As noted above, we previously concluded 5 that graphitic NPs become inert to O2 at high TNP because they, or at least their surface layers, evolve to have very few under-coordinated surface atoms, and speculated that carbon nano-onion (multi-wall fullerene) structure was most likely. Carbon nano-onions can be made in bulk by annealing nano-diamond with structural characterization by TEM. We, therefore, did a number of experiments to characterize the TNP and time dependence of O2 etching of NPs from nano-onion feedstock prepared by annealing nano-diamonds in a graphite crucible in an Ar-filled furnace at ~2100 K for 2 hours, giving material with clear nano-onion structure in TEM. Fig. shows a nano-onion NP oxidation experiment under conditions identical to those used in Figs. 4, 5, 6, and S11. The initial mass was ~9.9 MDa, corresponding to a nominal diameter of ~24.2 nm, assuming a density = 2.21 g/cm for annealed ultradispersed diamonds, and it may have been either a single nanoonion or small aggregate.