On the scavenging of gaseous nitrogen compounds by large and small rain drops I. A wind tunnel and theoretical study of the uptake and desorption of NH3 in the presence of CO2

Abstract

An experimental study has been carried out in the Mainz vertical wind tunnel to determine the rate at which NH3 in the presence of CO2 is absorbed by freely suspended water drops. The experimental uptake rates were found to be in good agreement with the rates predicted by the Kronig-Brink convective diffusion model and, for gas concentrations in the ppbv range also by the model in which it is assumed that the absorbed gas is well mixed inside the drop (henceforth called well mixed model). The same conclusion was shown to apply also to the desorption of NH3 from a drop previously exposed to NH3. The latter result is in contrast to the desorption of SO2 which must be described by a model which accounts for the diffusion of the species inside the drop. Comparison of our experimental results with theory show further that the uptake of NH3 in presence of CO2 is significantly overestimated if the slow reaction CO2(aq)+H2O⇌HCO3−+H+ is neglected in the theoretical computation.