Improved NO₂ spectral fits for TROPOMI and OMI by removing wavelengths around 430 nm

Abstract. The Fraunhofer absorption feature at 430 nm influences the retrieval of nitrogen dioxide (NO₂) from measurements by satellite-based instruments such as the Tropospheric Monitoring Instrument (TROPOMI) and Ozone Monitoring Instrument (OMI). The width and depth of the feature in the measured spectrum are affected by rotational Raman scattering (RRS) throughout the atmosphere and by vibrational Raman scattering (VRS) in open water bodies. RRS, or the Ring-effect, is accounted for in the Differential Optical Absorption Spectroscopy (DOAS) retrieval of the NO₂ slant column density (SCD) by means of a scalable reference spectrum, which will not fully pick up the variation of the depth of the 430 nm feature with the solar activity cycle. It is not possible to account for VRS with a scalable reference spectrum, since VRS characteristics depend on several aspects, including the viewing geometry and the material dissolved in the water, such as chlorophyll. From detailed inspection of DOAS fit residuals, the difference between the measured and modelled spectra, it is clear that the 430 nm feature disturbs the NO₂ SCD retrieval.

In this paper we investigate the benefits of removing the wavelength range 428–433 nm from the DOAS retrieval. This "NO2-gap approach" reduces the SCD error and the RMS error of the fit over water bodies by 10–20 % and the fit residual for the remaining parts of the window improves. Over some land scenes, where the residual outside the 428–433 nm range looks very good, the SCD error and RMS error are reduced by 5–10 %. For other areas the fit residual does not deteriorate by the NO2-gap approach. Over ocean waters the SCD is seen to decrease by a few percent, which leads to a decrease of the stratospheric NO₂ column of on average up to -2 μmol m^-2 in the tropics. Over land the change in SCD may be positive or negative by a few percent, which in combination with the change in the stratospheric column leads to changes in the tropospheric NO₂ column of on average ±2 μmol m^-2. These changes are too small to alter the general conclusions of the routine validation of TROPOMI data. Because of the improvement of the SCD error and systematic improvements over open water it has been decided to implement the NO2-gap approach in the forthcoming processor versions of TROPOMI and OMI.