and HCFs, as absolute intensity

measurements in the IR of C

2

H

2 ,

HONO, N

2

0

4 ,

and COS were performed. In the case of the

CFC and related families, the integrated cross-sections have been introduced into a two-

dimensional radiative-chemical-dynamical model in order to calculated the global warming

potential of each gas.

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1. INTRODUCTION

During the second half of the 20th century, increasingly sens1t1ve techniques have allowed to characterise, with improved accuracy, the mean composition of the Earth's atmosphere which, beside its two "major" gaseous components N2 and 0 2, revealed the presence of a series of "minor" constituents as well as even less concentrated "trace" gases detected today down to the pptv (parts per trillion by volume) level. Along this investigation, but more specifically since the late 1950s, quantitative changes were noticed and monitored with a high degree of precision. These included modifications in the concentrations of naturally existing gases, primarily CO

2,

CH

4 ,

CO and N

2

O, as well as the appearance of new compounds. Among this last category, some are "sources" released to the atmosphere at the ground, while others are resulting from the breakdown of these sources into "sink" and

"reservoir" gaseous constituents, mainly in the stratosphere through photolysis by solar UV radiation.

Since the early 1970s, further understanding of the complexity of our environment (i.e., the importance of NOx, CIOx and BrOx catalytic cycles, ... ), coupled with experimental evidences have indicated that man-related activities many induced environmental changes likely to lead to adverse conditions for Life on Earth. The two most striking examples which, meanwhile, confirmed these worries, are the weakening of the protective ozone layer in the stratosphere and the enhanced greenhouse effect in the troposphere.

The anthropogenic destruction of ozone in the stratosphere was predicted in 1974 by Molina and Rowland, who showed that long-lived chlorinated source gases released at the ground (i.e., CFC-11/CChF, CFC-l 2/CChF2) can ultimately be transported to the stratosphere, the region of our atmosphere located between about 15 and 50 km altitude. At such altitudes, these complex molecules are photodissociated by solar UV radiation of wavelengths shorter than about 250 nanometers, thus freeing carbon (C), fluorine (F) and chlorine (Cl) in the direct vicinity of the ozone layer.

The chlorine atoms so released are then available to feed the ClOx cycle which proceeds according to the pair of catalytic reactions:

and

Net result:

Cl+ 03 ClO+O

➔ ClO+O2

➔ Cl+ 02

(1) (2)

At the end of reaction (2), the active Cl atom is released and the cycle can proceed catalytically with subsequent destructions of ozone (0 ₃₎ molecules. The cycle is slowed if Cl atoms gets neutralised momentarily into a chlorine "reservoir" (i.e., ClONO2, HOCI, ... ) or more permanently into a "sink", i.e., HCI. Monitoring the changes in the atmospheric HCl burden, thus provides a means of evaluating the impact of anthropogenic chlorine-bearing sources on the stratospheric chlorine loading and ultimately on the erosion of the ozone layer.

During the last decade, sufficient undisputed scientific evidences have been

accumulated that link the depletion of the protective ozone layer (including the ozone hole

over the Antarctic continent) to the increasing loading of stratospheric chlorine caused by

anthropogenic CFCs and HCFCs ... ; by the mid-1980s, they became so convincing that they

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10 led to the definition and implementation of scenarios for progressive phase-out of the most dangerous halogen-containing products through the Montreal Protocol of 1987 and its Amendments of London ( 1990), Copenhagen ( 1992) and Vienna ( 1995).

Notice here that the fluorine atoms also freed along the photodissociation of the CFCs are not involved in an ozone-destroying catalytic cycle, because they are quickly neutralised into hydrogen fluoride, HF, which is their ultimate, very stable sink gas in the stratosphere.

Monitoring HF (for which no potentially natural source is known), also allows to evaluate the amounts of CFCs brought into the stratosphere and photodissociated there. The detection of HF in 1974 by the ULG, and of HCl in 1975 by the IASB/BIRA both by means of balloon observations was an undisputed evidence for CFCs to have already photodissociated in the stratosphere at that time.

As identified by P. Crutzen in the early 1970s, the NOx compounds (i.e., NO and NO2) also participate in a catalytic cycle similar to that described above, i.e.:

and

Net result:

NO + 03 ➔ NO2 + 02 NO2 + 0 ➔ NO + 02

with main sinks/reservoirs being HNO3, N2Os, NO3, and ClONO2.

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The concern about an anthropogenic enhancement of ozone depletion via the NOx catalytic cycle results from the slow but steady increase of the main source gas N2O at the ground, by about 0.3 %/yr, which is primarily caused by the increasing use of nitrogen fertilisers in modern, forced agriculture.

Different destruction processes of ozone predominate in the lower stratosphere. Since the discovery of the dramatic seasonal depletion of ozone during austral springs over Antarctica, additional processes were needed to explain the rapid reduction in ozone concentration between 15 and 20 km altitude. They involve heterogeneous reactions of reservoir species at the surface of solid particles, the so-called Polar Stratospheric Clouds (PSCs). These particles are a mixture of nitric acid and water vapour or only pure water, depending on the temperature prevailing inside the Antarctic polar vortex during the polar night. The chlorine reservoirs like ClONO2 and HOCI are converted into active chlorine according to the following reactions (the letter g stands for gas phase and s for solid phase):

ClONO2(g) + _H2O(s) ➔ HOCl(g) + HNO3(s) C!zONO2(g) + HCl(s) ➔ Ch(g) + HNO3(s)

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HOCl(g) + HCl(s) ➔ Ch(g) + H2O(s) (7)

In addition, the nitrogen partitioning is strongly affected by the conversion of N2O 5 in HNO3:

N2Os(g) + HCl(s) ➔ ClNO2(g) + HNO3(s)

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When the sunlight is coming back after the polar night, photodissociation reactions of Ch, HOCI and CIN02 are producing Cl and CIO radicals. Because the concentration N0 2 is lower than usual due to N

₂

0

₅

conversion into HN03 (reactions 8 et 9), a large amount of ClO remains available to increase the ozone loss rate. The ozone destruction cycles are the following:

2(Cl + 0

₃

➔ CIO + 02) CIO+CIO

➔

Cb02 Ch02 + hv

➔

Cl +ClOO

CIOO ➔ Cl+02

net 2 03 + hv ➔ 302

Br+ 03

➔

BrO + 02 Cl +03

➔

ClO + 02 ClO + BrO

➔

Cl+ Br + 02

➔

Br+OC!O

➔

BrCl + 02 BrCI + hv

➔

Br+Cl

net 203 + hv

➔

302 (I 0)

( I I) ( 12) ( 13)

( 14) ( 15) (16) (17) (18) (19)

These photolysis processes are initiated by ultraviolet solar radiation of wavelength larger than 300 nm.

Similar heterogeneous reactions on the surface of sulphuric acid aerosol droplets present at all latitude play an important role in the ozone reduction observed at mid-latitudes :

ClON02 + H20 (aerosol) ➔ HOCI + HN03

N20s + H20 (aerosol) ➔ 2HN0 ₃ (21)

(20)

The role of these two reactions was enhanced after major volcanic eruptions like the Mount Pinatubo eruption in June 1991, leading to significant N02 reductions at all latitudes.

The net efficiency of nitrogen oxide conversion into nitric acid is larger at high latitude

during winter because the decrease of photodissociation rates related to the solar irradiation

(14)

12 prevails in those conditions. In addition, the conversion of chlorine nitrate is strongly temperature dependent and efficient only at low temperature. Therefore reaction 20 can generally be neglected at mid-latitudes.

Based on comparison between model simulation and observations, it appears that heterogeneous processes occurring at the surface of stratospheric sulphuric acid aerosol are responsible for stratospheric ozone losses at mid-latitudes, due to the release of anthropogenic halocarbons in the atmosphere (Brasseur and Granier, 1992; Solomon et al., 1995, and references therein).

Ground-based remote sensing measurements of stratospheric species have been extensively used to increase the scientific understanding of stratospheric processes at high and mid-latitudes. In addition to the existing long-term observations performed at the NDSC Alpine station, several campaigns have been carried out in the northern hemisphere during winter and spring periods. The European Arctic Stratospheric Experiment (EASOE) co- ordinated by the European Commission (EC) during the winter 1991/ l 992 has demonstrated the need for such a complementary approach between high quality ground-based observations, aircraft and balloon measurements, combined with satellite data and model calculations.

As part of an overall aim to study the evolution of the Arctic stratosphere in that winter in order to understand northern hemisphere ozone loss, EASOE objectives included:

i) the measurements of the change in ozone concentration with altitude throughout m winter;

ii) the measurements the concentrations of other trace chemical species ( especially chlorine and nitrogen species);

iii) the investigation of the role of polar stratospheric clouds, and m particular to study dehydration and denitrification of the Arctic stratosphere;

iv) the study of the meteorological processes which move chemically-perturbed air southward.

Measurements were made using ground-based instruments at 16 sites ranging from the Arctic Circle to southern Europe, from large stratospheric balloons and from three research aircrafts.

The observations performed at the ISSJ from November 1991 to March 1992 contributed to the European Arctic Stratospheric Ozone Experiment (EASOE) and provided data as a reference for the middle latitude stratosphere during that winter.

In addition, ground-based observations of 0

3

and NO

2

total amounts were performed in Keflavik, Iceland (64°N) during EASOE from November 15, 1991 to March 30, 1992, the results are reported in section 3.5.1.

Because the stratospheric aerosol loading was significantly enhanced after the Mt.

Pinatubo eruption in June 1991, the impact of heterogeneous chemistry was more specifically

addressed during EASOE.

(15)

Despite the international effort and the important results obtained during EASOE, several scientific issues mainly related to ozone depletion outside the polar regions remained controversial due to the complex coupling between chemistry and dynamics, specifically in the northern hemisphere.

Understanding the observed long term trends in middle latitude ozone remained the highest scientific priority for the Second European Stratospheric Arctic and Mid-Latitude Experiment (SESAME) which started in January 1994 and continued until the end of 1995.

This overall objective required the study at a variety of times throughout the year of processes occurring in, and connecting, the lower stratosphere of high and middle northern latitudes.

This required the study of the interaction of chemistry and dynamics calling for high quality measurements of ozone, active chemical species and dynamical tracers, in conjunction with detailed meteorological studies. Specific measurement objectives of SESAME included:

i) the study of chlorine partitioning in high latitudes before, during and after the coldest winter period and, in middle latitudes, at least before and after the winter cold period.

ii) the study of the nitrogen partitioning. The greatest need in high latitudes is now for detailed studies of how polar stratospheric clouds and aerosols affect the nitrogen partitioning.

iii) the study of PSCs and aerosols. More specially, the nitrogen partitioning in middle latitudes is influenced by aerosol.

iv) ozone. A network of ozone measurements is required to understand the middle and high latitude changes on a seasonal time scale in terms of both dynamics and chemistry. The network should include the regular ground-based measurements supplemented by a large number of ozone sonde measurements.

As with EASOE, it was important to see how the various objectives outlined above change spatially and temporally.

It was therefore decided for SESAME to expand the ground-based instruments for stratospheric monitoring around 60° N to fill in the geographical gap between the observing sites situated beyond the Arctic circle and the mid-latitude NDSC Alpine stations near 45° N, in order to study the connection between these latitudinal ranges. The selected observing sites could be at or near the edge of the polar vortex during winter in order to determine the extent of chemically-induced loss of stratospheric ozone and to investigate transport from high latitude, mainly during late winter-early spring. The observations are based on two main techniques, the Fourier transform spectroscopy in the infrared (FfIR) and the UV-visible differential absorption spectroscopy. These measurements include total amounts of several reservoir species (FTIR) with ozone, nitrogen dioxide, OClO and BrO (UV-Visible). In total, ground-based instruments were operated at about 35 sites, mainly in Europe, including the ISSJ and Harestua (Norway, 60°N) where ULG, IASB/BIRA and ULB performed important observations. The results of SESAME are reported in section 3.5.2.

The coupling between the stratosphere and the troposphere is of primary importance.

For instance, the role of the tropospheric source gases on the stratospheric ozone budget is

well demonstrated, through processes including their oxidation by the atomic oxygen in its

(0

¹

D) excited state and their photodissociation (mainly the halocarbons) by UV radiation only

available in the stratosphere (wavelength lower than 250 nm).

(16)

14 The tropospheric ozone processes need also to be quantified to understand why ozone concentrations in the lower part of the atmosphere have increased by a factor of two or more since the end of the nineteen century, in the northern hemisphere, with an enhanced rate since

1950.

The tropospheric ozone budget is partly controlled by transport of stratospheric air and by in situ photochemical production and destruction. The production of ozone is obtained through complex mechanism involving the photo-oxidation of volatile organic compounds (RH) and carbon monoxide in presence of NOx (NO+NO _2). This process is initiated by the hydroxyl radical during daytime, the net results being the following:

(22) The degradation of the aldehydes (R'CHO) can produce additional ozone.

NOx acts as a catalyst in this oxidation process. The conversion of NO to NO2 by peroxy radicals (HO ₂ and RO2) is of fundamental importance because the NO2 photodissociation at wavelengths below 400 nm yields the atomic oxygen required to produce ozone in the troposphere. The conversion of NO to NO 2 is due, to a large extent, to the following reaction:

(23) The photochemical loss of tropospheric ozone is explained by the following reactions

➔ 20H

(24) (25)

(26) (27) These photochemical reactions (24) and (25) are directly responsible for the OH production in the troposhere. Therefore, depletion of stratospheric ozone which increases the UV irradiance levels around 300 nm is expected to decease the ozone concentration in pristine areas and to increase the oxidising capacity of the troposphere.

The hydroxyl radical, often called the "detergent" of troposheric trace species, can trigger reactions with a large number of trace gases (i.e., CH4, CO, non-methane hydrocarbons such as C ₂ H6, C2H2, HCFCs such as CHC1F2, CH 3 CCh, ... ) that affect, either directly or indirectly, both climate change and stratospheric ozone depletion. Its temporal variability may have been the cause of yet unexplained fluctuations in rates of change recently observed for CO, CH4 and CHClF2. For details dealing with that matter, see the recent IPCC Report

"Climate Change 1994", J.T. Houghton et al., eds., Cambridge University Press, 1995.

These scientific studies contribute to several international programmes, namely:

(17)

The Network for the Detection of Stratospheric Change (NDSC);

The Stratospheric Processes and their Role in Climate (SPARC), a project of the World Climate Research Programme (WCRP);

The International Global Atmospheric Chemistry Project (IGAC) a core project of the International Geosphere Biosphere Programme (IGBP)

The Tropospheric Ozone Research (TOR) and Tropospheric Optical Absorption Spectroscopy (TOPAS) sub-projects of the EUROTRAC project (EUREKA);

The Environment Programme of the European Commission (EC), DG XII;

The correlative measurement programme of the Upper Atmosphere Research Satellite (UARS) launched in September 1991.

The validation of the first European ozone space sensor, namely the Global Ozone Monitoring Experiment (GOME) on board ERS-2 launched the 21 st April 1995.

In addition, results obtained in the frame of this project have been extensively used in

several assessment reports like the "Scientific Assessment of Ozone Depletion: 1994" (WMO,

report no 37, 1995) and the "Climate Change 1994" (IPCC report, 1995)

(18)

16 2. INFRARED SPECTROSCOPY OF THE STRATOSPHERE

The ULG has had a long experience in high quality solar observations at the ISSJ which started as early as 1950. Aiming initially at the production of photometric solar atlases covering the visible and infrared domains, it reformulated its main activities around the mid- 70s, such as to devote increasing time to atmospheric studies. That move was in response to strong recommendations to measure and monitor some stratospheric key gases such as HCI, HF, ClONO 2, ... that would further substantiate the Molina and Rowland theory and provide basis for its quantification.

Subsequently, more molecules were added as target gases needing or deserving monitoring. As a commitment to NDSC (Network for the Detection of Stratospheric Change) which, in 1989, had selected the ISSJ as the infrared component for its primary "Alpine Station" to monitor the stratosphere at northern mid-latitudes, the nitrogen species NO, NO _2, HNO ₃ and HO 2 NO 2 , 0 _{3 ,} COF 2 , as well as long-lived trace gases such as N 2 O, CChF 2,

CHCIF ₂ and SF ₆ were progressively added to the list. Also, around that time, the EUROTRAC program was defined and funded, and the ISSJ station became involved in the EUROTRAC sub-project TOR (Tropospheric Ozone Research) to monitor a series of tropospheric constituents among which CH4, C ₂ H _6, C ₂ H _2, CO, HCN, OCS, H2CO, H ₂ CO _{2, ... ,} are known to influence the tropospheric oxidising capacity.

As indicated in Table 1.1 some data extend as far back in time as 1950-51, resulting from initial isolated observations which were made at that early stage, more for satisfying curiosity and feasibility criteria than as a sustained monitoring effort; however, these early measurements have proved quite useful in deducing a few trends over almost half a century.

Analyses including such extended data bases have been published in papers to which we refer for details and conclusions.

2.1. INSTRUMENTATION

Since 1990, two high performance infrared Fourier transform spectrometers (FTS) are operated at the ISSJ. They use the sun as source of background radiation to study the absorption characteristics of the Earth's atmosphere through the well known atmospheric windows that exist between I and 14 µm.

2.1.1. Home-made ULG FTS

This instrument, which works in air, has been entirely developed in the shops of the

Institute of Astrophysics of Liege and has been operational at the ISSJ since 1984. It is

installed at the coude focus of the Jungfraujoch 76 cm telescope. The FTS maximum optical

path difference is 2 meters, thus providing an unapodised spectral resolution of 0.0025 cm·

¹

(2.5 mKayser, mK). The instrument can be operated in two domains: 1 to 5.5 µm and 5 to 14

µm, using either an InSb detector and CaF ₂ optics, or a HgCdTe detector and ZnSe optics. A

typical interferogram is recorded in 3 to 5 minutes. A Hewlett Packard I 000F computer

controls the instrument and the data acquisition, while another HP 1 000F makes all the

computations (phase correction and Fourier transformation).

(19)

2.1.2. Commercial BRUKER IFS 120-HR

This new FTS was installed at the ISSJ in 1990, replacing the double-pass grating spectrometer which was put out of operation in 1989 (following good services during over 20 years for both solar and atmospheric studies). It can operate under vacuum and allows to achieve 0.001 cm-

¹

(1 mK) unapodised resolution (5 meters path difference). The BRUKER FTS is fed with solar radiation via a coelostat installed on the roof of the station. It makes use of either an InSb detector and CaF 2 optics or an HgCdTe detector and KBr optics, for operation in the 1 to 5.5 µm or the 5 to 14 µm domains, respectively. Typical observing time of an interferogram is on the order of 50 seconds to reach a resolution of 4 mK; through a I 00 cm- ¹ narrow bandpass filter, one scan provides a signal-to-noise ratio in excess of 1000 in the

1 to 5.5 µm region.

Phase corrections and Fourier transformations are made in quasi-real time, on site. Part of the hardware (in particular the data acquisition and handling system, based on the use of INMOS "transputers") and all of the software needed to operate the instrument have been developed in Liege.

2.1.3. Improvements over the last five years

- The BRUKER IFS 120-HR FTS was installed in 1990 in a minimal configuration and has been completed progressively by different accessories allowing to work in the near- and mid- infrared. New detectors and new beam-splitters have been installed to increase sensitivity and minimise fringing effects.

- In a FTS, the quality of the results relies essentially on the precision of the sampling grid of the interferogram, which is based on the stability of the wavelength of a reference laser line. Under the pressure conditions of the ISSJ (30% lower than sea level pressure, causing insufficient convection cooling), the original control laser installed by the Bruker factory showed lasing mode instability and had to be replaced by a Hewlett-Packard Model 5518A version; this improved the frequency of the instrument well beyond the minimum needed.

- A local fiber optics network, interconnecting the different "PCs" of the laboratory, enables data and programs to be exchanged among them, as well as other useful information such as the "exact time" received by radio from a transmitter located near Frankfurt; it further prevents all interconnected items to be at risk when lightning strikes the observatory.

- Another very important improvement has been the replacement of the old coelostat

installed in 1954, by a completely new system designed and build in Liege. This instrument

uses two flat mirrors in an altazimuthal configuration rotating slowly, actuated by a special

controller receiving, every second, the co-ordinates of the Sun computed by a "Personal

Computer", with an accuracy of a few arc seconds. The mirrors follow the Sun from sunrise to

sunset, without any manual intervention. A correction taking into account the light refraction

by the lower layers of the earth atmosphere is added to the astronomical computation of the

Sun position. This correction becomes larger and larger and cannot be neglected when the Sun

is observed near the horizon, at sunrise and sunset. This coelostat, installed in January 1993

on the upper terrace of the Sphinx Observatory, is working at the entire satisfaction of the

users, despite the severe conditions under which it has to operate (in winter, the temperature

may drop as low as -30°C).

(20)

18 - The meteorological data from the satellites METEOSAT - IR, -EIR and VIS can be received by a computer through a special interface box connected to a radio receiver. The maps allow to follow the evolution of the continental meteorological conditions and thus anticipate local conditions for future days observations; they can be stored for later print-out, if needed.

- A Hewlett Packard HP730 workstation has been purchased and is part of the local network linking all the PCs of the laboratory; it is mainly used for computations needing high speed and precision such as the calculation of atmospheric spectra and their comparison with observed ones.

- In 1994, we acquired a cryostat with compressor to cool infrared detectors down to 15°K, or to maintain the internal shield at low temperature, to keep (during several months) a few liters of liquid helium necessary to cool detectors down to 4 K for maximum sensitivity.

This system has been developed to our own specifications by INFRARED LABO RA TORIES (Tucson, USA). That firm installed three different detectors in the cryostat: one to extend the observations up to 40 µm, and two others to replace the HgCdTe detector used now in the 7 to 14 microns region, with a better sensitivity and a better linearity response. The compressor is water cooled by a closed loop system with heat exchanger build in Liege.

- To make observations in the ultra-violet, the Bruker FTS needs new optics (beam splitter and compensating plate), a specific detector and additional boards. Preliminary tests have shown that the system dividing the reference fringes to operate in the UV is accurate and stable enough for the instrument to be used in that domain. During 1995, constructions to the Sphinx building prevented us from making the high-resolution UV observations planned for comparison with the observations of the SAOZ (IASB-BIRA Jungfraujoch installation); they will be made when all changes to the facility will be completed ( end- 1996).

- A Benson digital plotter has been connected to the PC controlling the Bruker 120-HR FTS; the necessary software package is under development.

- In the home-made FTS, a new digital filter, using a transputer system installed in a PC and connected to the acquisition Hewlett-Packard HP-1000 computer, has been installed in replacement of a previous hardware version.

- In a near future, the various HP-IO00F, still in use around the home-made FTS, will be replaced by PCs. That replacement is needing a lot of new programming to adapt the "old"

software (computer of different architectures, different assembly language, ... ). Up to now, all the interferograms and the computed spectra recorded with the "home-made" FTS were archived on classical magnetic tapes (12.7 mm). The use of such magnetic tape drives is no more supported anywhere, and it is necessary to transfer all our data on another substrate.

This procedure is in progress, using also the transputer connection mentioned in the previous point; it will need another year of work for verification and reorganisation. The local network is proving very useful and reliable for these transfers.

The above reported modifications and improvements have been made without

stopping the observation campaigns and the data analysis.

(21)

2.2. IR OBSERVATION MISSIONS AT THE ISSJ

2.2.1. Long-term activities

Basically, infrared observation campaigns at the ISSJ are organised and performed by ULG scientists; they are complemented most appropriately by colleagues from the Belgian Institute for Space Aeronomy (IASB/BIRA) and from the Royal Observatory of Belgium (ORB/KBO) from Brussels.

Table 2.1 lists the numbers of days per month during which infrared observations have been made at the ISSJ between 1991 and 1995 with either the ULG or the BRUKER instrument, often both. Details on observational activities have been provided in the successive annual progress reports, but as a example, Table 2.2 gives the days of 1995 when either or both of the ISSJ instruments were operated . With only a few exceptions, observations have been made on every month of 1991 to 1995, with numbers of days varying from I to 18. For a monthly coverage to be acceptable in terms of secular monitoring, it is felt that good measurements ought to be made on at least 4 days per month (this depends obviously on the stability, lifetime and possible local pollution of the gases under investigation); for shorter than seasonal variability investigations and intensive campaign-type activities, observations need to be made on as many days as possible. It should be noted here that poor observational conditions also occur at (windy days with flying snow) and above (cloudy sky periods) the ISSJ, which often prevent achieving observations on many days of staying at the site.

YEARS

Month: 1991 1992 1993 1994 1995 TOTA

JANUARY 6 8 9 5 7 35

FEBRUARY I 13 8 7 8 37

MARCH 7 3 7 4 17 38

APRIL 4 11 10 8 9 42

MAY 13 8 ---- 5 6 32

JUNE 7 6 11 5 13 42

JULY 3 ---- 6 5 8 22

AUGUST 8 6 ---- 4 12 30

SEPTEMBER 8 13 9 4 2 36

OCTOBER 12 9 8 13 18 60

NOVEMBER 7 4 ---- 7 3 21

DECEMBER 3 7 5 10 8 33

TOTAL: 79 88 73 77 111 428

Table 2.1 : Days per month with IR observations at the ISSJ.

(22)

20 January 1995 : 14B, 15B, 16B, 19L,21L,26L,27L February 1995 : 7B, 8B, 10B, 19L, 21L, 22L, 27L, 28L

March 1995: lL, 2L, 5B, 7B, 9B, 10B, 11B, 12B, 13B, 14B,16B, 22B+L, 23B+L, 24B+L, 25B+L, 26B+L,28B+L

April 1995: 5B, 7B, 9B, 11B, 12B, 13B, 14B, 28B, 29B May 1995: 5B, 6B, 7B, 8B, 15B, 19B

June 1995 : 9L, I 0L, l 4L, 15B+L, 16B+L, 17B+L, 19B+L,20B+L, 22B, 23B, 25B, 26B, 27B

July 1995: 19B,20B,21B,22B,23B,24B,25B,26B

August 1995 :4L, 5L, 7L, SL, 9L, IOL, l lL, 15B, 16B, 17B, 18B, 21B September 1995 :29B, 30B

October 1995 : 2B, 3B, 7B, 9B+L, 10B+L, 11B+L, 12B+L, 13B+L, 14B+L, 15B+L, 16B+L, 17B+L, 19B+L,20B+L, 21B+L, 22B+L, 23B+L, 24B+L

November 1995 14B, 29B, 30B

December 1995 1B, 2B, 4B, 5B, 6B, 10B+L, 12B+L, 13B+L

• The letter Land/or B beside the dates indicate which of the Liege home-made (L) or Bruker-l 20HR (B) FfS was operated.

Table 2.2 : ISSJ Observations during 1995.

Mention should be made here of a new BRUKER IFS 120-HR, high resolution FfS recently installed and operated at the Zugspitze station near Garmisch-Partenkirchen (Germany) by the Fraunhofer Institut ftir Atmospharische Umwelt Forschung (IFU).

Maximum co-ordination and observational synergy between the ISSJ and Zugspitze has been recommended by the NDSC Steering Committee, in order to achieve complementarity and enhance correlative studies of atmospheric variability.

2.2.2. Campaign Support

During the period covered by the SMAC project, co-ordinated observations have been made in the frame of the two campaigns co-ordinated by the EC:

- EASOE (European Arctic Stratospheric Ozone Expedition), which was programmed for intensive measurements during the winter period of 1991-1992, i.e., September 1991 to March 1992;

- SESAME (Second European Stratospheric and Mid-latitude Experiment) campaign, with intensive observational periods during the winters of 1994 and 1994-95, and summer of 1994.

Figure 2.1 illustrates the type of contribution of ULG to the EASOE and SESAME

campaigns. It summarises the results in terms of vertical column abundances (in number

molecules per cm

²)

gathered above the ISSJ between 1990 and 1995 of seven molecules

primarily present in the stratosphere, as well as the long-lived N 2 0 constituent which is

included as a tracer of atmospheric dynamics. This presentation shows the gross

characteristics

(23)

Sample vertical column abundances above ISSJ

1990 1991 1992 1993 1994 1995

JMMJSNJMMJSNJMMJSNJMMJSNJMMJSNJMMJSN

-

^QC

4.2

~

4.0

0

_N

_3.8

z

' . ^. ~

• ... ~ ^.. ~ . i. __ .. _• ---~:~.•az ₁₁ __ a ._;f. • .:_ .; ^1>}_ l; .. ! .... ;.,._ !!. -l.:~-¥J--~itl. .. ~

0 0

l

O O

g s

Ô Ôô⁰

^a ^J 1

^Cb; ^{I •} ^..

r· • ⁶

⁰ ⁱ ^O ^{~ . •}

^~

• -- --· -·--- - - · · - · - - 0 •-··-··--· 0 • • ---t:t

8

oo 0•1·--- ---- ,-O •• 0 f' o· ___ .. ________________ .. _ _ _ _ _ 0 0 C\ • 0 ' ~ , · • o....__ _____ _

i

• 0 . •

3.6 ---.--·-- --··-·•··---·---·-··-·---. ---·.----.-' --. ---.-.---. --- ^.---

12

i:-"""-:"'.".'."r-"1'."""."tc:r"r...-:-: .• '.:': .• T" .. ""." .•. :r:.T:.T"'.'., -:: .• :-"'.1.:-J. -::-i.-:-: •• -::_:-::: •• ~ .. '.'l:.~:".:-=-T:.~--=··•r. .. =--=·-:1'. ::, .. -:". ... :"".:TT~::",:l'.'".,:O, __ ,:-,_::-,_.-:-: __ ::-_=-·-=-=-'""'-T:•-:r:.r:::::::.c-:: .•

==:--C'.':=:=

-

^CIC

_-

:ail

-

^I"')

0 -

^ir._:.;i

^6.0 _5.0

^I>

j

⁰

^J

- ⁰ ^4.0

⁰^!r^.. ⁰

z

3.0 - 2.5

',Q

,...,

0 z

~

:.;i

2.0 - 1.5

a, 0

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~ B~

1.0 -- ·-·

2.5

;:;- 2.0 ~---- - ..

0 z

0 0

~

1.5 .

1.0

^{-o,~ -}

5.0 __ g ..

0

I

Q

·-0 0

0

•· ►----o- ... - .•.. , .• - ·• ·• _ _. __ - ... .,_ __ -- - •. - - ' 7 - - . . ·• -- -- . - - ·-•->-·• -- -- • • • •

•·

0,, 0 ...

0

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^-- ^--

g __ ~ __ __ _o _ _ . o, oo

___ ---~--'-s._a .. --~--•

0 0 _ 8

. : ___ :

L.,:}." __ ~---·-t

I._--•_. ___ A- -- ··- _,

---·-·

-

- ---- ·---. ---- . ___

.

-- .

-·

8'- • 8

:;.,;i

4.0 - ^. ^· ^...

^~

,,.. . r

· ··· -. ! · --- · -, 3 -,,-s-:.--·-:-t.;"i-:-yi•·-,'!": -;.;• ·,;,a" ••i·"··:-_"': ;.:-~~:-,•.e:a-'··JJ·•i•~

0

g • f •

0 )

~~•

og•tlfa 1^{0 1 0} \\:J 0 0 • o S• •

i'j~

0 _3.0

D

rA

^.Ja ^O

--T ·- -:· --- - 0 °'Go·-J ··-·--- - ·--·

o·-·g . . . . ·-- ·-- - - -· - - · - ·- ·--- - - . - -- ---- ...

~o ';:::;:::===;:::::;:::::=~:;:;:;:::;=::::::::;:::;=·!·~=-·:::;::::::;::;:;:::::;::::;::::===;:::;::;:~-=--·~-=-·=-~--=-=·-=··~-~-~-==::::::::;:::::=:::::::~

- 1.6

VJ