Sulfur dioxide reactions on ice surfaces: implications for dry deposition to snow

Abstract

Controlled exposure of ice to a reactive gas, SO 2, demonstrated the importance of the chemical composition of the ice surface on the accumulation of acidity in snow. In a series of bench-scale continuous-flow column experiments run at four temperatures (−1, −8, −30 and −60°C), SO 2 was shown to dissolve and to react with other species in the ice-air interfacial region at temperatures approaching the melting point of ice. Experiments consisted of passing air containing SO 2 through glass columns packed with 100μm ice spheres of varying bulk composition (0–5μM H 2O 2, and 0–1 mM NaCl), and analysing SO 2 in the air and SO 4 2− in the ice. At all temperatures (−60 to −1°C), increased retention volumes were found for increasing ionic strength and oxidant concentration. At the coldest temperatures and with no NaCl, increased retention volumes for −60 vs −30°C are consistent with SO 2 uptake by physical adsorption. At warmer temperatures, −8 and −1°C, the observed tailing in the sorption curves indicated that other processes besides physical adsorption were occurring. The desorption curves showed a rapid decrease for the warmer temperatures, indicating the sorbed SO 2 is irreversibly oxidized to SO 4 2−. Results indicate that aqueous-phase reactions can occur below −8°C (i.e. −30 and −60°C). Results for different salt concentrations show that increasing ionic strength facilitates SO 2 oxidation at colder temperatures, which is consistent with freezing point depression. One environmental implication is that snowpacks in areas with background SO 2, can accumulate acidity during the winter months. As acidity accumulates, the solubility of SO 2 will decrease causing a concomitant decrease in the air-to-surface flux of SO 2. Modeling dry deposition of gases to snow surfaces should incorporate the changing composition of the ice surface.