During oxidation measurements, PO2 was typically set to 4.3 x 10 -5 Torr, while the trap chamber background pressure, Pbackground, was typically ~10 Torr, with residual gas analysis showing significant background pressures of O2 and water, along with inert gases (Ar and N2). As a result, small background rates of NP mass loss (Rbackground) are observed with only Ar added to the trap, even at low TNP, where sublimation is negligible. With increasing TNP, the mass loss rates in Ar increase rapidly, as sublimation (Rsublimation) increasingly dominates the mass loss. The sum of the two, i.e., Rsublimation + Rbackground, is defined as the baseline rate: Rbase. To measure oxidative mass loss rates (Roxid), the rate of mass loss with O2 added is corrected by subtracting Rbase measured just before and after each O2 dose at the same TNP. Raw mass loss rates are given in Da/second, but to allow rates for different NPs to be compared, surface-area-normalized rates (Da/sec/nm 2 ) are given, obtained by dividing the raw rates by the estimated NP surface area, approximated for a sphere of the measured mass with the bulk density. The actual surface areas may be larger, thus the area-normalized rates are upper limits. Finally, to allow comparison of experiments at different PO2 values, the Roxid values are further converted to O2 etching efficiencies, EEO2, given in units of Da/O2 collision, by dividing Roxid by the gas-kinetic collision rate for each pressure used. For the most common PO2 value (4.3 x 10 -5 Torr), the collision rate is ~154 O2 collisions/sec/nm 2 .