A new model for the basal pressure of Io's atmosphere is developed. This model takes into account the previously ignored fact that much of Io's surface has very high porosity, typically ~90%. Such porosity allows efficient subsurface cold trapping of atmospheric gases which tends to keep ambient surface pressures very low. SO2 is the only gas identified on Io, and the basal pressures for atmospheric models are usually pegged to local surface temperature via the SO2 vapor pressure equilibrium curve. Near Io's subsolar point the pressure in equilibrium with a surface SO2 frost deposit is ~10-7 bar. Porous surface models of the type we develop invoke equilibrium with the colder, subsurface permafrost (at ~3-cm depth) and yield pressures of ~10-12 bar. The subsurface cold trapping model explains many but not all observations relevant to Io's atmosphere. The new subsurface cold trapping model and the earlier surface frost equilibrium model, when taken together, provide lower and upper limits, respectively, on the basal SO2 pressure of Io's atmosphere.