Improved spectral fitting of nitrogen dioxide from OMI in the 405–465 nm window

Abstract. 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.