A sensitivity study of a theoretical model fo SO2 scavenging by water drops in air

Abstract

The rate at which SO2 is removed from air by a water drop has been investigated by solving numerically the convective diffusion equation for SO2 diffusing through air into a water drop where the species SO2.H2O, HSO3-, SO3- and SO4- were assumed to form. The drops were assumed to be either at rest or falling at terminal velocity. The falling drops had Reynolds numbers of 0.01, 4, 20 and 100, corresponding to drop radii of 10, 70.8, 142.3 and 305.5 μm, respectively. For the case of a falling drop the authors assumed either Hadamard-Rybczinski (HR) flow inside of the drop and Stokes flow outside of the drop, or flow fields described by the authors' own numerical solutions to the complete Navier-Stokes equation of motion. The computations were carried out for three models: model A which assumed that the resistance to diffusion lies entirely outside the drop (drop is a perfect sink), model B which assumed that the resistance to diffusion lies entirely inside the drop, and model C which assumed that the diffusion fields outside and inside the drop are coupled. From their studies the authors concluded that (1) model A does not realistically describe the scavenging of SO2 by a water drop; (2) although diffusion inside the drop controls the overall diffusion of SO2 into the drop, the coupling between the diffusion fields inside the drop to that outside the drop cannot be disregarded; (3) forced convection inside and outside a drop significantly affects the diffusion field and enhances transfer of SO2; (4) Stokes flow and HR flow, combined to describe low Reynolds number flow inside and outside a drop, underestimated the convective diffusion of SO2; (5) the diffusion field and transfer of SO2 are significantly affected by the drop size; (6) the S(IV) species formed inside a drop contribute differently to the diffusion field and transfer of SO2; and (7) raindrops and large cloud drops which fall through atmospheric pollution layers and industrial plumes cannot be considered at equilibrium with their environment. Therefore, for these conditions the concentration of SO3- available for conversion to SO4- must be computed from a solution of the time-dependent equations for transfer of SO2 by convective diffusion.