Deriving the slit functions from OMI solar observations and its implications for ozone-profile retrieval

Abstract

Abstract. The Ozone Monitoring Instrument (OMI) has been successfully measuring the Earth's atmospheric composition since 2004, but the on-orbit behavior of its slit functions has not been thoroughly characterized. Preflight measurements of slit functions have been used as a static input in many OMI retrieval algorithms. This study derives on-orbit slit functions from the OMI irradiance spectra assuming various function forms, including standard and super-Gaussian functions and a stretch to the preflight slit functions. The onorbit slit functions in the UV bands show U-shaped crosstrack dependences that cannot be fully represented by the preflight ones. The full widths at half maximum (FWHM) of the stretched preflight slit functions for detector pixels at large viewing angles are up to 30 % larger than the nadir pixels for the UV1 band, 5 % larger for the UV2 band, and practically flat in the VIS band. Nonetheless, the on-orbit changes of OMI slit functions are found to be insignificant over time after accounting for the solar activity, despite of the decaying of detectors and the occurrence of OMI row anomaly. Applying the derived on-orbit slit functions to ozone-profile retrieval shows substantial improvements over the preflight slit functions based on comparisons with ozonesonde validations.

Improved slant column density retrieval of nitrogen dioxide and formaldehyde for OMI and GOME-2A from QA4ECV: intercomparison, uncertainty characterisation, and trends

“…All four OMI fitting approaches convolve high-resolution absorption cross section spectra with the OMI slit function (Dirksen et al, 2006; this pre-flight slit function is slightly modified to match the observed irradiances in OMNO2-NASA), which has proved to be stable throughout the OMI mission period (Schenkeveld et al, 2017;Sun et al, 2017). OMNO2A v1 uses a fixed slit function for all 60 rows, where in OMNO2A v2 the slit function has been updated with respect to OMNO2A v1 to better represent the across-track average (Van Geffen et al, 2015).…”

Nitrogen dioxide and formaldehyde measurements from the GEOstationary Coastal and Air Pollution Events (GEO-CAPE) Airborne Simulator over Houston, Texas

“…These include the 45 CMAQ layers up to ∼ 19 km and 11 additional layers to 0.3 hPa. We use the MODIS BRDF (bidirectional reflectance distribution functions) gap-filled MCD43GF V005 Band 3 product (Schaaf et al, 2002;Q. Sun et al, 2017) to represent surface reflectance in the VLIDORT model.…”

Cross-evaluation of GEMS tropospheric ozone retrieval performance using OMI data and the use of an ozonesonde dataset over East Asia for validation

“…Figure 1 is a schematic diagram of the ozone profile algorithm. With the input of satellite measurements, the slit function is parameterized through cross-correlation between satellite irradiance and a high-resolution solar reference spectrum to be used for wavelength calibration and for high-resolution cross section convolution (Sun et al, 2017;Bak et al, 2019); a normalized Gaussian distribution is assumed to derive analytic slit functions for OMI. To remove the systematic errors between measured and calculated radiances, "soft calibration" is applied to measured radiances and then the logarithms of sun-normalized radiances are calculated as measurement vectors (Liu et al, 2010a;Cai et al, 2012;Bak et al, 2017).…”

“…Ozone soundings have been taken for more than 50 years, since the 1960s. The accuracy of ozonesonde measurements has been reported as 5 %-10 % with a precision of 3 %-5 %, depending on the sensor type, manufacturer, solution concentrations, and operational procedure (Smit et al, 2007;Thompson et al, 2007Witte et al, 2017Witte et al, , 2018. Three types of instruments have been carried on balloons, i.e., the modified Brewer-Mast (MBM), the carbon-iodine cell (CI), and the electrochemical concentration cell (ECC).…”