A ground-based water-vapor differential absorption lidar (DIAL) has been applied to investigate aerosol properties in the convective marine boundary layer. The high resolution and accuracy of this instrument enables measurement of the particle backscattering coefficient βpar and the absolute humidity under a single boundary layer cloud, so the hygroscopic growth of the aerosol can be investigated in a single profile over a large range of the relative humidity RH. Arguing that the convective boundary layer was adiabatic and using the DIAL absolute humidity measurements, the relation βpar(RH) has been determined. This relation is compared with a simple aerosol model. Assuming that the simplifications are valid, it follows that βpar(RH)is mainly sensitive to variations in the mass fraction of soluble material &egr; of an internally mixed aerosol. Applying this model to the experimental data, we found &egr; was in the range of 0.05--0.27 during the measurements. These values are consistent with those from in situ particle analyses at other anthropogenically influenced sites. These rather low soluble mass fractions have strong implications for the magnitude of both aerosol direct and indirect effects. ¿ 2000 American Geophysical Union<