Das Projekt "Untersuchung des Haushaltes der Halogenverbindungen in der Troposphaere (HALOBUD)" wird vom Umweltbundesamt gefördert und von Universität-Gesamthochschule Essen, Fachbereich 8 Chemie, Institut für Physikalische und Theoretische Chemie durchgeführt. General Information: The distribution and fate of volatile organohalogen compounds in the atmosphere has recently attracted considerable attention. In particular, it has been suggested that phytotoxic chloroacetic acids found in connifer needles arise from atmospheric precursors and that they have made a significant contribution to forest decline in a number of European regions. It is frequently assumed that halogenated organic compounds found in water air and soil are almost all of anthropogenic origin and hence it has been suggested that the use of organohalogen compounds should be reduced or even phased out. The scientific basis for such statements remains vague and often unproven. The purpose of this programme is to provide reliable scientific data concerning the fate of halogenated compounds in the atmosphere and the possibility of formation of organohalogen compounds, including haloacetic acids, from biogenic hydrocarbons. The objective of the first part of the programme is to provide mechanisms for the atmospheric degradation of a range of structurally selected organohalogen compounds and to determine kinetic and photochemical parameters required to quantify these processes This work will give rise to structure-reactivity relationships which will be used to make reliable predictions concerning the atmospheric oxidation of organohalogen species in general. Testing of these mechanisms will be carried out at the EUPHORE outdoor smog chamber facility. Secondly, the yields of chlorinated products arising from either the homogeneous reaction of Cl atoms with unsaturated hydrocarbons of biogenic origin or from heterogeneous photochemical reactions in the presence of sea-salt aerosols will be investigated. Finally, the chemical mechanisms developed in this programme will be used in tropospheric models to assess the relative importance of anthropogenic source to biogenic emissions in the formation of adsorbable organic halogen. The results of this study will enable regulatory authorities and producers to put anthropogenic emissions into a realistic global perspective. This information could reduce the costs incurred in regulation of important industrial uses of organohalogen compounds without detriment to the environment. Prime Contractor: Risoe National Laboratory, Department of Plant Biology and Biogeochemistry; Roskilde/Denmark.