Finally, the EEO2 for the carbon nano-onion NP, which was initially just above 10 -3 Da/O2 collision, similar to the other black carbon NPs, quickly dropped by 2 orders of magnitude as the NP lost ~4% of Minitial. As already discussed, we believe the initial high reactivity reflects the presence of reactive (non-onion) carbon on the NP surface, with reactivity dropping quickly as this material oxidized away or isomerized to onion structure.