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| Titel |
Evaluation of the operational Aerosol Layer Height retrieval algorithm for Sentinel-5 Precursor: application to O2 A band observations from GOME-2A |
| VerfasserIn |
A. F. J. Sanders, J. F. de Haan, M. Sneep, A. Apituley, P. Stammes, M. O. Vieitez, L. G. Tilstra, O. N. E. Tuinder, C. E. Koning, J. P. Veefkind |
| 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 ; 8, no. 11 ; Nr. 8, no. 11 (2015-11-25), S.4947-4977 |
| Datensatznummer |
250116694
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| Publikation (Nr.) |
 copernicus.org/amt-8-4947-2015.pdf |
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| Zusammenfassung |
An algorithm setup for the operational Aerosol Layer Height product
for TROPOMI on the Sentinel-5 Precursor mission is described and
discussed, applied to GOME-2A data, and evaluated with lidar
measurements. The algorithm makes a spectral fit of reflectance at the
O2 A band in the near-infrared and the fit window runs from
758 to 770 nm. The aerosol profile is parameterised by
a scattering layer with constant aerosol volume extinction coefficient
and aerosol single scattering albedo and with a fixed pressure
thickness. The algorithm's target parameter is the height of this
layer. In this paper, we apply the algorithm to observations from
GOME-2A in a number of systematic and extensive case studies, and we
compare retrieved aerosol layer heights with lidar
measurements. Aerosol scenes cover various aerosol types, both
elevated and boundary layer aerosols, and land and sea surfaces. The
aerosol optical thicknesses for these scenes are relatively
moderate. Retrieval experiments with GOME-2A spectra are used to
investigate various sensitivities, in which particular attention is
given to the role of the surface albedo.
From retrieval simulations with the single-layer model, we learn that
the surface albedo should be a fit parameter when retrieving aerosol
layer height from the O2 A band. Current uncertainties in
surface albedo climatologies cause biases and non-convergences when
the surface albedo is fixed in the retrieval. Biases disappear and
convergence improves when the surface albedo is fitted, while
precision of retrieved aerosol layer pressure is still largely within
requirement levels. Moreover, we show that fitting the surface albedo
helps to ameliorate biases in retrieved aerosol layer height when the
assumed aerosol model is inaccurate. Subsequent retrievals with
GOME-2A spectra confirm that convergence is better when the surface
albedo is retrieved simultaneously with aerosol parameters. However,
retrieved aerosol layer pressures are systematically low (i.e., layer
high in the atmosphere) to the extent that retrieved values no longer realistically represent actual extinction profiles. When
the surface albedo is fixed in retrievals with GOME-2A spectra,
convergence deteriorates as expected, but retrieved aerosol layer
pressures become much higher (i.e., layer lower in atmosphere). The
comparison with lidar measurements indicates that retrieved aerosol
layer heights are indeed representative of the underlying profile in
that case. Finally, subsequent retrieval simulations with two-layer
aerosol profiles show that a model error in the assumed profile (two
layers in the simulation but only one in the retrieval) is partly
absorbed by the surface albedo when this parameter is fitted. This is
expected in view of the correlations between errors in fit parameters
and the effect is relatively small for elevated layers (less than
100 hPa). If one of the scattering layers is near the
surface (boundary layer aerosols), the effect becomes surprisingly
large, in such a way that the retrieved height of the single layer is above the
two-layer profile.
Furthermore, we find that the retrieval solution, once retrieval
converges, hardly depends on the starting values for the
fit. Sensitivity experiments with GOME-2A spectra also show that
aerosol layer height is indeed relatively robust against inaccuracies
in the assumed aerosol model, even when the surface albedo is not
fitted. We show spectral fit residuals, which can be used for further
investigations. Fit residuals may be partly explained by spectroscopic
uncertainties, which is suggested by an experiment showing the
improvement of convergence when the absorption cross section is scaled
in agreement with Butz et al. (2013) and Crisp et al. (2012), and
a temperature offset to the a priori ECMWF temperature profile is
fitted. Retrieved temperature offsets are always negative and quite
large (ranging between −4 and −8 K), which is not expected
if temperature offsets absorb remaining inaccuracies in meteorological
data. Other sensitivity experiments investigate fitting of stray light
and fluorescence emissions. We find negative radiance offsets and
negative fluorescence emissions, also for non-vegetated areas, but
from the results it is not clear whether fitting these parameters
improves the retrieval.
Based on the present results, the operational baseline for the Aerosol
Layer Height product currently will not fit the surface albedo. The
product will be particularly suited for elevated, optically thick
aerosol layers. In addition to its scientific value in climate
research, anticipated applications of the product for TROPOMI are
providing aerosol height information for aviation safety and improving
interpretation of the Absorbing Aerosol Index. |
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