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

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

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

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

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

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

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

On-orbit temporal variation: cross track dependence

Solar function change or solar cycle? DeLand and Marchenko, http://sbuv2.gsfc.nasa.gov/solar/omi/

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

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

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

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

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

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

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

Ozone profile validations

Stratospheric ozone columns validation

Tropospheric ozone columns validation

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

UV1 UV2 UV2 UV1