The RAL retrieval scheme derives profiles of number density on a set of pressure levels, spaced approximately every 4-6 km in altitude (taken from the SPARC-DI grid). The optimal estimation method is used. Averaging kernels are provided on this basis. It is noted that the vertical resolution of the retrieval is relatively coarse compared to the vertical grid and that tropospheric levels in particular have significant bias towards the assumed a priori state. It is therefore important to take account of the characterisation of the retrieval provided by the averaging kernels when comparing these results to other data-sets, particularly where those are more highly vertically resolved.