We point out that in addition to a vapor pressure equilibrium source for SO2 in an atmosphere of Io, recent laboratory sputtering results, combined with Voyager-measured particle fluxes, indicate that SO2 could also result from charged particle erosion of frost deposits on the satellite's surface. On the nightside, and for dayside frost patches where the temperature may be ≤100 ¿K, such erosion will be a dominant mechanism.