As TNP was initially stepped to 1400 K, Rbase was constant at ~0.02 Da/sec/nm 2 , due to background O2 and H2O reactions, then increased rapidly for TNP ≥ 1500 K, signaling the onset of sublimation. When O2 was added, the mass loss rates increased substantially (Roxid ≫ Rbase) for low TNP, but with increasing TNP the effect of O2 addition decreased, becoming negligible at 1600 K (Roxid ≈ zero). Then, during the 2 nd ramp, the mass losses in both Ar and O2 were essentially zero, i.e., the NP had changed irreversibly during the first TNP ramp, substantially reducing both the sublimation and oxidation rates.