To study by field experiments the chemical behaviour of two important natural VOCs: isoprene in continental regions and DMS in maritime areas.
The photochemistry of short lived substances emitted in the lower troposphere cannot be described by simple schemes. Highly reactive volatile organic compounds (VOC's) are rapidly oxidized in the boundary layer. These substances not only react with hydroxyl radicals, but also with nitrate radicals and have probably an important role as precursors of peroxy radicals which in turn influence the photochemical budget of hydroxyls and ozone. In this concern, and due to their high emission rates, the importance of dimethylsulfide (DMS) produced by the ocean surface and isoprene produced by vegetation in forested areas is a major fate of the photochemical budget of the lower atmosphere. The degradation mechanisms and kinetics of oxidation of isoprene and DMS are not well known, particularly, the interactions of the compounds and their secondary products with O3, HOX, and NOX. This will be validated through laboratory experiments (see companion project LABVOC), but also validated in the field near strong surface sources, under low NOX and high NOX conditions, and under the influence of variable photochemical conditions with a particular emphasis on the difference between daytime and night time chemistry. Particularly, the hydroxyl and peroxy radicals budget will take into account the reaction of peroxy and nitrate radicals. Field experiments will be done for the study of DMS degradation on a coastal site "Pointe de Penmarc'h" located in western Britanny (France). This site has been proved to be an almost ideal site for the measurements of DMS photochemical degradation under these various influences. Isoprene degradation will be studied on a eucalyptus forest site.
Intensive field experiments in 1993 and 1994 will include the measurements of primary substances and their degradation products (sulfur dioxide, methane sulfonic acid, sulfate aerosols, organic acids, aldehydes) at different levels, the monitoring of background concentrations of the species involved in the photochemical system (NOX, O3, CH4, CO) and the determination of free radicals (RO2, HO2, NO3,). These data will be connected to radiative parameters (photolysis frequency of NO2 and NO3), meteorological and micrometeorological measurements in order to assess the fluxes, lifetimes and the reaction pathways of primary and secondary substances in the lower troposphere.
Funding SchemeCSC - Cost-sharing contracts
LA1 4YQ Lancaster /High Bantham
LS2 9JT Leeds