During the initial short etching periods, the baseline-subtracted etching rates were small, as shown in the "Rates-baseline" text box, and Rbaseline measured between each etching period was roughly constant. Both observations indicate that the NP surface site distribution did not evolve significantly during this part of the experiment. During the first few minutes of the long etching period, the baseline-subtracted etching rate was -170 Da/sec, consistent with the rates measured at lower N2O pressures ("a" -"d"), assuming roughly linear dependence on PN2O. The mass loss rate gradually increased during the first part of etching period "e", indicating that the reactivity toward N2O was increasing. It reached a maximum at roughly the 7.5-hour mark, then decreased again, eventually becoming roughly constant after the ~11 hour mark. The final slope of M vs.