Uptake of Gas-Phase SO₂ and H₂O₂ by Ice Surfaces:  Dependence on Partial Pressure, Temperature, and Surface Acidity

The uptakes of gas-phase SO 2 and H 2 O 2 by ice surfaces have been investigated at temperatures from 213 to 238 K and from 10 -7 to 10 -4 Torr partial pressure. These experiments have been conducted in a low-temperature, coated-wall flow tube coupled to an electron-impact, quadrupole mass spectrometer which monitors changes in the SO 2 and H 2 O 2 partial pressure. The ice surfaces are formed by freezing liquid water. Unlike the uptakes of strong acids such as HNO 3 and HCl, the SO 2 and H 2 O 2 uptakes are fully reversible on the time scale of the experiment and the surface coverages are roughly a thousandth of a monolayer at 10 -6 Torr partial pressure and 228 K. The SO 2 uptakes scale with the square root of the partial pressure of the SO 2 gas, indicating that dissociation of the hydrated form of adsorbed SO 2 is occurring on the surface. The H 2 O 2 uptakes scale linearly with the H 2 O 2 partial pressure, indicating that dissociation does not occur. The uptakes are driven by H-bond interactions in this case. Support for these conclusions comes from uptake measurements with ice surfaces which were formed by freezing either acidic or basic aqueous solutions. Although the H 2 O 2 uptakes are independent of pH, the acidic ice surfaces considerably inhibit the SO 2 uptake and the basic surfaces enhance the SO 2 uptake. The results in this paper are consistent with atmospheric observations which show that both S(IV) and H 2 O 2 have low retention efficiencies after supercooled cloud droplets freeze, whereas the retention efficiency of HNO 3 is high. The uptakes are sufficiently small that scavenging of SO 2 and H 2 O 2 by ice clouds will not be significant.