One of the surprising observations for etching of pre-heated Si NPs was the low initial reactivity at 1200 K, which increased by an order of magnitude as the NPs etched. To account for this behavior, we noted that TNP during pre-heating was well above the bulk silicon Tmelt, so the NPs should have had thoroughly annealed surface structures. By analogy with the very low S0 values observed for Si(100), we proposed that the pre-melted Si NPs were smooth, with relatively few sites capable of dissociatively adsorbing O2, thus limiting EEO2. The observation of order-ofmagnitude increases in EEO2 during initial stage of etching was then attributed to atomic scale roughening of the surface, creating more reactive sites, similar to the increase seen under steadystate etching for Si(100). This scenario is also consistent with the observation that if the Si NPs were not pre-heated above Tmelt, then the initial EEO2 was substantially higher than for premelted NPs, but also increased much less as the NP etched. Since these NPs had not been not melted, their surfaces would have been rougher, making the initial reactivity higher, but also limiting the increase from surface roughening during etching.