Perfect nano-onions would have no under-coordinated surface sites, and therefore should be inert to O2 under these conditions (as is HOPG, for example), thus the surprise is that the initial EEO2 was so large. We conclude that the nano-onion NP initially had either a rather defective surface layer, or had significant surface contamination by non-onion-structured carbon. As this reactive carbon burned away, the NP quickly became inert. For an onion NP of this initial mass, roughly 5.3% of the mass would be in the surface layer, thus the ~4% mass lost as the NP became inert corresponds to only ~0.75 ML.