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| Titel |
An improved tropospheric NO2 retrieval for OMI satellite observations |
| VerfasserIn |
Y. Zhou, D. Brunner, K. F. Boersma, R. Dirksen, P. Wang, B. Buchmann |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250028146
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| Zusammenfassung |
| Nitrogen oxides are key precursors of both ozone and secondary aerosols, and they are
harmful to humans and ecosystems. The Ozone Monitoring Instrument (OMI) is a recent
UV/visible spectrometer on NASA’s Aura satellite with a comparatively small pixel size and
daily global coverage which makes it particularly suitable for air quality monitoring.
Information on vertical tropospheric columns (VTCs) of tropospheric trace gases is
derived from the spectroscopic data by means of a retrieval algorithm. However, the
retrieval depends on a number of so-called “a priori” assumptions which introduce
considerable uncertainties in the derived quantity. Current operational retrievals
are based on global a priori data sets at coarse spatial and temporal resolution,
which are much coarser than the resolution of individual OMI pixels. Furthermore,
there are simplified physical descriptions such as Lambertian surface assumed for
albedo data set. Therefore the improvement of the a priori assumptions used for the
computation is a main concern to obtain accurate values of NO2of high spatial
resolution.
In order to obtain more accurate vertical tropospheric columns of nitrogen dioxide (NO2)
than currently available, we are developing new data sets of critical retrieval parameters at
high temporal and spatial resolution for Europe, such as a high resolution surface pressure
map, illumination and viewing geometry dependent surface reflectance, and a-priori vertical
NO2 profiles from a regional model. As a first step, we analyzed the sensitivity of retrieved
NO2 to the surface pressure, and addressed the issue in a quantitative way by reprocessing
selected periods with accurate pixel-average surface pressures deduced from a high resolution
topography data set. The differences between original and enhanced retrieval were analyzed
for different seasons separately, and validated with in situ NO2 VTCs calculated from
ground-based measurements over the Swiss plateau and selected background stations in the
Po Valley in Italy. The results demonstrate the importance of an accurate (i.e. high
resolution) treatment of this parameter, in particular in the vicinity of complex
topography such the Alps. As a second step, a new illumination and viewing geometry
dependent surface reflectance data set is currently being developed for the OMI
observations based on high temporal and spatial resolution multiangular reflectance
observations from NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS).
As a first analysis, a comparison of this new surface reflectance data set with the
coarse albedo data set of the operational product will be shown and a preliminary
sensitivity analysis of the impact on the retrieved NO2 accuracy will be presented. |
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