Airborne data acquired with the Office of Naval Research Pelican aircraft during the second Aerosol Characterization Experiment are analyzed to address the issue of the relationship between aerosol number and volume concentration in the marine atmosphere. In agreement with past studies, the mean value of the number-to-volume ratio is found to be 168¿67 μm-3 for the size range 0.06≤r≤1.8 μm. The data were further explored with respect to the factors influencing both the magnitude of the ratio and the linearity of the relationship between the number and volume concentrations. For the experimental venue examined here, a number of factors appear to modulate the magnitude of the number-to-volume ratio, and while the data are not definitive, the linearity of the number-volume relationship appears to arise naturally owing to the dominant particle growth mechanisms in the boundary layer. ¿ 2000 American Geophysical Union