SULFUR-DIOXIDE REACTIONS ON ICE SURFACES - IMPLICATIONS FOR DRY DEPOSITION TO SNOW

Controlled exposure of ice to a reactive gas, SO2, demonstrated the im portance of the chemical composition of the ice surface on the accumul ation of acidity in snow. In a series of bench-scale continuous-flow c olumn experiments run at four temperatures (-1, -8, -30 and -60 degree s C), SO2 was shown to dissolve and to react with other species in the ice-air interfacial region at temperatures approaching the melting po int of ice. Experiments consisted of passing air containing SO2 throug h glass columns packed with 100 mu m ice spheres of varying bulk compo sition (0-5 mu M H2O2, and 0-1 mM NaCl), and analysing SO2 in the air and SO42- in the ice. At all temperatures (-60 to -1 degrees C), incre ased retention volumes were found for increasing ionic strength and ox idant concentration. At the coldest temperatures and with no NaCl, inc reased retention volumes for -60 vs -30 degrees C are consistent with SO2 uptake by physical adsorption. At warmer temperatures, -8 and -1 d egrees C, the observed tailing in the sorption curves indicated that o ther processes besides physical adsorption were occurring. The desorpt ion curves showed a rapid decrease for the warmer temperatures, indica ting the sorbed SO2 is irreversibly oxidized to SO42-. Results indicat e that aqueous-phase reactions can occur below -8 degrees C (i.e. -30 and -60 degrees C). Results for different salt concentrations show tha t increasing ionic strength facilitates SO2 oxidation at colder temper atures, which is consistent with freezing point depression. One enviro nmental implication is that snowpacks in areas with background SO2, ca n accumulate acidity during the winter months. As acidity accumulates, the solubility of SO2 will decrease causing a concomitant decrease in the air-to-surface flux of SO2. Modeling dry deposition of gases to s now surfaces should incorporate the changing composition of the ice su rface.