Uptake of SO2 by polycrystalline water ice

We have investigated two previous experimental studies (Clapsaddle and Lamb, 1989; Conklin et al. 1989) of SO2 uptake into polycrystalline ice the results of which seem to conflict. Both studies employed porous packed beds prepared by freezing 200-mum-diameter water drops in liquid nitrogen followed by aging. In the absence of oxidation, uptake was measured via frontal chromatography at various temperatures between -60 and -1 degreesC, with SO2 mixing ratios between 15 and 100 ppb. The experiments differed primarily in the ice surface areas and exposure times, yielding purportedly equilibrium surface coverages that differed by more than a factor of 50. The uptake increased with temperature and with a less than linear dependence on partial pressure. Our comparison shows that a kinetic model is needed for interpretation partly explaining the apparent discrepancy between the two investigated uptake experiments. The uptake rates, its amount, and its temperature dependence suggest that SO2 dissociates and diffuses into an internal reservoir for example comprised of veins and nodes, but not into a surface layer as previously hypothesized. Whereas slow diffusion may remain undetected during the relatively short time scales of laboratory experiments, it may dominate trace gas uptake by natural ice. We suggest that dry deposition schemes of SO2 onto snowpacks in climate models should include the kinetics of uptake and account for the temperature and pressure dependencies found in the laboratory studies reviewed here, (C) 2001 Academic Press.