The activity of point defects within cubic-ice nanocrystals has been probed using isotopic exchange of D2O molecules isolated intact within H2O aerosol particles of average radii ranging from 12 to 45 nm. The observed rates of conversion of D2O to dynamically coupled HDO molecules, and ultimately to isolated HDO, offer a direct evaluation of the activity of both the protonic and orientational (Bjerrum) defects within a few nm of the ice surface. We have sought answers to two interesting questions related to proton transport in cubic ice: (1) is Bjerrum-defect activity enhanced near the ice surface as a consequence of the structural defects associated with the surface, and (2) does proton trapping in the disordered surface region lead to a reduced proton activity, as reported for amorphous solid water? The new data indicate that Bjerrum defect activity within ice nanocrystals exceeds that of bulk ice for similar temperatures by more than an order of magnitude. In contrast, the observed behavior of protonic defects matched closely that of thick ice films.