To allow comparisons between NPs of different sizes, the absolute mass loss rates were normalized to the nominal NP surface areas, estimated from M by assuming spherical shape and the graphite bulk density. Since the actual surface areas are certainly higher, the area-normalized rates (Da/sec/nm 2 ) are upper limits on the true rate/area. To allow experiments to be compared for different reactants at different partial pressures, the area-normalized rates were further converted to etching efficiencies ("EEN2O" or "EEO2", in units of Da of mass lost/reactant collision) using gas kinetic collision rates per nm 2 calculated from the reactant partial pressures.