Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> The TOPAS (Tikhonov regularised Ozone Profile retrievAl with SCIATRAN) algorithm to retrieve vertical profiles of ozone from space-borne observations in nadir-viewing geometry has been developed at the Institute of Environmental Physics (IUP) of the University of Bremen and applied to the TROPOspheric Monitoring Instrument (TROPOMI) L1B spectral data version 2. Spectral data between 270 and 329 nm are used for the retrieval. A recalibration of the measured radiances is done using ozone profiles from MLS/Aura. Studies with synthetic spectra show that individual profiles in the stratosphere can be retrieved with an uncertainty of about 10 %. In the troposphere, the retrieval errors are larger depending on the a priori profile used. The vertical resolution above 18 km is about 6–10 km, and it degrades to 15–25 km below. The vertical resolution in the troposphere is strongly dependent on the solar zenith angle (SZA). The ozone profiles retrieved from TROPOMI with the TOPAS algorithm were validated using data from ozonesondes and stratospheric ozone lidars. Above 18 km, the comparison with sondes shows excellent agreement within less than <span class="inline-formula">±5</span> % for all latitudes. The standard deviation of mean differences is about 10 %. Below 18 km, the relative mean deviation in the tropics and northern latitudes is still quite good, remaining within <span class="inline-formula">±20</span> %. At southern latitudes, larger differences of up to <span class="inline-formula">+40</span> % occur between 10 and 15 km. The standard deviation is about 50 % between 7–18 km and about 25 % below 7 km. The validation of stratospheric ozone profiles with ground-based lidar measurements also shows very good agreement. The relative mean deviation is below <span class="inline-formula">±5</span> % between 18–45 km, with a standard deviation of 10 %. TOPAS retrieval results for 1 d of TROPOMI observations were compared to ozone profiles from the Microwave Limb Sounder (MLS) on the Aura satellite and the Ozone Mapping and Profiler Suite Limb Profiler (OMPS-LP). The relative mean difference was found to be largely below <span class="inline-formula">±5</span> % between 20–50 km, except at very high latitudes.

Highlights

  • The stratospheric ozone layer is of particular importance for humans because it protects the biosphere from biologically damaging ultraviolet radiation (UV)

  • We developed a new TOPAS algorithm based on the firstorder Tikhonov regularisation to retrieve the vertical distributions of ozone from TROPOspheric Monitoring Instrument (TROPOMI) measurements in the wavelength range from 270 to 329 nm

  • We estimated the retrieval quality using synthetic spectra adapted to TROPOMI

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Summary

Introduction

Ozone is one of the most important trace gases in the Earth’s atmosphere. The stratospheric ozone layer is of particular importance for humans because it protects the biosphere from biologically damaging ultraviolet radiation (UV). The first space-borne measurements of contiguous spectra in the ultraviolet and visible spectral regions at sufficiently high spectral resolution were made by the Global Ozone Monitoring Experiment (GOME) instrument (Burrows et al, 1999) These data were used to retrieve ozone profiles from measurements of GOME (Burrows et al, 1999). With the launch of Sentinel 5 Precursor (S5P) in October 2017, the TROPOspheric Monitoring Instrument (TROPOMI) is another nadir-viewing spectrometer operating in the UV–Vis and SWIR spectral range It is a followup of OMI and SCIAMACHY. The L1B version 1 TROPOMI data in band 3 (314– 340 nm) have been used by Zhao et al (2020) to determine tropospheric ozone and investigate its changing distribution due to the Covid-19 pandemic Their profile retrieval was limited to the UV3 band because of larger systematic radiance differences in band UV1 and larger fitting residuals in band UV2.

Measurement data
TROPOMI
Ozonesondes
Ozone lidar
Inversion technique
Retrieval algorithm
Synthetic retrievals of ozone profiles
Sensitivity study
Calibration correction for TROPOMI measurements
Validation
First results
Findings
Conclusion and summary
Full Text
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