Measurements of the Aerosol Light-scattering Coefficient at Ambient and 85% Relative Humidity on the ONR Pelican During ACE-2

The ultimate goal of the this project is to elucidate the impact of ambient relative humidity RH on the optical properties of the aerosols encountered in and above the Marine Boundary Layer MBL, and to do it in such a manner as to provide a basis for the parameterization of the impact of RH on satellite retrievals of aerosol properties. For example, several recent papers have suggested that the total light scattering from MBL aerosols will be a function not only of their composition but also of their initial dry size distribution cf., Hegg et al., 1992. Hence, one would expect considerable variability in the scattering hygroscopic growth factor and a need for measurements in a number of different MBL scenarios e.g., polluted, clean background, dustimpacted, etc. to fully characterize the importance of RH on MBL aerosol scattering. In a similar vein, Boucher and Anderson 1996 have pointed out that, contrary to assumptions commonly made in simple two-stream radiative transfer models, the various aerosol optical parameters necessary to estimate radiative forcing by aerosols can have considerable covariance and thus must be measured jointly to produce valid parameters. There are few such measurements in the marine atmosphere. These issues can best be resolved by measurements under a variety of conditions encompassing all of the aerosol types likely to be encountered in the MBL. Simultaneous with such in-situ measurements, remote retrieval of aerosol properties by for example AVHRR will permit assessment of the impact of aerosol hydration on satellite retrievals and thus support development of the parameterizations alluded to above. Recent studies have shown that water of hydration can dominate the aerosol optical properties in the marine atmosphere Hegg et al., 1997.