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

Abstract

Controlled exposure of ice to a reactive gas, SO₂, 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), SO₂ 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 SO₂ through glass columns packed with 100-μm ice spheres of varying bulk composition (0–5 μM H₂O₂, and 0–1 mM NaCl), and analysing SO₂ in the air and SO₄²⁻ 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 SO₂ 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 SO₂ is irreversibly oxidized to SO₄²⁻. 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 SO₂ oxidation at colder temperatures, which is consistent with freezing point depression. One environmental implication is that snowpacks in areas with background SO₂, can 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 snow surfaces should incorporate the changing composition of the ice surface.