UNCERTAINTY AND SENSITIVITY ANALYSES OF OH-INITIATED DIMETHYL SULFIDE (DMS) OXIDATION-KINETICS

A numerical experiment has been conducted on the OH-initiated tropospheric oxidation of DMS. This involved the selection of a set of reactions describing the OH-initiated oxidation kinetics and the conversion of the present level of uncertainty of the system into uncertainty ranges and distributions for the relevant system parameters (kinetic constants and initial concentrations). Uncertainties have been propagated through the model onto the output variables of interest. This has allowed (a) the uncertainty in model prediction to be quantified and compared with observations (uncertainty analysis) and (b) the relative importance of each input parameter in determining the output uncertainty to be quantified (sensitivity analysis). Output considered were the ratio of MSA/(SO2 + H2SO4) concentration at a given time, the ratio SO2/H2SO4, the total peroxynitrate species concentrations and the relative fraction of SO2 and H2SO4 formed through the various pathways. Conditional upon the model and data assumptions underlying the experiment, the following main conclusions were drawn: (1) The possibility of direct formation of SO3 without SO2 as intermediate as suggested by Bandy et al. (1992) and Yin et al. (1990), involving direct thermal decomposition of CH3SO3 . does not seem to play a major role in the overall generation of sulphate. This is relevant to the issue of gas to particle conversion over remote areas. (2) Reaction of CH3SOO . intermediate may be the most important pathway to the formation of SO2. (3) The dominating peroxynitrate is CH3S(O)(2)O2NO2. Through sensitivity analysis the kinetic constants have been identified which - because of their uncertainty and of their impact on the output - mostly contribute to the output uncertainty.