|da Rosa, MB; Behnke, W; Zetzsch, C: Study of the heterogeneous reaction of O3 with CH3SCH3 using the wetted-wall flowtube technique, Atmospheric Chemistry and Physics, 3, 1665-1673 (2003)|
This work presents the heterogeneous kinetics of the reaction of CH3SCH3 (di-Me sulfide, DMS) with O3 (ozone) in aq. solns. of different ionic strengths (0, 0.1 and 1.0M NaCl) using the wetted-wall flowtube (WWFT) technique. Henry's law coeffs. of DMS on pure water and on different concns. 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 detd. to be H (M atm-1) = 2.160.5 at 274.4 K, 1.470.3 at 283.4 K, 0.720.2 at 291 K, 0.570.1 at 303.4 K and 0.330.1 at 313.4 K on water, on 1.0M NaCl to be H = 1.570.4 at 275.7 K, 0.80.2 at 291 K and on 4.0M NaCl to be H = 0.440.1 at 275.7 K and 0.160.04 at 291 K, showing a significant effect of ionic strength, , on the soly. of DMS according to the equation In(H/M atm-1)=4061 T-1 -0.0522-50.9 T-1 - 14.0. At concns. of DMS(liq) above 50M, UV spectrophotometry of both O3(gas) and DMS(gas) enables us to observe simultaneously the reactive uptake of O3 on DMS soln. and the gas-liq. equilibration of DMS along the WWFT. The uptake coeff., (gamma), of O3 on aq. solns. of DMS, varying between 1 and 15 10-6, showed a square root-dependence on the aq. DMS concn. (as expected for diffusive penetration into the surface film, where the reaction takes place in aq. soln.). The uptake coeff. was smaller on NaCl soln. in accord with the lower soly. of O3. The heterogeneous reaction of O3(gas) with DMS(liq) was evaluated from the observations of the second order rate const. (kII) for the homogeneous aq. reaction O3(liq) + DMS(liq) using a numerical model of radial diffusion and reactive penetration, leading to kII kII (in units of 108 M-1 s-1) = 4.1 1.2 at 291.0 K, 2.150.65 at 283.4 K and 1.80.5 at 274.4 K. Aside from the expected influence on soly. and aq.-phase diffusion coeff. of both gases there was no significant effect of ionic strength on kII, that was detd. for 0.1M NaCl, leading to kII kII (108 M-1 s-1) = 3.2 1.0 at 288 K, 1.70.5 at 282 K and 1.30.4 at 276 K, and for 1.0M NaCl, leading to 3.21.0 at 288 K, 1.30.4 at 282 K and 1.20.4 at 276 K, where the error limits are estd. from the output of the model calcns., taking the variability of individual runs at various DMS levels into account.
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