DOAS Retrievals of Stratospheric O 3 and NO 2 from Odin / OSIRIS Limb-Radiance Measurements

1. DOAS Retrievals of Stratospheric O3 and NO2 from Odin / OSIRIS Limb-Radiance Measurements Samuel Brohede Craig S. Haley and the Odin team Chalmers University of Technology and York University

2. OSIRIS onboard Odin Odin SMR OSIRIS OS IRI OSIRIS = Optical Spectrograph and InfraRed Imager System SMR = Sub Millimeter Radiometer

3. Odin orbit specifications • Polar orbiting • Sun synchronous • Near terminator • 18.00 ascending node • Altitude ~600 km • Limb scanning between 7 and 70 km

4. The Optical Spectrograph • Wavelengths: 280-800 nm (UV-Vis) • Resolution: ~1 nm • FOV: 1 km vertical, 40 km horizontal OSIRIS

5. OS spectra from one scan NO2 window O3 window

6. DOAS analysis smoothed cross section  ’  • Differential cross sections, ’, are used. Differential Optical Absorption Spectroscopy • Smoothing: Polynomial or Boxcar

7. Advantages of DOAS • Broad features like Mie and Rayleigh-scattering does not need to be modelled • No need to estimate real I0 / self calibrated • Multiple species can be measured simultaneously • High th spectrum (reference) used as I0

8. DOAS windows NO2 window O3 window

9. DOAS Least Squares Fit

10. The I0-effect • Structures inthe reference spectrum (I0) . • Ratioing does not cancel out. • Taking care of in the x-sections convolution.

11. The Ring effect • Filling in of Fraunhofer and Telluric lines • Two approaches: 1 Backward modelling [ Sioris et al ] 2 Pseudo absorber [ Chance et al ] • Including Ring corr. did not improve the fit.

12. Polarization • Differential structures in response for perp. light • Included as a pseudo absorber in O3-window

13. Tilt/Undersampling • Different tilt of reference and measured spectra + Fraunhofer structures + undersampling => ratioing won’t cancel out. • Included as a pseudo absorber in NO2-window

14. T-dependent x-sections • Two possible approaches: 1) Use T at tangent height 2) Non linear fit for T • Only important for NO2-window

15. Wavelength shift • Calibration differences between the OS and cross sections. • Taking care of in a non-linear fit • Only important in the NO2-window • No stretching/squeezing corrections

16. Retrieval procedure Effective column density, c(th) Vertical number density, n(z) [ molecules cm-3 ] [ molecules cm-2 ]

17. Optimum Estimation (MAP) • A Priori climatology [ McLinden ] • Non linear iteration • RTM = LIMBTRAN pseudo-3D [ Griffoen ]

18. Calculating K and F • K and F are calculated numerically • CPU-Time consuming calculations • Two approaches: 1) 2- DOAS (ok for K in O3-window) 2) Sparse wavelength grid

19. Method summary O3-region NO2-region Wavelengths: 571-617 nm 435-451 nm Species: O3, NO2, O4 O3, NO2, O4 Corrections: I0, Pol I0, Tilt, T-dep, -shift Ref. height: 50 km 50 km Estimating K: 2- 2.5 nm step Estimating F: 4 nm step 1 nm step Alt range: ~15-40 km ~20-35 km

20. O3-Results

21. NO2-Results

22. Off-plane orbit

23. Ozone hole splitting

24. Comparison 3 March 2002 OSIRIS: lat:68.1o N lon:10.6o W 17:23 UTC POAM III: lat:67.5o N lon:20.5o W 18:27 UTC

25. Comparison 8 Aug 2001 OSIRIS: lat:76.9o N lon:13.8o E 10:59 UTC Sonde: lat:78.9o N lon:11.9o E 10:59 UTC

26. Comparison 22 Aug 2001 OSIRIS: lat:28.9o N lon:16.1o W 18:48 UTC Sonde: lat:28.3o N lon:16.5o W 11:17 UTC

27. Comparison Paired Radiances Zonal means Nov 18 2001 DOAS Percentage difference

28. Comparison Zonal means Nov 3 2002 Paired Radiances DOAS Percentage difference

29. Conclusions and outlook • O3 compares well to Flittner and sondes • NO2 looks promising • Further validation needed • Article soon submitted