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Introduction

Water vapor and ozone are of crucial importance in both, climate and chemistry of our Earths atmosphere. As most important greenhouse gas water vapor and in a minor sense ozone play a dominating role in determining climate whereby particularly the radiative feedback of clouds is of great importance. In addition atmospheric water vapor acts as engine of atmospheric dynamics. As chemical substances ozone and water vapor are strongly involved in the chemical oxidation capacity of the atmosphere [Thompson, 1992]. Climate and chemistry of the atmosphere are strongly linked to each other. Climatic changes will influence the chemistry and vice versa. Ozone and water vapor play a key role in the interaction between the chemistry and climate of the atmosphere [Ramanathan et al., 1985]. Long term changes of ozone and water vapor can have a climatic impact on our earth system [IPCC (=Intergovernmental Panel on Climate Change)-report 1994, 1995].

At present there are large deficiencies in the knowledge of the global distribution of ozone and water vapor on spatial as well as temporal scale, particularly in the upper troposphere and lower stratosphere. In order to make reliable predictions of the potential climatic change caused by changes of ozone and water vapor there is a strong need for accurate measurements of the spatial and temporal distribution of tropospheric and stratospheric ozone and water vapor in both hemispheres. Particularly, there is an urgent need for improved data quality for ozone [ WMO Scientific Assessment of Ozone Depletion 1998, 1999] as well as for water vapor measurements [IPCC-report 1994, 1995]. This can be achieved by calibration or comparison of the sensing devices with accurate reference instruments.

The environmental simulation facility at the Forschungszentrum Jülich (FZJ) enables control of pressure, temperature, ozone and water vapor concentration. Under realistic atmospheric conditions the airborne ozone or water vapor sensing devices can be compared to accurate reference instruments. A fast response dual beam UV-photometer serves as ozone reference. For the calibration of the water vapor sensors a dew point is used for lower/middle tropospheric water vapor conditions, while for middle/upper tropospheric conditions a Lyman (a) fluorescence hygrometer serves as reference.

Since 1996 the facility is established as  World Calibration Facility for Ozone Sondes (=WCCOS): a facility for quality assurance of ozone sondes used in the GAW [Global Atmosphere Watch, 1993] program of the WMO (World Meteorological Organization) focusing on ozone sonde precision, accuracy and long term stability [WMO-report No. 104, 1995]. Several WMO sponsored international intercomparison experiments of ozone sondes, JOSIE = Jülich Ozone Intercomparison Experiment, have been conducted at the facility. The aim was to assess the performance of the major types of ozone sondes that are used in the global network of sounding stations [Smit et al., 1998, Smit and Kley, 1998].
An important task of the facility since 1994 is the regular (monthly) calibration of water vapor sensing devices which are flown aboard 5 civil "in-service" aircraft (Airbus-A340) within the frame of the European project MOZAIC (Measurement of Ozone and Water Vapor on Airbus In-Service Aircraft) for automatic monitoring tropospheric water vapor, particularly in the middle/upper part of the troposphere. The facility is also used to characterize humidity sensing devices deployed on radiosondes. Further, the facility is a platform to investigate the performance of new developed airborne sensing devices.

References

Global Atmosphere Watch Guide, GAW-Report No. 86, WMO/TD-No. 553, World Meteorological Organization, Geneva, 1993.

IPCC (=Intergovernmental Panel on Climate Change), Climate Change 1994: Radiative Forcing of Climate Change and An Evaluation of the IPCC IS92 Emissions Scenarios, Report of working groups 1I and III of the Intergovernmental Panel on Climate Change, Cambridge University Press, 1995.

Ramanathan, B. Subasilar, C.H. Whitlock,D.F. ,Young, and Y. Zhou, Absorption of solar radiation by clouds: observation versus model, Science, 267, 496-499, 1995.

Smit H.G.J., W. Sträter, M. Helten, D. Kley, D. Ciupa, H.J. Claude, U. Köhler, B. Hoegger, G. Levrat, B. Johnson, S.J. Oltmans, J.B. Kerr, D.W. Tarasick, J. Davies, M. Shitamichi, S.K. Srivastav, C. Vialle, and G. Velghe, JOSIE: The 1996 WMO International intercomparison of ozone sondes under quasi flight conditions in the environmental simulation chamber at Jülich, Proceedings of the XVIII Quadrennial Ozone Symposium, Eds. R. Bojkov , and G. Visconti, LAquila, Italy, September 1996, 971-974, 1998.

Smit, H.G.J., and D. Kley, Jülich Ozone Sonde Intercomparison Experiment (JOSIE), WMO Global Atmosphere Watch report series, No. 130 (Technical Document No. 926), World Meteorological Organization, Geneva, 1998-B.

Thompson, A.M., The oxidizing capacity of the earths atmosphere: Probably past and future changes, Science, 256, 1157-1165, 1992.

WMO report No. 104, Report of the fourth WMO meeting of experts on the quality assurance/science activity centers (QA/SACs) of the global atmosphere watch. Jointly held with the first meeting of the coordinating committees of IGAC-GLONET and IGAC-ACE at Garmisch-Partenkirchen, Germany, 13-17 March 1995, WMO TD.No. 689, World Meteorological Organization, Geneva, 1995.

WMO (=World Meteorological Organization) , Scientific Assessment of Ozone Depletion: 1998, Global Ozone Research and Monitoring Project - Report No. 44, World Meteorological Organization, Geneva, 1999.


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