Albedo measurements and optical sizing of single aerosol particles

Aerosols play an important role in global climate change by their interactions with incoming solar radiation and outgoing longwave radiation from the planetary surface. The climate effects of aerosols depend on their scattering and absorption properties. This article describes the development of an instrument (ASTER: Aerosol Scattering To Extinction Ratio) that simultaneously measures the scattering and extinction of single aerosol particles. ASTER uses a high-Q cavity to amplify the extinction signal and innovative optics to collect the scattered light. It can distinguish many partially absorbing particles from a few black ones even if the bulk absorption is the same. Optical sizing and single-scattering albedo measurements were made for laboratory-generated particles with diameters from about 300 nanometers to above one micrometer. Using this prototype instrument, changes in albedo for single particles of 20% or greater were detected by measurement of the scattering and extinction. Optical sizing of the individual particles to within similar to 50 nm was accomplished using the ratio of the forward scattered light to the total scattering. Initial measurements of laboratory air showed a mode of highly absorbing particles. This article reports design and early laboratory tests on ASTER.