Background aerosol light scattering characteristics: nephelometric observations at Mauna Loa Observatory compared with results at other remote locations

Background aerosol may be defined as that which exists in remote locations, well away from local variable sources. The total contribution to the optical aerosol extinction coefficient due to scattering and that contribution due to the scattering into the backward hemisphere (hemispheric backscatter) are important due to their role in determining atmospheric extinction and albedo respectively. Specially adapted instruments are described for measuring these parameters in remote sites at various altitudes in the troposphere. Preliminary field measurements are described and data compared for several sites and altitudes. The Mauna Loa Observatory, Hawaii, is of particular interest since it has been used for studies of CO₂ increases. The measured light scattering coefficient due to aerosol at 500 nm wavelength is ~ 10% of the Rayleigh scattering of the gas molecules and provides a base line to which future changes may be referred. The results for the hemispheric backscatter (or albedo) indicate a value occasionally above that coefficient predicted by the Mie formulae and assumed size distribution of spherical particles.