Dry deposition of sulphur dioxide and ammonia on wet surfaces and the surface oxidation kinetics of bisulphite

The dry deposition of (SO2)g in air on a thin water layer was measured in a small wind tunnel at wind velocities from 0.5 to 6.0 m s−1 and water-layer thicknesses of 4–4600 μm. A reasonable estimate could be made of the concentrations in the gas and in the liquid phase, based on chemical analysis of the water layer and on Fickʹs first law. The interface is regarded as a reactive surface for gases, and the reactions with that can be described with uptake- and release-rate constants. For NH3 and SO2 these constants are strongly dependent on pH; NH3 was used as a pH regulator. We concluded from the experiments with different water-layer thicknesses that during dry deposition, oxidation of HSO3− occurs at the surface of the water. The greater part of the S(IV) oxidation does not take place in the bulk of the liquid and the results cannot be explained with the oxidation of the SO32− ion. The kinetic equation for the surface oxidation was found to be [S(IV)]1Vt−1=kox[HSO3−]102/3[Ox]gOnmols−1 with n = 2/3 for ozone and unknown for oxygen. The apparent rate constants (m4mol−1s−1) for oxygen (with arbitrarily n = 2/3) and ozone oxidation between 5 and 20°C are for oxygen: kO2=3.3×1021exp(6820/T) for ozone: kO3=7.7×107exp(1750/T) The oxidation of S(IV) with both O2 and O3 can be described with the reaction of an activated HSO3− or HS2O5− surface complex. The reaction-rate constants are independent of pH and water-layer thickness. The positive apparent activation energy points to a more complicated mechanism than is represented. With ozone, the reaction is of second order. The represented results can possibly be of importance for the modelling of the dispersion of air pollution. The results point to an influence of the drop size (available reactive surface) on the S(IV) oxidation rate.