To more clearly separate the effects of time and TNP on NP evolution, a series of experiments were done in which NPs were allowed to evolve over time under constant reaction conditions. An example is shown in Fig. , for a carbon black NP with initial mass of ~59.9 MDa, which if spherical with the bulk density (~1.8 g/cm 3 ) would have diameter of ~47.3 nm. Before the start of the data shown, the NP had been heated to 1900 K for 10 minutes for cleaning, then at the beginning of the data record, it was being held at 1300 K for Q-stepping (purple background). TNP was then set to 1500 K, and held for > 8 hours. Rbase was measured (unshaded background), then PO2 was set to 4.3 x 10 -5 Torr for the rest of the experiment (cyan background). The M vs. time record was divided into periods with near-constant mass loss rates, and fit to obtain the average slopes during each period. Periods with slopes greater than the initial Rbase are indicated by red fit lines and roman numeral labels. Periods with slopes near Rbase (i.e., Roxid ≈ zero) are shown with blue regression lines without labels. Fig. shows how the EEO2 values varied as the NP lost mass, using the same colors and labels as in frame A. The blue points with downward arrows indicate time periods when the slope of M vs. time ≈ Rbase, corresponding to Roxid and EEO2 ≈ zero. From our ability to detect small slope differences, we roughly estimate that our sensitivity limit for EEO2 is ~10 -6 Da/O2 collision.