2015-04-16: Improved spectral fitting of nitrogen dioxide from OMI in the 405 - 465 nm window

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.

This study revises and for the first time fully documents the OMI NO2 retrieval (called OMNO2A) 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 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.

Read the full article here.
Comparison of the OMI NO2 vertical column densities (lines with symbols) of the new v2 OMNO2A (red circles) and old v1 OMNO2A (blue squares) retrieval for the Pacific Ocean orbit of 1 July 2005, and the average SCIAMACHY data (black triangles) over Pacific Ocean of the same day. The two lines without symbols show differences between the NO2 vertical column densities. A difference of about 0.6×1015 molec/cm2 between OMI and SCIAMACHY due to diurnal increases in stratospheric NO2. A comparison between OMI and SCIAMACHY should be limited to latitudes below 45 degrees, because for higher latitudes the instruments cover different geographic areas.
World map of the liquid water coefficient (in m) based on all 15 OMI orbits of 1 July 2005; the Pacific Ocean orbit from the other figure is marked by a black triangle. All ground pixels with solar zenith angle less than 75 degrees are plotted; no filtering for cloudy pixels was applied.