Ozone Profile and Tropospheric Ozone Retrievals from Joint UV and Visible Measurements by the TEMPO

Abstract:

Sensitivity to boundary layer ozone from UV measurements in the Hartley/Huggins ozone bands is limited due to insufficient photon penetration down to the lower troposphere especially at larger viewing geometry. Sensitivity studies demonstrated that adding visible measurements in the Chappuis ozone band can significantly enhance the sensitivity to boundary layer ozone. This additional sensitivity motivates the first NASA EVI (Earth Venture Instrument) TEMPO (Tropospheric Emissions: Monitoring of Pollution) instrument to measure earthshine radiance in the visible (540-740 nm) in addition to UV (~290-490 nm) to make better ozone pollution measurements. However, this retrieval is challenging due to weak ozone absorption in the visible and strong interferences from other trace gases (O₄, H₂O, O₂), surface reflectance and aerosols and the requirement of accurate radiometric calibration across different spectral channels.

We present the latest TEMPO UV/visible algorithm development in this study. Temperature-parameterized O₃ and O₄ cross sections will be taken from BDM and Thalman datasets, respectively. The HITRAN 2018 database will be used to make a look-up table for H₂O and O₂ cross sections in the visible as a function of temperature and pressure instead of time-consuming line-by-line on-line calculation. We will also improve the characterization of the wavelength dependence of surface albedo using surface albedo/BRDF climatology derived from a combination of ASTER, USGS, MODIS, SCIAMACHY databases. To perform accurate and fast radiative transfer calculation, we combine fast 2 half stream PCA-VLIDORT RTM with look-up table corrections to approximations of scalar, 2-half stream, and coarser vertical grid. On-orbit slit functions are derived assuming an asymmetric super Gaussian slit function, and linearization of slit functions are included in the fitting to account for slit function variations on the pixel-by-pixel basis. Ozone profile a priori based on a tropopause-based climatology merged with hourly resolved CTM ozone profiles in the lower troposphere is used to stabilize the retrievals and account for the diurnal variation of tropospheric ozone. We test the algorithm and present the retrieval performance using synthetic TEMPO data (290-345 nm and 540-650 nm).