One surprise was that under O2 etching conditions, with TNP ≤ 1600 K, all graphitic and carbon black NPs eventually passivated, i.e., became inert toward further O2 attack, implying that the NP surfaces had transformed to be nearly free of reactive, under-coordinated sites. This was attributed to evolution of the NPs to have surface layers with fully-coordinated, nano-onion (fullerene-like) structures, which are known to form when carbon NPs are annealed. Experiments with carbon nano-onion NPs found, as expected, that some were initially inert to O2, while others had non-zero initial reactivity due to contamination by non-onion carbon, but passivated rapidly as this contamination etched away.