Torr) for NPs of similar mass (hence similar surface area/volume ratios). These two NPs showed the same qualitative etching behavior, with EEO2 initially increasing then decreasing toward passivation, however, there were also obvious differences: 1. The initial EEO2 was more than an order of magnitude lower for the silicon NP compared to the silica NP. 2. The initial increase in EEO2 and the time to reach peak EEO2 were larger for Si (~10x in ~80 minutes) than for silica (~2x in ~15 minutes). 3. Both reached similar levels of passivation (EEO2 < 10 -5 Da/collision) after similar amounts of mass loss (~75% of Minitial), but passivation took more than twice as long for the silicon NP.