Halogenoxide in ihrer Eigenschaft als Radikale spielen in der Atmosphärenchemie eine wichtige Rolle. Insbesondere die Auswirkung auf das Treibhausgas Ozon ist höchstsignifikant und kann zu dessen vollständiger Zerstörung führen. Wir beabsichtigen, mit Hilfe von Feldmessungen unser Verständnis der chemischen Abläufe sowie vertikaler Transportprozesse zu vertiefen. Der in Ostafrika (Republik Djibouti) befindliche hypersaline See Lac Assal zeichnet sich von allen anderen Salzseen der Welt dadurch aus, dass er der einzig Salzsee ist, der ausschließlich und ohne wesentliche zeitliche Verzögerung von Meerwasser gespeist wird, dabei jedoch eine zehnfache Anreicherung von Halogeniden gegenüber dem Meerwasser aufweist. Gemeinsam mit seiner außergewöhnlichen Topographie (Riftlage) und Meteorologie (extreme Hitze bei vergleichsweise hoher Luftfeuchtigkeit) machen ihn diese Bedingungen zu einem optimalen Labor für Untersuchungen der atmosphärischen Halogenchemie. Neben der Frage nach Vertikalprofilen von Halogenoxiden, gemessen mittels speziell entwickelter DOAS-Technologie, stellen Quellen und Freisetzungsmechanismen von Halogenoxiden, Wechselwirkungen von Halogenoxiden mit Ozon, wie auch mit Stickstoffdioxid zur Überprüfung eines postulierten Zusammenhanges zwischen Halogenchemie und der Chemie der Stickoxide den Forschungsschwerpunkt dar.
Die großskalige räumliche Verteilung und Variabilität von Eisblumen auf dem Meereis in der Arktis und Antarktis wurde bisher noch nicht untersucht. Eisblumen haben möglicherweise einen großen Einfluß auf die troposphärische Chemie. Außerdem könnten sie als Quelle von Meersalz-Aerosolen Auswirkungen auf die Interpretation von Eisbohrkern-Daten haben. Die Bromid-Konzentration ist in Eisblumen etwa dreimal so hoch wie in Meerwasser. Durch heterogene Reaktionen kann gasförmiges Brom exponentiell zunehmend freigesetzt werden ('Bromine Explosion'). Bromoxid ist beteiligt an Prozessen des troposphärischen Ozonabbaus und der Ablagerung von Quecksilber in der polaren Biosphäre. Ziel des Projektes ist die Entwicklung eines Verfahrens zur Kartierung von Eisblumen mittels des Ku-Band Scatterometers Sea Winds. Die Validation wird mit Hilfe der kombinierten Analyse von passiven (AMSR) und aktiven (ASAR) satellitengetragenen Mikrowellen-Sensoren durchgeführt. Die Hypothese vom Einfluß der Eisblumen auf die troposphärische Chemie soll anhand des Vergleichs mit Bromoxid-Satellitenmessungen (GOME und SCIAMACHY) überprüft werden.
Dead Sea Investigated are release mechanisms and the influence of Reactive Halogen Species (RHS) on tropospheric chemistry and their consequences on the atmospheric composition at mid latitudes. Field campaings in the Dead Sea Valley in Israel are planned. Because of its geographical position and topography it can be regarded as a large scale natural 'laboratory'. Questions to be answered include the details of the mechanisms releasing RHS to the atmosphere, in particular the 'Bromine Explosion' mechanism, a possible anthropogenic influence on RHS release, determination of the total amount of halogens released, and the involvement of halogen oxides in regional ozone destruction. From these data a better assessment of the influence of RHS on the chemistry of the troposphere and the global ozone budget can be attained. The main measurement technique will be Differential Optical Absorption Spectroscopy (DOAS).
Objective/Problems to be solved: The proposed research programme is designed to resolve many of the outstanding key issues concerning the chemical transformation of DMS so that a reliable quantitative appraisal can be made of its contribution to CCN formation and consequently an assessment of the magnitude of its regulatory role in climate. Past work on the atmospheric chemistry has been instrumental in highlighting very specific processes, which need to be investigated in detail if a reliable assessment of the relationship between DMS, CCN and climate is to be made. The continuing improvement in analytical techniques now makes it possible to make high quality and high time resolution measurements of many species, both in the laboratory and in the field, which were previously either not possible or only with large error limits and poor time resolution. Scientific objectives and approach: The major objectives of the project are 1) to put constrains on the large uncertainties associated with current photochemical models by providing more accurate gas-phase kinetic and photochemical data on DMS oxidation chemistry. 2) Investigate particle formation from both DMS and DMSO. 3) Simultaneous high-time resolution measurements of dimethyl sulphide, oxidation products, halogen oxides, NO3 radical, and aerosol number/size distribution in 3 campaigns at sites with different geographical locations reflecting distinct aspects of DMS chemistry. 4) Use the data to determine the relative importance of the oxidants OH, NO3 and halogen oxides under different atmospheric conditions. 5) Use the laboratory data to construct a DMS chemistry module for CT-models capable of describing both the remote and polluted marine atmosphere and test of the models against the field data. The objectives will be achieved by a closely co-ordinated amalgamation of laboratory, field and modelling investigations. Expected impacts: The main deliverables of the project will initially be progressive constraints on kinetic/ mechanistic aspects of the oxidation chemistry of DMS and DMSO from laboratory and field experiments. This will be accompanied by high-time resolution field measurements of DMS, oxidation products, aerosols and other products relevant to the photo-chemistry. Based on this laboratory and field information a comprehensive gas/aerosol DMS-halogen-chemistry mechanism (g/a-DMS-HALO) module for incorporation in CT-models will be developed, which will be capable of describing DMS chemistry in both the remote and polluted marine atmosphere. The information can eventually be incorporated into global climatic models.
1. The kinetics of the reaction O(3P)+Cl2O=2ClO was studied using a fast flow system and ESR detection. Model calculations were employed to take into account secondary reactions of the intermediates. A rate constant of k=(3.1 +- 0.5) x 10 xx (-10) qcm sec xx (-1) was found at room temperature. An apparent activation energy of EA=(5.8 +- 1.2)kJ x Mol xx (-1) was derived. 2. The ECR-technique has been employed to follow the interaction of low-energy electrons with the molecules ClO2 and Cl2O. The following k-values for the capture of thermal electrons and activation energies for these processes were obtained: ClO2: k=(11.5 +- 0.5) x 10 xx (-10) qcm sec xx (-1); EA=(1.1 +- 0.4)kcal x Mol xx (-1); Cl2O: k=(2.6 +- 0.5) x 10 xx (-10) qcm sec xx (-1); EA=(0.5 +- 0.1)kcal x Mol xx (-1). The energy dependence of the capture process has been studied for electron energies less or equal 0.4 eU. 3. Electron capture properties are determined to be: k(HNO3)-(1.4 +- 0.7) x 10 xx (-8) qcm sec xx (-1) (1) k(ClONO3)=(3.9 ;- 1.2) x 10 xx (-9) qcm sec xx(-1) (2). From studies of the energy dependence of the processes and from thermodynamic considerations it is concluded that (2) is a dissociative reaction. Reactions of ClONO3 with O2 xx (-) are considered.