The reactions and fate of many natural and pollutant species in the atmosphere are influenced by the interaction between gas and liquid phase. Despite the acknowledged significance o f these heterogeneous processes, some factors determining the uptake of gases have still to be elucidated.
Although the equilibrium water-air partitioning is characterised by the Henry’s law coefficient, the kinetics of the uptake process are more complicated. The Resistance Model approach describes uptake as a combination of the individual transfer processes, i.e. gas diffusion, mass accommodation, liquid solubility and liquid reaction. The mass accommodation is defined as the probability of incorporation at the phase boundary on collision with the surface.
In this work, a new vertical wetted-wall flow reactor was constructed to study the accommodation of gas molecules by aqueous solutions. The trace gas was brought into contact with a liquid surface slowly flowing down the inside of the flow tube using a movable injector. Changes in gas phase concentration due to uptake by the liquid were monitored by wavelength-resolved UV absorption spectroscopy.
The apparatus was tested by measuring the reactive uptake of ozone on Na2S203 solutions. The derived mass accommodation coefficient of a = 4.3 x 10'2, with a lower limit of > 2.7 x 10°, at 293 K agrees with previously published data. A rate coefficient of k" = 3 .7 x 10s 1 m of1 s' 1 for the reaction of ozone with Na2S203 in water at 293 K was calculated.
The main focus of this work was on phenols since some are phytotoxic and have been observed in the ambient atmosphere in the gas phase as well as in fog and rain water. The uptake was measured on water and bromine water to distinguish between solubility-limited and gas and interface controlled regimes. Mass accommodation coefficients o f phenol, 2-nitrophenol and m-cresol were derived as a function of
temperature. At 293 K, measured values of a were 8.3 x 10'3, 9.4 x 10‘4 and6.3 x 1CT3, respectively, for these compounds. These values imply that under tropospheric conditions mass accommodation could be the rate limiting process for transfer across the interface.
In the Resistance Model mass accommodation is depicted as a continuous process where only clusters of critical size are taken up by the liquid phase. From the temperature dependence of a it was possible to derive the enthalpy and entropy for the transition (mass accommodation) between gas and solvated state. The size of the critical cluster (defined by N*) was determined for each phenol.
A general lack of experimental data regarding the Henry’s law coefficient of 2- nitrophenol necessitated independent measurement of this parameter in this work. A newly built bubble column was used to purge 2-nitrophenol from an aqueous solution. The Henry’s law coefficient was derived from the observed loss in the liquid phase as measured by UV absorption spectroscopy. Highly consistent data as a function of temperature were obtained, ranging from 420 M atm' 1 at 278 K to 60 M atm' 1 at 303 K. The enthalpy and entropy of solubility were determined as -50.0 kJ mol' 1 and -104.0 J mol' 1 K 1, respectively.