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 res ults of which seem to conflict. Both studies employed porous packed beds pr epared by freezing 200-mum-diameter water drops in liquid nitrogen followed by aging. In the absence of oxidation, uptake was measured via frontal chr omatography at various temperatures between -60 and -1 degreesC, with SO2 m ixing 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 inc reased with temperature and with a less than linear dependence on partial p ressure. Our comparison shows that a kinetic model is needed for interpreta tion partly explaining the apparent discrepancy between the two investigate d uptake experiments. The uptake rates, its amount, and its temperature dep endence suggest that SO2 dissociates and diffuses into an internal reservoi r for example comprised of veins and nodes, but not into a surface layer as previously hypothesized. Whereas slow diffusion may remain undetected duri ng the relatively short time scales of laboratory experiments, it may domin ate trace gas uptake by natural ice. We suggest that dry deposition schemes of SO2 onto snowpacks in climate models should include the kinetics of upt ake and account for the temperature and pressure dependencies found in the laboratory studies reviewed here, (C) 2001 Academic Press.