1. Deriving the Instrument Transfer Function from OMI Solar Observations and Its Implications for Ozone Retrievals
Kang Sun, Xiong Liu, Zhaonan Cai, Guanyu Huang, Gonzalo González Abad, and Kelly Chance
Harvard-Smithsonian Center for Astrophysics
Kai Yang, UMD
GSICS Annual Meeting, Mar. 22, 2017

2. Instrument transfer function (slit function)
OMI detector array, UV1 band
Nadir
Native resolution spectrum:
Observation by OMI:

3. Why is ITF a challenge?
OMI slit functions were thoroughly measured preflight (Dirksen et al. 2006)
Is the on-orbit slit function the same to preflight measurements?
Is the slit function stable on-orbit?
Is it possible to derive on- orbit slit function and improve retrieval?
Wavelength direction
Cross-track (spatial) direction

4. Derive ITF by solar observations
The slit function shape retrieved by fitting OMI solar spectra with a high- resolution solar reference spectrum
Slit function
Inverse
Forward
Measured solar spectra
High resolution reference solar spectrum

5. Fitting on-orbit slit function
Gaussian function:
Super Gaussian function:
Preflight ITF with homogeneous stretch:
k = 2, Changing w
Stretch
w = constant, Changing k
1. spectrally averaged
2. spectrally resolved
Beirle et al. AMT 2017

6. On-orbit temporal variation: fitting windows
Divide each OMI band into four windows
Assume standard Gaussian slit function for each window

7. On-orbit temporal variation: cross track dependence
Fitted slit function width

8. On-orbit temporal variation: cross track dependence

9. Solar function change or solar cycle?
DeLand and Marchenko,

10. On-orbit temporal variation: solar cycle and RA
Non-RA rows
Non-RA rows

11. On-orbit temporal variation: solar cycle and RA
RA rows
RA rows

12. On-orbit slit functions differ from preflight!
Cross-track (1-30)
Cross-track (1-60)
Cross-track (1-60)

13. SAO ozone profile retrievals
Stretched preflight 2
Test SAO ozone profile retrievals using
Gaussian, operational
Super Gaussian
Preflight slit function
Stretched preflight (spectrally averaged)
Stretched preflight (spectrally resolved)
Stretched preflight 1

14. Ozone profile validations – cross-track pattern
Operational SAO ozone profile data
Many other products!
Note: each column is the medium bias of 50 – 200 pairs of collocated OMI- sonde profiles

15. Summary
Derived OMI on-orbit slit functions complicated by solar activities at certain wavelengths
Slit functions of non-RA rows stable over time; RA had impact on slit functions for < 300 nm
On-orbit slit function widths differ from preflight, varying by cross-track positions
Derived slit functions show better cross-track consistency in ozone retrieval

16. Backup: validation using ozone sondes (2004 – 2008)
Cloud fraction < 0.3
Dt < 6 hr
Dlat, Dlon <1
SZA < 75˚
RMS < 1.5
Use averaging kernels

17. Ozone profile validations

18. Stratospheric ozone columns validation

19. Tropospheric ozone columns validation

20. Ozone profile validations – UV2 only
Same issue in UV2
A single “stretch” of preflight
Note: each column is the medium bias of 10 – 120 pairs of collocated OMI- sonde profiles

21. UV1
UV2
UV2
UV1