Aerosol number-to-volume relationship and relative humidity in the eastern Atlantic

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 mu m(-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.