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| Titel |
Accounting for surface reflectance anisotropy in satellite retrievals of tropospheric NO2 |
| VerfasserIn |
Y. Zhou, D. Brunner, R. J. D. Spurr, K. F. Boersma, M. Sneep, C. Popp, B. Buchmann |
| 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 ; 3, no. 5 ; Nr. 3, no. 5 (2010-09-01), S.1185-1203 |
| Datensatznummer |
250001285
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| Publikation (Nr.) |
 copernicus.org/amt-3-1185-2010.pdf |
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| Zusammenfassung |
| Surface reflectance is a key parameter in satellite trace gas retrievals in
the UV/visible range and in particular for the retrieval of nitrogen dioxide
(NO2) vertical tropospheric columns (VTCs). Current operational
retrievals rely on coarse-resolution reflectance data and do not account for
the generally anisotropic properties of surface reflectance. Here we present
a NO2 VTC retrieval that uses MODIS bi-directional reflectance
distribution function (BRDF) data at high temporal (8 days) and spatial (1 km × 1 km)
resolution in combination with the LIDORT radiative transfer
model to account for the dependence of surface reflectance on viewing and
illumination geometry. The method was applied to two years of NO2
observations from the Ozone Monitoring Instrument (OMI) over Europe. Due to
its wide swath, OMI is particularly sensitive to BRDF effects. Using
representative BRDF parameters for various land surfaces, we found that in
July (low solar zenith angles) and November (high solar zenith angles) and
for typical viewing geometries of OMI, differences between MODIS black-sky
albedos and surface bi-directional reflectances are of the order of 0–10% and
0–40%, respectively, depending on the position of the OMI
pixel within the swath. In the retrieval, black-sky albedo was treated as a
Lambertian (isotropic) reflectance, while for BRDF effects we used the
kernel-based approach in the MODIS BRDF product. Air Mass Factors were
computed using the LIDORT radiative transfer model based on these surface
reflectance conditions. Differences in NO2 VTCs based on the Lambertian
and BRDF approaches were found to be of the order of 0–3% in July and
0–20% in November with the extreme values found at large viewing angles.
The much larger differences in November are mainly due to stronger BRDF effects
at higher solar zenith angles. To a smaller extent, they are also caused by the
typically more pronounced maximum of the NO2 a priori profiles in the boundary layer
during the cold season, which make the retrieval more sensitive to radiation changes near the surface.
However, BRDF impacts vary considerably across Europe due to differences
in land surface type and increasing solar zenith angles at higher latitude.
Finally, we compare BRDF-based NO2 VTCs with those retrieved using the
GOME/TOMS Lambertian equivalent reflectance (LER) data set. The relative differences
are mostly below 15% in July but in November the NO2 VTCs from TOMS/GOME are
lower by 20–60%. Our results indicate that the specific choice of albedo data set
is even more important
than accounting for surface BRDF effects, and this again demonstrates the
strong requirement for more accurate surface reflectance data sets. |
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