Comparing just the three pre-heated silica NPs, EEO2 increased initially as the NPs began to etch, peaking after 30 -40 minutes of etching, then decreasing by several orders of magnitude as the NPs passivated. Passivation for the two silica NPs studied at PO2 = 4.3 x 10 -5 Torr occurred on similar time scales (despite the 100 K difference in TNP), with EEO2 dropping below 10 -4 Da/O2 collision after ~2 hours of oxidation. The other silica NP (green triangles) passivated on a much slower time scale, but this was the experiment (Fig. ) in which PO2 was halved after ~10 minutes and halved again between the 1.25 and 1.5 hour points, to test the effect of varying PO2 on the etching kinetics. Although EEO2 was found to be PO2 independent, i.e., the etching rate was linear with PO2, clearly the lower oxygen pressure substantially slowed the passivation process, which is not surprising, given that SiO2 nucleation and growth depend on the Oads coverage.