Investigation of the uptake rate of ozone and methyl hydroperoxide by water surfaces

The uptake kinetics of ozone (O-3) and methyl hydroperoxide (CH3OOH, MHP) by aqueous solutions were studied as a function of temperature using the droplet train technique combined with mass spectrometry detection. The uptake of ozone by pure water was found to be too small to be directly measured. Using NaI as a scavenger increased the uptake coefficient gamma from below the detection limit to a range from 0.0037 to 0.0116 for I- activities in the range from 0.3615 to 2.889 at 282 K. From these experiments, we estimated the second-order rate constant for the reaction O-3 + I- --> products to be in the range 3.2 x 10(8) to 2.4 x 10(9) M-1 s(-1) for temperature between 275 and 293 K. The activation parameters for this reaction were also estimated. For methyl hydroperoxide, the uptake rate on pure water was fast enough to be directly measured. According to the physicochemical properties of this hydroperoxide, the uptake was mainly due to the diffusion and accommodation processes. It was therefore possible to measure its mass accommodation coefficient alpha as a function of temperature. The observed values are in the range 0.92 x 10(-2) to 2.08 x 10(-2) for temperature between 281 and 261 K. The activation parameters for the accommodation were also determined.