Study of the heterogeneous reaction of O3 with CH3SCH3 using the wetted-wall flowtube technique

[摘要] This work presents the heterogeneous kinetics of the reaction ofCH₃SCH₃ (dimethyl sulphide, DMS) with O₃ (ozone) in aqueous solutions of different ionic strengths (0, 0.1and 1.0M NaCl) using the wetted-wall flowtube (WWFT) technique. Henry's lawcoefficients 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 gasphase, using a numerical transport model of phase exchange, were determined to beH±s (Matm^-1) = 2.16±0.5 at274.4 K, 1.47±0.3 at283.4 K, 0.72±0.2 at291 K, 0.57±0.1at 303.4 K and 0.33±0.1 at 313.4 K on water, on 1.0M NaCl to be H = 1.57±0.4 at275.7 K, 0.8±0.2 at 291 K and on 4.0M NaCl to be H = 0.44±0.1 at 275.7 K and0.16±0.04 at 291 K, showing a significant effect of ionic strength, m, on the solubility ofDMS according to the equation ln (H/M atm^-1)= 4061 T^-1 - 0.052m² - 50.9mT^-1- 14.0. At concentrations of DMS_(liq) above 50mM, UV spectrophotometry of bothO_3(gas) and DMS_(gas) enables us to observe simultaneously the reactive uptake ofO₃ on DMS solution and the gas-liquid equilibration of DMS alongthe WWFT. The uptakecoefficient, g(gamma), of O₃ on aqueous solutions of DMS, varying between 1 and15·10^-6, showed a square root-dependence on the aqueous DMS concentration (as expectedfor diffusive penetration into the surface film, where the reaction takes place inaqueous solution). The uptake coefficient was smaller on NaCl solution in accord with the lowersolubility of O₃. The heterogeneous reaction of O_3(gas) withDMS_(liq) was evaluated from the observations of the second order rate constant(k^II) for the homogeneous aqueous reaction O_3(liq) +DMS_(liq) using a numerical model of radial diffusion and reactivepenetration, leading to k^II ±Dk^II(in units of 10⁸ M^-1s^-1) =4.1±1.2 at291.0 K, 2.15±0.65at 283.4 K and 1.8±0.5 at 274.4 K. Aside from the expected influence on solubility andaqueous-phase diffusion coefficient of both gases there was no significanteffect ofionic strength on k^II, that was determined for 0.1M NaCl, leading tok^II ± Dk^II(10⁸M^-1s^-1)= 3.2±1.0 at 288 K, 1.7±0.5 at 282 K and 1.3±0.4 at 276 K, and for 1.0M NaCl, leading to3.2±1.0 at 288 K, 1.3±0.4 at 282 K and 1.2±0.4 at 276 K, where the error limits areestimated from the output of the model calculations, taking the variability of individualruns at various DMS levels into account.

[学科分类] 大气科学