The etching efficiency, EEN2O, is plotted just below the M vs. time plot, correcting for both the baseline mass loss rate from sublimation, and the NP surface area, which would have decreased as M decreased. EEN2O at the start of the final etching period was ~6 x 10 -6 Da/collision, fluctuating but generally increasing over the first ~3 hours of the period to ~2.4 x 10 -5