Study of the heterogeneous reaction of O3 with CH 3SCH3 using the wetted-wall flowtube technique

Abstract

This work presents the heterogeneous kinetics of the reaction of CH 3SCH3 (dimethyl sulphide, DMS) with O3 (ozone) in aqueous solutions of different ionic strengths (0, 0.1 and 1.0M NaCl) using the wetted-wall flowtube (WWFT) technique. Henry's law coefficients of DMS on pure water and on different concentrations of NaCl (0.1M - 4.0M) in the WWFT from UV spectrophotometric measurements of DMS in the gas phase, using a numerical transport model of phase exchange, were determined to be H ±σ(Matm-1) = 2.16±0.5 at 274.4 K, 1.47±0.3 at 283.4 K, 0.72±0.2 at 291 K, 0.57±0.1 at 303.4K and 0.33±0.1 at 313.4K on water, on 1.0M NaCl to be H = 1.57±0.4 at 275.7 K, 0.8±0.2 at 291K and on 4.0M NaCl to be H = 0.44±0.1 at 275.7K and 0.16±0.04 at 291 K, showing a significant effect of ionic strength, μ, on the solubility of DMS according to the equation ln(H/Matm-1)=4061 T-1-0.052μ2-50.9μ T-1-14.0. At concentrations of DMS(liq) above 50μM, UV spectrophotometry of both O3(gas) and DMS(gas) enables us to observe simultaneously the reactive uptake of O3 on DMS solution and the gas-liquid equilibration of DMS along the WWFT. The uptake coefficient, γ (gamma), of O3 on aqueous solutions of DMS, varying between 1 and 15 10-6, showed a square root-dependence on the aqueous DMS concentration (as expected for diffusive penetration into the surface film, where the reaction takes place in aqueous solution). The uptake coefficient was smaller on NaCl solution in accord with the lower solubility of O3. The heterogeneous reaction of O3(gas) with DMS(liq) was evaluated from the observations of the second order rate constant (κII) for the homogeneous aqueous reaction O3(liq) + DMS(liq) using a numerical model of radial diffusion and reactive penetration, leading to κII ±ΔκII (in units of 108M-1 s-1) = 4.1±1.2 at 291.0 K, 2.15±0.65 at 283.4K and 1.8±0.5 at 274.4 K. Aside from the expected influence on solubility and aqueous-phase diffusion coefficient of both gases there was no significant effect of ionic strength on κII, that was determined for 0.1M NaCl, leading to κII ± 1κII (108M-1 s -1) = 3.2 ± 1.0 at 288K, 1.7±0.5 at 282K and 1.3±0.4 at 276 K, and for 1.0M NaCl, leading to 3.2±1.0 at 288 K, 1.3±0.4 at 282K and 1.2±0.4 at 276 K, where the error limits are estimated from the output of the model calculations, taking the variability of individual runs at various DMS levels into account. © 2003 European Geosciences Union.