For example, the variation in EEO2 for the graphite NPs was almost a factor of 10, with average EEO2 of ~1.6 x 10 -3 Da/O2 collision. Previously reported 1500 K O2 etching rates for graphitic NPs 5 also give EEO2 values in the same broad range. Note, however, that the smallest graphite NP (~11 MDa, raw data in Fig. ) had EEO2 about 10 times lower than average, and if this NP is excluded, the EEO2 values varied by only a factor of ~three, with average EEO2 of ~1.75 x 10 -3 . NPs with graphitic structures should always be reactive toward O2. For example, an idealized graphite NP, consisting of a stack of graphene layers, would have unreactive basal planes on the top and bottom faces, but the sides would expose the edges of the stacked graphene layers, of which ~50% of the atoms are under-coordinated and reactive toward O2 in our temperature range. Real graphite NPs should have more sites for a given mass NP due to defects and irregular shapes (i.e. higher surface area). The small graphite NP was, thus, anomalously unreactive, and possible reasons are discussed below.