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Improving the DOAS NO2 retrieval for the Aura/Ozone Monitoring Instrument

This study focuses on improving the retrieval of nitrogen dioxide (NO2) using the Aura/Ozone Monitoring Instrument (OMI). The authors propose adjustments in the instrumental wavelength shifts, iterative removal of spectral features, and correction of instrumental noise, resulting in a reduction of the OMI NO2 retrieval bias. The findings have implications for improving the accuracy of atmospheric trace gas measurements.

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Improving the DOAS NO2 retrieval for the Aura/Ozone Monitoring Instrument

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  1. Improving the DOAS NO2 retrieval for the Aura/Ozone Monitoring InstrumentMarchenko, S., Krotkov, N., Lamsal, L., Celarier, E., Swartz, B., Bucsela, E. 7th DOAS Workshop, 6-8/07/2015

  2. MLS Aura, as part of the “A-train” constellation: - launched July 15, 2004; - lagging Aqua by 8-15 min; - alt.=705 km sun-synchronous orbit, ~13:45 LST equator-crossing time

  3. Ozone Monitoring Instrument (OMI) Main goal: atmospheric trace gases (O3, SO2, NO2, etc.). Nadir-viewing, ‘pushbroom’ single monochromator: 264-504 nm spectral range (2 UV and 1 Vis channel); 0.4-0.6 nm spectral resolution; 30-60 x-track FOVs. Very stable instrument; over the mission lifetime (2004-present): 3-8 % change in the optical throughput; < 0.01 nm change in the wavelength registration. Global Earth coverage in 1-2 days (15 orbits/day): 13x24 km nadir footprint; ~2600 km swath; ~1650 2-sec exposures/orbit; S/N >~ 1000.

  4. The problem: ~30% high bias traceable to the OMI SCD(NO2) [1015mol cm-2] [1015mol cm-2] Monthly Pacific (140W- 180W) zonal-mean NO2 columns for March 2010 * SCIAMACHY NO2 data courtesy Andreas Richter (IEP, University of Bremen ) ** GOME-2 data courtesy Pieter Valks ( DLR ) * **

  5. Revising the OMI DOAS SCD(NO2) retrieval 1. Adjustment of the instrumental wavelength shifts combined with iterative removal of the Ring spectral featuresin multiple ‘micro-windows’; 7 windows in the NO2 retrieval range (402-465 nm). 2. Iterative, sequential estimates of SCDs of the trace gases (NO2, H2O, CHOCHO). 3. Iterative removal of the instrumental noise; correction of the fixed spectral patterns. --------------------------------------------------------------------------------------------------- The result: an overall reduction of the OMI SCD(NO2) by 10-35%.

  6. Sequential SCD retrieval OMI reflectances Iterative wavelength adjustment and Ring-pattern removal in ‘micro-windows’ OMI RS-free reflectances SCD(NO2) SCD(H2O) SCD(CHOCHO) Instrument noise removal

  7. Flexible wavelength adjustment and Ring removal: ‘micro-windows’ Ring H2ONO2 1 0 -1 Ring+NO2+… (open water) Diff. Opt. Depth [%] Ring+NO2+… (Beijing) ‘micro – windows’ OMI: 20 March 2005, orbit #03610

  8. Flexible wavelength adjustment and Ring removal: ‘micro-windows’ 0.001 nm wavelength error [molec cm-2] SCD(NO2) change (van Geffen et al. 2015, AMT 8, 1685) Our SCD(NO2) errors ~ [molec cm-2] I.e., we are [potentially] sensitive to Δλ > 0.002 nm errors (~1/100 OMI pix).

  9. Flexible wavelength adjustment and Ring removal: ‘micro-windows’ In each ‘micro-window’, irradiance spectrum is offset by Δλ, then splined to radiance wavelengths reflectance 2. Iterative polynomial (2nd order) smoothing of the reflectances. 3. Linear fit of the reflectance to the Ring spectrum (R1 , R2) 4. Removal of the RS : 5. Cost function = Standard Deviation of the RS-free reflectances. 6. Δλoptimalat min(Cost function) in each window. 7. In each window, application of Δλoptimal shifts to irradiances; final evaluation of RS amplitudes. 8. RS smoothing (running mean), then removal of the Ring patterns from the reflectances. Remember that RS amplitudes are wavelength-dependent.

  10. The Ring-pattern amplitudes: ‘micro-windows’ off-nadir FOV near-nadir FOV The Ring line-filling scales for two fields of view (rows). RS[1] - the first micro-window, 402-410 nm; RS[7] - the last micro-window, 451-465 nm. 20 March 2005, orbit #03610

  11. The wavelength corrections: micro-windows the farthest off-nadir FOV the closest to nadir FOV Orbital exposures: 200—599; 600—999; 1000—1399. The ±1σ bars characterize the spread within the orbital blocks. 20 March 2005, orbit #03610

  12. RMS of spectral residuals OMI orbit #03610, 20 March 2005 Single-window (402-465 nm) shift-and-squeeze ‘Micro-windows’

  13. Spectral cross-talk Cross-correlation: NO2 absorption and the Ring line-filling At the OMI 0.63 nm spectral resolution:

  14. Spectral cross-talk At the OMI 0.63 nm spectral resolution: NO2 absorptions are typically ~5-7 weaker than RS. CHOCHO is ~5 weaker than NO2. H2O is ~comparable to RS. Wavelength errors mayincrease the correlation between NO2 and RS. ~30% RS error may lead to ~10% NO2 bias. One should consider: the relative strengths of the trace-gas absorptions and RS patterns; the wavelength dependence of the RS and trace-gas spectra; the spectral resolution. RS may influence the NO2 retrievals. Sequential NO2 may affect the H2O and CHOCHO retrievals. SCD retrieval

  15. SCD retrievals: fixed Solar or dynamic Solar?

  16. Grey line: long-term (5 years) Solar variability (OMI resolution) Black line: short-term (27-day) Solar changes (OMI resolution) Marchenko, S. & DeLand, M., 2014, Astrophys. J., 789, 117

  17. Short-term Solar variability for Cycle 24: OMI vs. GOME-2 The 27-day Solar variability (black - OMI; blue - GOME-2). Red bars: representative 2-sigma uncertainties.

  18. The presented approach practically eliminates the ~30% OMI SCD(NO2) bias. • The new SCD(NO2) should be available by Fall 2015. • See more details and results in: • Marchenko, S.V., Krotkov, N.A., Lamsal, L.N., Celarier, E.A., W. H. Swartz, W.H., Bucsela, E.J., 2015, "Revising the slant-column density retrieval of nitrogen dioxide observed by the Ozone Monitoring Instrument", JGR-Atmospheres, in press • Celarier, E.A., “Effect of bias removal in OMI DOAS NO2 retrieval on vertical column densities”, this Workshop (poster)

  19. Backup

  20. Left panel: Bremen (courtesy Andreas Richter) and Goddard SCD(NO2) retrievals for different cloud conditions: OMI orbit #03610 from 20 March 2005. Right panel: the (Goddard / Bremen) ratios (black line) and (Goddard - Bremen) differences (orange line).

  21. Spectral cross-talk: typical absorption spectra @ OMI resolution

  22. Slant Column Density calculation Slant columns (1st approx.) 1st Trip Averaged OMI irradiances Fixed-pattern residuals in reflectances Slant columns (Final approx.) 2nd Trip OMI radiances Slant column uncertainties

  23. SCD calculation 1st Pass offset RRS signal removal 1st Trip 1st Pass Slant column estimation Noise removal Spike removal 2nd Pass 2nd Pass Low-pass filt. Calculation of fixed-pattern residuals 2nd Trip 1st Pass Slant Column estimation Noise removal Correlation Check 2nd Pass SCD Uncertainties

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