Article (Scientific journals)
Improved spectral fitting of nitrogen dioxide from OMI in the 405-465 nm window
van Geffen, J H G M; Boersma, K F; Van Roozendael, M et al.
2015In Atmospheric Measurement Techniques, 8, p. 1685-1699
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Keywords :
nitrogen dioxide; OMI; remote sensing
Abstract :
[en] 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 clearwater 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.
Disciplines :
Earth sciences & physical geography
Author, co-author :
van Geffen, J H G M
Boersma, K F
Van Roozendael, M
Hendrick, F
Mahieu, Emmanuel  ;  Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Groupe infra-rouge de phys. atmosph. et solaire (GIRPAS)
De Smedt, I
Sneep, M
Veefkind, J P
Language :
English
Title :
Improved spectral fitting of nitrogen dioxide from OMI in the 405-465 nm window
Publication date :
08 April 2015
Journal title :
Atmospheric Measurement Techniques
ISSN :
1867-1381
eISSN :
1867-8548
Publisher :
Copernicus, Katlenburg-Lindau, Germany
Volume :
8
Pages :
1685-1699
Peer reviewed :
Peer Reviewed verified by ORBi
European Projects :
FP7 - 284421 - NORS - Demonstration Network Of ground-based Remote Sensing Observations in support of the GMES Atmospheric Service
Funders :
CE - Commission Européenne [BE]
Available on ORBi :
since 21 October 2014

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