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., Hendrick, F., Mahieu, E., De Smedt, I., Sneep, M. and Veefkind, J.P.: 2015,
"Improved spectral fitting of nitrogen dioxide from OMI in the 405 - 465 nm window,"
Atmos. Meas. Tech. 8, 1685-1699.


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 0.85x10^15 molec/cm2, 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 0.35x10^15 molec/cm2, 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.


   1. Introduction
   2. Observations of NO2 column densities
      2.1 UV/Vis satellite-based NO2 observations
      2.2 DOAS retrieval of NO2 slant column densities
   3. Intercomparisons of stratospheric NO2 columns
   4. Improvements to the OMI NO2 retrieval
      4.1 Reference spectra
          4.1.1 Other absorption features
      4.2 Wavelength calibration
          4.2.1 Uncertainty in NO2 SCD related to calibration
   5. Results of the OMI NO2 retrieval improvements
      5.1 Current vs. updated NO2 SCD over the Pacific
      5.2 About including O2-O2 and liquid water
      5.3 Comparison between OMNO2A and QDOAS
      5.4 Reprocessed OMI NOtwo data of 2005
   6. Concluding remarks
contents of the supplement
   S1. DOAS retrieval of NO2 slant column densities
   S2. Comparison with ground-based NO2 data
       S2.1 Ground-based NO2 observations in UV-Vis and IR
       S2.2 Comparison with ground-based NO2 data
   S3. The OMI slit function & convolution of the reference spectra
       S3.1 The OMI slit function
       S3.2 Convolution of reference spectra
   S4. Reference spectra
       S4.1 Solar reference spectrum
       S4.2 Absorption reference spectra
       S4.3 Ring coefficient and detector row 0
   S5. OMNO2A processor version numbering
   S6. Comparison between OMNO2A and QDOAS
   S7. Reprocessed OMI NO2 data of 2005
PDF file of the paper (15 pages; 1.8 MB)
PDF file of the supplement (14 pages; 1.4 MB)

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created: 17 June 2014
last modified: 8 April 2015