Measurements acquired from the Office of Naval Research (ONR) Pelican research aircraft during the second Aerosol Characterization Experiment (ACE 2) are analyzed to derive values for the dry (RH=40%) aerosol number-to-volume ratio in the submicron size range. This ratio is found to be relatively constant, with a mean value of 168¿21 μm-3, in agreement with previous studies elsewhere. The impact of ambient relative humidity (RH) on the dry number-to-volume is also quantified and a procedure for estimating the dry from the ambient ratio established. Finally, the feasibility of a remote retrieval of the aerosol number concentration in the submicron size range, essentially the cloud condensation nucleus concentration active at a nominal 0.2% supersaturation, is partially assessed. ¿ 2000 American Geophysical Union