SO(2) UPTAKE ON ICE SPHERES - LIQUID NATURE OF THE ICE-AIR INTERFACE

The amount Of SO2 gas absorbed by ice of known surface area at equilib rium was used to estimate the volume of liquid water present at the ic e-air interface at temperatures from -1 to -60-degrees-C. Calculations were based on Henry's law and acid dissociation equilibrium. The liqu id volume is lowest at lower temperatures and ionic strength and under most conditions was greater than the volumes calculated based on free zing-point depression. The equivalent liquid layer thickness, assuming that liquid water is uniformly distributed around the grains, ranged from 3-30 nm at -60-degrees-C to 500-3000 nm at -1-degrees-C. Correspo nding ionic strengths for the two temperatures were 1.7-0.0012 M and 0 .005-0.00009 M. Lower values were for ice made from distilled water, a nd higher values were for ice made from 10(-3) M NaCl. Estimated pH va lues were from 2.9 at -60-degrees-C to 4.1 at -1-degrees-C. Results de monstrate that gas absorption can be used to estimate an equivalent li quid volume and thickness for the ice-air interfacial region. While no t directly comparable to physical measurements, the estimated values s hould be directly applicable to modeling uptake of SO2 and other trace gases by ice. Lack of good thermodynamic data for temperatures below 0-degrees-C is the main limitation to applying this method.