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| Titel |
Sensitivity of aerosol retrieval to surface albedo |
| VerfasserIn |
Felix Seidel, Alexander A. Kokhanovsky, Michael E. Schaepman |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250046652
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| Zusammenfassung |
| Atmospheric particles are the objective of intensive research. In addition to effects on our
health, they also have a significant influence on climate. Aerosols can be measured in situ or
retrieved using optical remote sensing instruments. They are made to measure the upwelling
solar radiance, reflected from the Earth’s surface, scattered and absorbed by atmospheric
molecules and aerosols. Separating measured upwelling radiance into its components is
one of the most challenging tasks for quantitative remote sensing applications in
general and aerosol retrieval in particular. The measured upwelling radiance is often
dominated by the surface component and surface albedo is therefore one of the most
influencing parameter to radiative transfer. Therefore, it is important to analyze and
understand the impact of surface albedo on quantities derived from remote sensing
data.
We present here an analysis of this influence with respect to the retrieval of
aerosol optical depth (AOD) using a fast and simple model for atmospheric radiative
transfer. Results show that larger AOD lead to increased reflectances R at sensor
level for underlying "dark" surfaces and to decreased R for "bright" surfaces. In
between, R does not depend strongly on AOD for surface albedo. It is therefore
very difficult to infer AOD from measured upwelling radiances or R with such an
underlying surface, which is sometimes called "critical" surface albedo. We use the
derivative of R with respect to AOD for different surface albedos to understand
better the relationship between R, AOD and surface albedo. Results with small
derivatives show that the critical surface albedo is found typically in the range of
approximately 0.2–0.4, which depends mainly on aerosol single scattering albedo and
therefore also on aerosol extinction, as well as scattering angle, wavelength and other
parameters.
Aerosol remote sensing relies on previously measured or estimated surface albedo, which
are prone to errors. The impact of such uncertainties is also evaluated in the presented work
by using the derivative of R with respect to AOD. The results confirm that the AOD
retrieval at "dark" surfaces is less influenced by surface albedo uncertainties than
"bright" surfaces. It is obvious that even smallest uncertainties in the surface albedo
may lead to large uncertainties in AOD retrieval for surfaces close to the critical
surface albedo. It was found for typical remote sensing conditions that only 0.01
surface albedo uncertainty leads to at least 0.2 AOD retrieval uncertainty. Assumed or
empirically estimated surface albedo values may lead therefore to unusable AOD
results.
It can be thus concluded that AOD retrieval is very sensitive to surface albedo and its
related uncertainties. |
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