On the relationship between relative humidity and particle backscattering coefficient in the marine boundary Layer

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 b and the absolute humidity under a single boundary layer cloud so that the deliquescence 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 b(rh) has been determined. This relation is compared with a simple aerosol model. Assuming that the simplifications are valid, it follows that b(rh)is mainly sensitive to variations in the mass fraction of soluble material e of an internally mixed aerosol. Applying this model to the experimental data, we found that e 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.