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| Titel |
Improved spectral fitting of nitrogen dioxide from OMI in the 405–465 nm window |
| VerfasserIn |
J. H. G. M. van Geffen, K. F. Boersma, M. Van Roozendael, F. Hendrick, E. Mahieu, I. De Smedt, M. Sneep, 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. 4 ; Nr. 8, no. 4 (2015-04-08), S.1685-1699 |
| Datensatznummer |
250116294
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| Publikation (Nr.) |
 copernicus.org/amt-8-1685-2015.pdf |
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| Zusammenfassung |
| An improved nitrogen dioxide (NO2) slant column density retrieval for
the Ozone Monitoring Instrument (OMI) in the 405–465 nm spectral region is
presented. Since the launch of OMI on board NASA's EOS-Aura satellite in
2004, differential optical absorption spectroscopy (DOAS) retrievals of NO2 slant column densities have been the
starting point for the KNMI DOMINO and NASA SP NO2 vertical column data
as well as the OMI NO2 data of some other institutes. However, recent
intercomparisons between NO2 retrievals from OMI and other UV/Vis and
limb spectrometers, as well as ground-based measurements, suggest that OMI
stratospheric NO2 is biased high.
This study revises and, for the first time, fully documents the OMI NO2
retrieval in detail. The representation of the OMI slit function to convolve
high-resolution reference spectra onto the relevant spectral grid is
improved. The window used for the wavelength calibration is optimised,
leading to much-reduced fitting errors. Ozone and water vapour spectra used
in the fit are updated, reflecting the recently improved knowledge of their
absorption cross section in the literature. The improved spectral fit also
accounts for absorption by the O2–O2 collision complex and by
liquid water over clear-water areas.
The main changes in the improved spectral fitting result from the updates
related to the wavelength calibration: the RMS error of the fit is reduced by
23% and the NO2 slant column by
0.85 × 1015 molec cm−2, independent of latitude, solar
zenith angle and NO2 value. Including O2–O2 and liquid water
absorption and updating the O3 and water vapour cross-section spectra
further reduces NO2 slant columns on average by
0.35 × 1015 molec cm−2, accompanied by a further 9%
reduction in the RMS error of the fit.
The improved OMI NO2 slant columns are consistent with independent
NO2 retrievals from other instruments to within a range that can be
explained by photochemically driven diurnal increases in stratospheric
NO2 and by small differences in fitting window and approach. The
revisions indicate that current OMI NO2 slant columns suffered mostly
from an additive positive offset, which is removed by the improved
wavelength calibration and representation of the OMI slit function. It is
therefore anticipated that the improved NO2 slant columns are most
important to retrievals of spatially homogeneous stratospheric NO2
rather than to heterogeneous tropospheric NO2. |
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