Sensitivity of multiangle imaging to natural mixtures of aerosols over ocean

Multiangle remote sensing data can discriminate among aerosol air mass types, as represented by climatologically probable, external mixtures of component particles. Retrievals are performed over a comparison space of four-component mixtures, selected from six commonly observed components having assumed, fixed microphysical properties but with mixing ratios free to vary from 0% to 100%. We refer to this approach, which assumes climatologically probable component particles and derives aerosol mixtures from the observations, as a “climatological retrieval.” On the basis of simulated Multiangle Imaging Spectroradiometer (MISR) observations over dark water, the retrieval can distinguish mixtures containing large, spherical particles (sea salt), nonspherical particles (accumulation and coarse mode dust), and small, dark particles (black carbon) to within 20% or better of each component’s true mixing ratio. This is sufficient to distinguish maritime from continental aerosol air masses. The retrievals, which use all nine MISR angles and the two wavelengths least affected by ocean surface reflectance (672 and 867 nm), are not good at distinguishing medium, spherical, nonabsorbing (sulfate) from medium, spherical, absorbing (carbonaceous) particles. However, the sum is retrieved to within 20% of the true mixing ratio or better. This is significantly more detail about the properties of particle mixtures than has previously been retrieved from satellite data, and in all cases, the derived total column aerosol optical depth remains well constrained, to at least 0.05 or 20%, whichever is larger. We expect the MISR data, with its frequent global coverage, to complement in situ and field data, which can provide greater detail about aerosol size and composition locally. This combined effort should advance our knowledge of aerosol behavior globally and our ability to model the impact of aerosols on the climatically important solar radiation budget.