On the relationship between relative humidity and particle backscattering coefficient in the marine boundary layer determined with differential absorption lidar

A ground-based water-vapor differential absorption lidar (DIAL) has been ap plied to investigate aerosol properties in the convective marine boundary l ayer. The high resolution and accuracy of this instrument enables measureme nt of the particle backscattering coefficient beta(par) and the absolute hu midity under a single boundary layer cloud, so the hygroscopic growth of th e aerosol can be investigated in a single profile over a large range of the relative humidity RH. Arguing that the convective boundary layer was adiab atic and using the DIAL absolute humidity measurements, the relation beta(p ar)(RH) has been determined. This relation is compared with a simple aeroso l model. Assuming that the simplifications are valid, it follows that beta( par)(RH) is mainly sensitive to variations in the mass fraction of soluble material epsilon of an internally mixed aerosol. Applying this model to the experimental data, we found epsilon was in the range of 0.05-0.27 during t he measurements. These values are consistent with those from in situ partic le analyses at other anthropogenically influenced sites. These rather low s oluble mass fractions have strong implications for the magnitude of both ae rosol direct and indirect effects.