624 I ThCZ-l
STRATOSPHERIC MONITORING STATIONS IN EUROPE P.C. SIMON and M. DE MAZIERE
Institut d'Aeronomie Spatiale de Belgique 3, Avenue Circulaire, B-1180 BRUXELLES, Belgium
L. DELBOUILLE and G. ROLAND
Institut d'Astrophysique de l'Universite de Liege Avenue de Cointe, 5, B-4200 OUGREE-LIEGE, Belgium
S. GODIN
Service d'Aeronomie du CNRS
B.P. n° 3, F-9l370 VERRIERES-LE-BUISSION, France K. KUNZI
Universitat Bremen, FBI
Postfach 330440, D-2800 BREMEN 33, FRG.
J. de LA NOE
Observatoire. de Bordeaux B.P. 89, F-33270 FLOIRAC, France
P.T. WOODS
National Physical Laboratory, Division of Quantum Metrology Queens Road, TEDDINGTON, Middlesex TWll OLW, UK
Measurements of the trends in ozone and other stratospheric trace species require a coordinated scientific effort in establishing ground based observations coupled with satellite measurements in order to quantitatively detect early changes in stratospheric composition and structure and to improve the understanding of short term processes needed to validate either long term observations or model simulations.
Over the observations which
last three decades, ground-based spectroscopic use the Sun and/or the zenith sky as a source of radiation have provided a large amount of information on the composition and the structure of the stratosphere. Very significant results have been obtained for example with the Dobson network which have been used to determine trends in ozone total content from 1957 and in adjusting satellite measurements of total column ozone performed since 1979. This has been extended to global coverage in total ozone by removing from the satellite data the effect of instrument degradation which occurred in orbit. High-resolution infrared spectroscopic measurements from high altitude observatories have also been successful in the discovery of and detection of trends of anthropogenic and natural origin in some strato- spheric species.
ThC2-2/625
More recently, new powerful instruments like lidars and micro- wave radiometers have been developed. Lidar has already been shown to have the capability to measure vertical profiles of ozone, aerosols and temperature whilst microwave radiometers have measured chlorine monoxide, water vapour and ozone.
A systematically developed ground-based network is required in Europe for monitoring the stratosphere at high and mid-latitudes with the specific objectives of :
(i) providing the earliest detection of possible changes in strato- spheric composition and structure ;
(ii) understanding the aeronomic processes involved in short-term variations of ozone and related trace species ;
(iii) providing by comprehensive measurements of stratospheric constituents, and thereby supplying "ground truth" for the current and planned satellite observations.
These objactives require a comprehensive combination of experimental methods including lidar, microwave radiometry, UV-visible spectroscopy at medium and high resolution, and IR high resolution spectroscopy. IR laser heterodyne spectrometry will also be investigated because of its ability to detect some important stratospheric reservoirs and other trace gases. Because of the variety of instruments spread over different locations, a major effort is needed to coordinate the various aspects defined in the objectives. Furthermore, the new techniques need to be carefully compared with proven methods before being included in a Global Network. The aim is to compare, calibrate and validate the various observations and to interpret the data in an integrated fashion. An assessment of the critical quantities of each individual technique such as stability, accuracy, precision (or repeatability) and sensitivity to perturbations due to the specific characteristics of each site will be performed.
The mid-latitude station includes the "Observatoire de Haute Provence" (44N, 6E, France), the "Station Scientifique Internationale" at the Jungfraujoch (46.5N, 8E, Switzerland), the "Deutsche Wetterdienst" in Hohenpeissenberg (47N, llE, FRG) and the "Observatoire de Bordeaux" (44N, lW, France). Contributions from Arosa, Payerne and Bern (Switzerland) are also expected.
A new station in Spitsbergen (79N, l2E, Norway) which is already carrying out lidar observations will be expanded with a permanent microwave sensor and Fourier-transform spectrometer within the next few
6Z6/ThCZ-3
years. This Arctic station will therefore extend the European effort by adding, at high latitude, similar stratospheric monitoring capabilities to those already existing at mid-latitude.
the two stations is required to obtain
Strong coordination between maximum benefit from the observations and to analyse the data relating to different stratospheric conditions.
The lidar measurements which provide the vertical distribution of ozone are based on the Differential Absorption Laser (DIAL) technique and are currently performed at the "Observatoire de Haute Provence" (OHP, France) at Hohenpeissenberg (FRG) and in Ny-Alusund (Spitsbergen). In addition, stratospheric aerosol measurements are also made at the OHP.
Details on the lidar network are given in a review paper by S. Godin (this volume).
The ultraviolet, visible and infrared Fourier Transform Spectrometry (FTS) is located at the "Station Scientifique Internationale", Jungfraujoch (altitude : 3580 m). The main objectives will be the measurements of column ,content of ozone and the various related species (N0
2, HF, HC1, CH
4, HN0
3, ClON0
2, and, tentatively, OH ... ) with their variabilities and their trends. When relevant, vertical concentration profiles will be retrieved. For some species (N
20, CH
4, CO, C2H
6) , trends have been established over 40 years.
The available equipment consists of a non-commercial Fourier Transform spectrometer, yielding a resolution limit of 0.0025 cm-l between 1 and 14 micrometer and a new BRUKER IFS 120 Fourier Transform spectrometer, yielding a resolution limit of 0.001 cm-l
, between 0.3 and 500 micrometer, to be operated in early 1990. In addition UV-visible spectrometers with high and medium resolution will be installed for comparison purposes. Further details are given in a companion paper by R.
Zander (this volume).
The microwave radiometers measures the thermal emission from rotational transitions of constituents like ozone, chlorine monoxide and water vapor in the millimetric wavelength range. They are currently operating at the "Observatoire de Bordeaux" (France) and in Bern (Switzerland).
Microwave radiometers are well suited to perform long-term trend measurements of total stratospheric ozone and its vertical distribution,
since they are easily calibrated with black body radiators, and thereby have responses that can be made stable over long periods. The altitude
ThC2-4/627
range over which high quality ozone profiles can be determined is typically 15-70 km with an altitude resolution of 10 km. However, at present, only little experience is available with this type of sensor for ozone research and more intercomparisons and experience with sites in different climatic regions are needed.
France
CIO measurements are also being (Pyrenees, 1990) at an altitude
implemented at stations higher than 2000 m and
in in Switzerland (Bern, 1992). Another site in the French Alps is also being investigated. Plans exist to have CIO radiometers in Hohenpeissenberg and Spitsbergen after 1992.
Laser heterodyne spectroradiometer (LHS) works by mixing radiation from a tunable laser with solar radiation on a high-speed photodetector. The absorption spectra of atmospheric constituents are obtained with high resolution (10- 3 10-4 -1
and high
very cm )
sensitivity at wavelengths ranging from 2.5 to 15 ~m. Important examples include HOCI and ClON0
2. Its high spectral resolution also enables the shape of absorption lines to be measured with high sensitivity. This provides information on the altitude distribution of the gas and enables any tropospheric contribution to be separated out. Examples include HCl, HN03 and 0
3.
The National Physical Laboratory spectrometer is being developed as a portable test-bed to demonstrate the technology. It is configured with a number of diode lasers to allow rapid sequential measurements of a number of important stratospheric gases. These will include ClON0
2 and HOCI to provide complementary scientific information to that obtained with other methods, and HCl, HN0
3 and 0
3 for intercomparison measurements.
The concept of the European mid-latitude station could be disputed because of the spreading of the various observation sites.
However, distances between sites of less than 500 km are acceptable provided that the measurements of various constituents are well coordinated and consideration is given to the stratospheric dynamics.
This problem will carefully be addressed by coordination. In addition, i t is our intention to install in all sites "portable" powerful equipment such as, for instance, the UV-visible diode array spectrometer and, when proven, the LHS, in order to have data directly comparable in accuracy and repeatability.
