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SPARC INITIATIVES : Chemistry and Microphysics in the Lower Stratosphere and Upper Troposphere

Chair: A.R.Ravishankara (USA), e-mail: ravi@al.noaa.gov).

Trace species in the upper troposphere/lower stratosphere, in particular ozone, are crucial in determining the overall climatic effects of the stratosphere and of stratospheric changes, and in assessing the potential impact of aircraft emissions. The observation that, contrary to original expectations, the most significant ozone depletion has occurred in the region just above the tropopause rather than in the upper stratosphere has given added importance to an improved understanding of the chemistry in this region of the atmosphere. The type of activities to be undertaken include study of:

• NOx distribution in the upper troposphere,
• the parameterization of heterogeneous chemistry in models,
• comparisons of chemical models,
• comparative modelling studies of the impact of aircraft emissions,
• use of aircraft and satellite measurements (especially for model validation),
• parameterization of climate-chemistry interactions,
• supporting laboratory investigations.

The topics concerned are of direct interest to both SPARC and the (IGBP) International Global Atmospheric Chemistry (IGAC) project, and close co-operation is maintained to see that key issues are properly addressed.

The objectives of this initiative are described in the SPARC Implementation Plan.

Two SPARC/IGAC joint tasks:

1. Analysis and evaluation of data on peroxy radicals.
   Peroxy radicals play a major role in the production of ozone in the troposphere and also affect the chemistry of the lower stratosphere. Because of various experimental difficulties, laboratory measurements have been confined to a few methods. Progress in other aspects of this chemistry has not been evaluated and taken into account to define the reactivity of peroxy radicals. Therefore, the group, whose members have contributed significantly to the previous studies, are evaluating the data systematically. The outcome of this project will be critically evaluated data that can be used for atmospheric modeling. From the outset, the task required a paper published in a high-profile refereed journal (with prominent acknowledgement to SPARC).
   
   Leader of the task: Dr. Geoffrey Tyndall, NCAR, Boulder, CO, USA
   Members: Dr. R. A. Cox, Cambridge, UK - Prof. Robert Lesclaux, CNRS-Bordeaux, France - Dr. Geert Moortgat, MPI-Chemie, Mainz, Germany - Dr. Timothy Wallington, FORD motor Co., USA - Dr. A. R. Ravishankara, NOAA Aeronomy Lab, Boulder CO, USA.
   
   
2. Quantum yield for O(1D) production in ozone photolysis.
   Photolysis of ozone to give the first electronically excited oxygen atom, followed by its reaction with water and methane, is the primary source of the OH radicals in the lower and middle atmosphere. There is a lot of new data from physical chemistry and chemical physics communities on this process. This data has to be evaluated and presented in a form accessible to atmospheric chemists. Therefore, a joint SPARC/IGAC task was started. As in the case of the first task, a peer-reviewed publication is planned. The data assembly process has begun and the project is expected to be completed late this year. The members are likely to meet in England before or after the IUGG meeting in Birmingham.
   
   Leader of the task: Prof. Yutaka Matsumi, Nagoya University
   Members: Prof. Gus Hancock, Oxford University, UK - Dr. Anthony Hynes, University of Miami, USA - Prof. A. Comes, Frankfurt, Germany - Dr. A. R. Ravishankara, NOAA Aeronomy Lab, USA.

Workshops

• Workshop on UT/LS Heterogeneous Chemistry, Strasbourg, France, 21-23 October 1996.
• Special session at the Spring AGU (Atmospheric Sciences section), Boston, May 31 - June 4, 1999. Conveners: A.R. Ravishankara and P.Sander.
• Workshop on Nitrogen Oxides in the Lower Stratosphere and Upper Troposphere, Heidelberg, Germany, 19-22 March 2001. Convener : K. Pfeilsticker.

Publications

Report of the Strasbourg Workshop on UT/LS Heterogeneous Chemistry
A.R.Ravishankara, SPARC Newsletter 9, July 1997.

Small Organic Peroxy Radicals: Big Player in Ozone Production,
Geoffrey Tyndall et al., SPARC Newsletter 15, July 2000.

The atmospheric chemistry of small organic peroxy radicals,
G.S. Tyndall, R. A. Cox, C. Granier et al., J. Geophys. Res., in press, 2001.

Quantum yields for production of O(¹D) in the ultraviolet photolysis of ozone: Recommandation based on evaluation of laboratory data,
Y. Matsumi, F.J. Comes, G. Hancock et al., J. Geophs. Res, submitted 2001.