Because passivation was observed only after hours of etching and loss of >50% of Minitial, it is clear that Oads must almost all desorb, much of it as SiO to account for the mass loss. O or O2 desorption may also be significant channels for Oads loss. For example, for hot carbon surfaces, Minton and co-workers have shown that CO and CO2 are major Oads desorption channels only for T < ~1300 K, with O desorption dominating at higher T. Neither our experiments nor the Si(100) etching studies were sensitive to O or O2 desorption, which result in no net change to the surface, and simply decrease the net O2 sticking probability. The net sticking probabilities extracted by modeling Si NP etching and passivation are in the 10 -4 range, and therefore are not inconsistent with substantial O or O2 loss. Formation of surface SiO2 must occur to explain the eventual passivation but must be a minor channel, at least during the initial hours-long phase of fast etching, because otherwise the NP would passivate quickly, after little total mass loss. A similar consideration applies to Oads diffusion to subsurface sites. If this were highly efficient, the NP mass would increase initially, which was never observed, and for even modest diffusion efficiency, the NP bulk would saturate quickly.