MDa. As in the 1200 K example in Fig. , and in all similar experiments at TNP ≤ 1300 K, the initial EEO2 was reasonably fast but increased during initial etching, in this case almost doubling during the first ~half hour of etching. EEO2 then decreased during the remainder of the experiment, dropping ~two orders of magnitude below the peak value after ~2.5 hours of oxidative etching, at which point the experiment was terminated. Because the initial fast etching was slower than for the NP in Fig. , the total mass lost was only ~50% of Minitial. The fact that this NP did not passivate as completely as the example in Fig. may reflect differences in the initial size and structure of the NPs, however, we also expect that higher temperatures should retard passivation by decreasing the surface lifetime of adsorbed oxygen species, inhibiting formation of the passivating SiO2 surface layer.