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| Titel |
Sensitivity of aerosol retrieval to geometrical configuration of ground-based sun/sky radiometer observations |
| VerfasserIn |
B. Torres, O. Dubovik, C. Toledano, A. Berjón, V. E. Cachorro, T. Lapyonok, P. Litvinov, P. Goloub |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 14, no. 2 ; Nr. 14, no. 2 (2014-01-24), S.847-875 |
| Datensatznummer |
250118309
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| Publikation (Nr.) |
 copernicus.org/acp-14-847-2014.pdf |
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| Zusammenfassung |
A sensitivity study of aerosol retrievals to the geometrical configuration of
the ground-based sky radiometer observations is carried out through inversion
tests. Specifically, this study is focused on principal plane and almucantar
observations, since these geometries are employed in AERONET (AErosol RObotic
NETwork). The following effects have been analyzed with simulated data for
both geometries: sensitivity of the retrieval to variability of the observed
scattering angle range, uncertainties in the assumptions of the aerosol
vertical distribution, surface reflectance, possible instrument pointing
errors, and the effects of the finite field of view. The synthetic
observations of radiometer in the tests were calculated using a previous
climatology data set of retrieved aerosol properties over three AERONET
sites: Mongu (Zambia) for biomass burning aerosol, Goddard Space Flight
Center (GSFC; Maryland, USA) for urban aerosol and Solar Village (Saudi
Arabia) for desert dust aerosol. The results show that almucantar retrievals,
in general, are more reliable than principal plane retrievals in presence of
the analyzed error sources. This fact partially can be explained by practical
advantages of the almucantar geometry: the symmetry between its left and
right branches that helps to eliminate some observational uncertainties and
the constant value of optical mass during the measurements, that make
almucantar observations nearly independent of the vertical variability of
aerosol. Nevertheless, almucantar retrievals present instabilities at high
sun elevations due to the reduction of the scattering angle range coverage,
resulting in decrease of information content. It is in such conditions that
principal plane retrievals show a better stability, as shown by the
simulation analysis of the three different aerosol models.
The last part of the study is devoted to the identification of possible
differences between the aerosol retrieval results obtained from real AERONET
data using both geometries. In particular, we have compared AERONET
retrievals at the same sites used in the simulation analysis: Mongu (biomass
burning), GSFC (urban) and Solar Village (desert dust). Overall, this
analysis shows robust consistency between the retrievals from simultaneous
observations in principle plane and almucantar All identified differences are
within the uncertainties estimated for the AERONET operational aerosol
retrieval. The differences in the size distribution are generally under
10% for radii between 0.1 μm and 5 μm, and outside
this size range, the differences can be as large as 50%. For the
absorption parameters, i.e., single scattering albedo and the imaginary part
of the refractive index, the differences are typically under 0.01 and
0.003, respectively. The real part of the refractive index showed a
difference of 0.01 for biomass burning and urban aerosol, and a difference
of around 0.03 for desert dust. Finally, it should be noted that the whole
data set includes only 200 pairs, which have been taken under very stable
atmospheric conditions; therefore, in a general case, differences between
principal plane (PPL) and almucantar (ALM) are expected to be higher. Though
the observed differences between ALM and PPL are rather small, it should be
noted that this analysis has been conducted using a limited set of 200
observation pairs selected under stable atmospheric conditions. |
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