Absorbing aerosol radiative effects in the limb-scatter viewing geometry

[摘要] The limb-scatter satellite viewing geometry is well suited to detectinglow-concentration aerosols in the upper troposphere due to its longobservation path length (~200 km), high vertical resolution(~1–2 km) and good geographic coverage. We use the fullythree-dimensional radiative transfer code SASKTRAN to simulate thesensitivity of limb-scatter viewing Odin/OSIRIS satellite measurements toabsorbing mineral dust and carbonaceous aerosols (smoke and pure soot), aswell as to non-absorbing sulfate aerosols and ice in the upper troposphere.

At long wavelengths (813 nm) the addition of all aerosols (except soot) toan air only atmosphere produced a radiance increase as compared to air only,on account of the low Rayleigh scattering in air only at 813 nm. Theradiance reduction due to soot aerosol was negligible (<0.1%) atall heights (0–100 km).

At short wavelengths (337, 377, 452 nm), we found that the addition ofany aerosol species to an air only atmosphere caused a decrease insingle-scattered radiation due to an extinction of Rayleigh scattering inthe direction of OSIRIS. The reduction was clearly related to particle sizefirst, with absorption responsible for second-order effects only.Multiple-scattered radiation could either increase or decrease in thepresence of an aerosol species, depending both on particle size andabsorption. Large scatterers (ice, mineral dust) all increasedmultiple-scattered radiation within, below and above the aerosol layer.Small, highly absorbing pure soot particles produced a negligiblemultiple-scattering response (<0.1%) at all heights, primarilyconfined to within and below the soot layer. Medium-sized scatterersproduced a multiple-scattering response that depended on their absorbingproperties. Increased radiances were simulated as compared to air only atall short wavelengths (337, 377 and 452 nm) for sulfate aerosolparticles (non-absorbing) while decreased radiances were simulated for smokeparticles (absorbing) at 337 and 377 nm, where multiple scatteringinvolving the medium-sized carbonaceous particles amplified their absorbingproperties. At 452 nm, however, this effect was attenuated andalbedo-dependent.

At short wavelengths, the combined effect of single scattering decreases andmultiple scattering increases led to complex total radiance signatures thatgenerally could not unambiguously distinguish absorbing versus non-absorbingaerosols. Smoke aerosols led to a total radiance decrease (as compared toair only) at all altitudes above the aerosol layer (15–100 km). This uniquesignature was a result of the aerosols' strong absorbing properties,non-negligible scattering efficiency, and number concentrations high enoughto make multiple scattering effects due to the aerosol itself significant.Thus, in the limb-scatter viewing geometry scene darkening above the aerosollayer is unambiguously due to absorption whereas scene darkening within andbelow the aerosol layer can simply be the result of a reduction insingle-scattered radiance. Our simulations show a greater scene darkeningfor decreasing wavelengths, increasing surface albedo, decreasing solarzenith angle, and increasing particle number concentration, however, at 337 nmthis effect did not exceed 0.5% of the total radiance due to air only,making the unique identification of medium-sized carbonaceous aerosols,i.e., smoke, difficult. Scene darkening (or brightening) varies linearlywith particle number concentration over three orders of magnitude.

A fortuitous, unexpected implication of our analysis is that limb-scatterretrievals of aerosol extinction are not sensitive to external informationabout aerosol absorption.

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