We report low temperature cathodoluminescence spectroscopy measurements of the band edge emission from ZnO nanostructures grown by vapour phase transport on Si. A range of donor bound exciton emission lines are found and the Al-related emission at 3.3605 eV in particular shows a marked inhomogeneity in its distribution throughout the sample. Increased 3.3605 eV emission is seen at a range of locations in nanorods and nanosheets where different nanostructures cross or coalesce, suggesting aggregation of Al donors in ZnO at regions of crystal structure disruption. However, localized crystal structure disruption appears to be a necessary rather than a sufficient condition for Al aggregation, since increased 3.3605 eV emission is seen only at such regions, but not all such regions show increased emission, implying that the microscopic nature of such regions is important in determining Al aggregation. Supporting data are presented from well-aligned, non-crossing, nanorods on a-sapphire.