The overall objective of this project is to examine by means of laboratory experiments, the chemical behaviour of two important, naturally emitted volatile organic compounds (VOCs): isoprene (in continental regions) and dimethyl sulphide (DMS, in marine areas).
Hydrogen-abstraction fractions for the reactions of hydroxyl radicals with isoprene, 1-butene and isobutene have been determined from the amount of water formed per hydroxyl reacted at large excess alkene/diene. Using discharge flow - molecular beam mass spectrometry (DF-MBMS) techniques, abstraction fractions of 16+/-8, 37+/-14 and 24+/-8% were obtained for isoprene, 1-butene and isobutene respectively (preliminary data). In a second part of the work, an additivity based structure activity relationship has been proposed that allows predictions of the total rate constant and the detailed primary product distributions of alkene/diene plus hydroxyl radical reactions.
The additivity approach for the prediction of total rate constants and products distributions can be a powerful tool in modelling the tropospheric hydroxyl radical initiated oxidation of biogenic and anthropogenic unsaturated volatile organic compounds (VOC). Regarding primary product distributions, experimental data are needed for full validation.
The project deals with the study of the (photo) degradation products of isoprene and DMS in their reactions with OH- and NO3-radicals, and with O3 (only isoprene). These studies will be performed in the absence of NOX (NO and NO2) or with various concentrations of NOX, in order to identify the major intermediate species which are generated in the different reaction systems. Especially the role played by the various peroxy radicals (RO2) will be examined, in order to establish their chemical fate under tropospheric conditions. In the case of isoprene, the yields of the products will be determined: carbonyl compounds, nitrates, peroxynitrates, organic hydroperoxides, organic acids, etc. In the case of DMS, the yields of the important products, SO2, methylsulphonic acid, dimethylsulphoxide, sulfate, aerosol (CCN) will be investigated.
In addition, separate kinetic studies will be carried out on specific radicals involved in the (photo)degradation mechanism of isoprene and DMS, or on radicals possessing similar functional and/or structural groups: e.g. the kinetics and product distribution of the reactions RO2 + RO2, RO2 + HO2, RO2 + NO3; study of the reactivity of CH3,S, CH3SO and CH3SO2 radicals with O2, O3 and NO2.
Finally, the development of a chemical box-model to describe the atmospheric transformation of isoprene and DMS will be part of this project with use of data obtained in both laboratory and field measurements. Since isoprene is used as representative VOC in modelling studies of the photochemistry occurring in the continental boundary layer, a prediction of the resulting ozone and other photo-oxidants is needed. The model involving DMS oxidation is required to assess the sulphur budget and its role in aerosol formation. The work will be performed by nine European research groups, which will deploy a variety of complementary kinetic, spectroscopic and chromatographic techniques.
Funding SchemeCSC - Cost-sharing contracts
OX11 0RA Didcot - Oxfordshire