Combined UV and IR ozone profile retrieval from TROPOMI and CrIS measurements

International audienceVertical ozone profiles from combined spectral measurements in the ultraviolet and infrared spectral range were retrieved by using data from the TROPOspheric Monitoring Instrument on the Sentinel-5 Precursor (TROPOMI/S5P) and the Cross-track Infrared Sounder on the Suomi National Polar-orbiting Partnership (CrIS/Suomi-NPP), which are flying in loose formation 3 min apart in the same orbit. A previous study of ozone profiles retrieved exclusively from TROPOMI UV spectra showed that the vertical resolution in the troposphere is clearly limited (Mettig et al., 2021). The vertical resolution and the vertical extent of the ozone profiles is improved by combining both wavelength ranges compared to retrievals limited to UV or IR spectral data only. The combined retrieval particularly improves the accuracy of the retrieved tropospheric ozone and to a lesser degree stratospheric ozone up to 30 km. An increase in the degrees of freedom (DOF) by 1 DOF was found in the UV + IR retrieval compared to the UV-only retrieval. Compared to previous publications, which investigated combinations of UV and IR observations from the Ozone Monitoring Instrument and Tropospheric Emission Spectrometer (OMI and TES) and Global Ozone Monitoring Experiment version 2 and Infrared Atmospheric Sounding Interferometer (GOME-2 and IASI) pairs, the degree of freedom is lower, which is attributed to the reduced spectral resolution of CrIS compared to TES or IASI. Tropospheric lidar and ozonesondes were used to validate the ozone profiles and tropospheric ozone content (TOC). In their comparison with tropospheric lidars, both ozone profiles and TOCs show smaller biases for the retrieved data from the combined UV + IR observation than from the UV observations alone. For the ozone profiles below 10 km, the mean differences are around ±10 % and the mean TOC varies around ±3 DU. We show that TOCs from the combined retrieval agree better with ozonesonde results at northern latitudes than the UV-only and IR-only retrievals and also have lower scatter. In the tropics, the IR-only retrieval shows the best agrement with TOCs derived from ozonesondes. While in general the TOCs show good agreement with ozonesonde data, the profiles have a positive bias of around 30 % between 10 and 15 km. The reason is probably a positive stratospheric bias from the IR retrieval. The comparison of the UV + IR and UV ozone profiles up to 30 km with the Microwave Limb Sounder (MLS) demonstrates the improvement of the UV + IR profile in the stratosphere above 18 km. In comparison to the UV-only approach the retrieval shows improvements of up to 10 % depending on latitude but can also show worse results in some regions and latitudes

HELSTOP: A Project Design for the Harmonization and Evaluation of Lower Stratospheric and Tropospheric Ozone Vertical Profiles

International audienceTropospheric ozone is an important trace gas, as it is both a greenhouse gas and a pollutant detrimental to human health, crop yields and ecosystem productivity. As a highly reactive gas, ozone is an important component of air quality in the lower troposphere at a regional scale but also in the middle and upper troposphere at larger scales. The relative greenhouse effect of ozone is greatest in the upper troposphere (UT) and lower stratosphere (LS), the most climate sensitive region of the atmosphere. Measuring and characterizing the vertical distribution of ozone and understanding the underlying processes are two essential activities in atmospheric research. Techniques to measure the vertical distribution of ozone in the troposphere and lower stratosphere are the conventional ozonesondes, UV-photometers onboard commercial & research aircraft, Lidars, and FTIR's . However, since more than a decade, ground-based UV-visible spectrometers (e.g. MAXDOAS, PANDORA) and satellite instruments (e.g. OMI, TROPOMI), which are based on the remote sensing technique, play an increasing role in the characterization of tropospheric ozone. Besides clear regional differences, the distribution and trends of ozone in the troposphere and lower stratosphere are not always consistent between the different datasets obtained from the different standard ozone observing techniques . As a matter of fact, measuring the vertical profile of tropospheric and lower stratospheric ozone concentrations from satellites remains very challenging and have to rely on ground-based retrievals of ozone for validation. Here we will present the project design HELSTOP (Harmonizing and Evaluate Lower Stratospheric and Tropospheric Ozone Profiles) to harmonize and evaluate the data of lower stratospheric and tropospheric ozone profiles obtained from the different ozone measuring techniques. Presently, HELSTOP is submitted as a proposal for a COST action. In HELSTOP we want to bring scientists and engineers belonging to different ozone observation communities together, not only to strengthen, speed up and expand existing activities of harmonization of instruments and datasets within a technique, but also to compare Quality Assurance/Quality Control (QA/QC) procedures, operation best practices, harmonization efforts, and retrieval algorithms between the different ozone measuring techniques. As end objective, HELSTOP aims at providing a harmonized and consistent dataset of vertical ozone profiles in the troposphere and lower stratosphere, retrieved by the different techniques listed above. Together with a data cross-comparison at dedicated sites, HELSTOP will also create the framework for future intercomparisons and assessments