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

摘要

An improved nitrogen dioxide (NO₂) slant column density retrieval forthe Ozone Monitoring Instrument (OMI) in the 405–465 nm spectral region ispresented. Since the launch of OMI on board NASA's EOS-Aura satellite in2004, differential optical absorption spectroscopy (DOAS) retrievals of NO₂ slant column densities have been thestarting point for the KNMI DOMINO and NASA SP NO₂ vertical column dataas well as the OMI NO₂ data of some other institutes. However, recentintercomparisons between NO₂ retrievals from OMI and other UV/Vis andlimb spectrometers, as well as ground-based measurements, suggest that OMIstratospheric NO₂ is biased high.This study revises and, for the first time, fully documents the OMI NO₂retrieval in detail. The representation of the OMI slit function to convolvehigh-resolution reference spectra onto the relevant spectral grid isimproved. The window used for the wavelength calibration is optimised,leading to much-reduced fitting errors. Ozone and water vapour spectra usedin the fit are updated, reflecting the recently improved knowledge of theirabsorption cross section in the literature. The improved spectral fit alsoaccounts for absorption by the O₂–O₂ collision complex and byliquid water over clear-water areas.The main changes in the improved spectral fitting result from the updatesrelated to the wavelength calibration: the RMS error of the fit is reduced by23% and the NO₂ slant column by0.85 × 1015 molec cm−2, independent of latitude, solarzenith angle and NO₂ value. Including O₂–O₂ and liquid waterabsorption and updating the O₃ and water vapour cross-section spectrafurther reduces NO₂ slant columns on average by0.35 × 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 independentNO₂ retrievals from other instruments to within a range that can beexplained by photochemically driven diurnal increases in stratosphericNO₂ and by small differences in fitting window and approach. Therevisions indicate that current OMI NO₂ slant columns suffered mostlyfrom an additive positive offset, which is removed by the improvedwavelength calibration and representation of the OMI slit function. It istherefore anticipated that the improved NO₂ slant columns are mostimportant to retrievals of spatially homogeneous stratospheric NO₂rather than to heterogeneous tropospheric NO₂.