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| Titel |
Absorbing aerosol radiative effects in the limb-scatter viewing geometry |
| VerfasserIn |
A. Wiacek, R. V. Martin, A. E. Bourassa, N. D. Lloyd, D. A. Degenstein |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 6, no. 10 ; Nr. 6, no. 10 (2013-10-22), S.2761-2776 |
| Datensatznummer |
250085088
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| Publikation (Nr.) |
 copernicus.org/amt-6-2761-2013.pdf |
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| Zusammenfassung |
The limb-scatter satellite viewing geometry is well suited to detecting
low-concentration aerosols in the upper troposphere due to its long
observation path length (~200 km), high vertical resolution
(~1–2 km) and good geographic coverage. We use the fully
three-dimensional radiative transfer code SASKTRAN to simulate the
sensitivity of limb-scatter viewing Odin/OSIRIS satellite measurements to
absorbing mineral dust and carbonaceous aerosols (smoke and pure soot), as
well as to non-absorbing sulfate aerosols and ice in the upper troposphere.
At long wavelengths (813 nm) the addition of all aerosols (except soot) to
an 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. The
radiance reduction due to soot aerosol was negligible (<0.1%) at
all heights (0–100 km).
At short wavelengths (337, 377, 452 nm), we found that the addition of
any aerosol species to an air only atmosphere caused a decrease in
single-scattered radiation due to an extinction of Rayleigh scattering in
the direction of OSIRIS. The reduction was clearly related to particle size
first, with absorption responsible for second-order effects only.
Multiple-scattered radiation could either increase or decrease in the
presence of an aerosol species, depending both on particle size and
absorption. Large scatterers (ice, mineral dust) all increased
multiple-scattered radiation within, below and above the aerosol layer.
Small, highly absorbing pure soot particles produced a negligible
multiple-scattering response (<0.1%) at all heights, primarily
confined to within and below the soot layer. Medium-sized scatterers
produced a multiple-scattering response that depended on their absorbing
properties. Increased radiances were simulated as compared to air only at
all short wavelengths (337, 377 and 452 nm) for sulfate aerosol
particles (non-absorbing) while decreased radiances were simulated for smoke
particles (absorbing) at 337 and 377 nm, where multiple scattering
involving the medium-sized carbonaceous particles amplified their absorbing
properties. At 452 nm, however, this effect was attenuated and
albedo-dependent.
At short wavelengths, the combined effect of single scattering decreases and
multiple scattering increases led to complex total radiance signatures that
generally could not unambiguously distinguish absorbing versus non-absorbing
aerosols. Smoke aerosols led to a total radiance decrease (as compared to
air only) at all altitudes above the aerosol layer (15–100 km). This unique
signature was a result of the aerosols' strong absorbing properties,
non-negligible scattering efficiency, and number concentrations high enough
to make multiple scattering effects due to the aerosol itself significant.
Thus, in the limb-scatter viewing geometry scene darkening above the aerosol
layer is unambiguously due to absorption whereas scene darkening within and
below the aerosol layer can simply be the result of a reduction in
single-scattered radiance. Our simulations show a greater scene darkening
for decreasing wavelengths, increasing surface albedo, decreasing solar
zenith angle, and increasing particle number concentration, however, at 337 nm
this 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 linearly
with particle number concentration over three orders of magnitude.
A fortuitous, unexpected implication of our analysis is that limb-scatter
retrievals of aerosol extinction are not sensitive to external information
about aerosol absorption. |
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