Comparison of aerosol single scattering albedos derived by diverse techniques in two North Atlantic experiments

Abstract

Aerosol single scattering albedo ω (the ratio of scattering to extinction) is important in determining aerosol climatic effects, in explaining relationships between calculated and measured radiative fluxes, and in retrieving aerosol optical depths from satellite radiances. Recently, two experiments in the North Atlantic region, the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) and the Second Aerosol Characterization Experiment (ACE-2), determined aerosol ω by a variety of techniques. The techniques included fitting of calculated to measured radiative fluxes: retrievals of ω from skylight radiances: best fits of complex refractive index to profiles of backscatter, extinction, and size distribution; and in situ measurements of scattering and absorption at the surface and aloft. Both TARFOX and ACE-2 found a fairly wide range of values for ω at midvisible wavelengths (~550 nm), with 0.85 ≤ ωmidvin ≤ 0.99 for the marine aerosol impacted by continental pollution. Frequency distributions of ω could usually be approximated by lognormals in ωmax -ω, with some occurrence of bimodality, suggesting the influence of different aerosol sources or processing. In both TARFOX and ACE-2, closure tests between measured and calculated radiative fluxes yielded best-fit values of ωmidvin of 0.90 ± 0.04 for the polluted boundary layer. Although these results have the virtue of describing the column aerosol unperturbed by sampling, they are subject to questions about representativeness and other uncertainties (e.g., thermal offsets, unknown gas absorption). The other techniques gave larger values for ωmidvin for the polluted boundary layer, with a typical result of ωmidvin = 0.95 ± 0.04. Current uncertainties in ω are large in terms of climate effects. More tests are needed of the consistency among different methods and of humidification effects on ω.