14 Jul 2021

14 Jul 2021

Review status: this preprint is currently under review for the journal AMT.

Retrieval algorithm for OClO from TROPOMI by Differential Optical Absorption Spectroscopy

Jānis Puķīte1, Christian Borger1, Steffen Dörner1, Myojeong Gu1, Udo Frieß2, Andreas Carlos Maier3, Carl-Fredrik Enell4,a, Uwe Raffalksi4, Andreas Richter3, and Thomas Wagner1 Jānis Puķīte et al.
  • 1Max Planck Institute for Chemistry, Mainz, Germany
  • 2Institute of Environmental Physics, Heidelberg, Germany
  • 3Institute of Environmental Physics, Bremen, Germany
  • 4Swedish Institute of Space Physics, Kiruna, Sweden
  • anow at: EISCAT Scientific Association, Kiruna, Sweden

Abstract. The TROPOspheric Monitoring Instrument (TROPOMI) is a UV-VIS-NIR-SWIR instrument on board of Sentinel-5P satellite developed for monitoring the Earth’s atmosphere. It was launched on 13 October 2017 in a near polar orbit. It measures spectrally resolved earthshine radiances at an unprecedented spatial resolution of around 3.5 x 7.2 km² (3.5 x 5.6 km² starting from 6 Aug 2019) (near nadir) with a total swath width of ~ 2600 km on the Earth's surface providing daily global coverage. From the measured spectra high resolved trace gas distributions can be retrieved by means of differential optical absorption spectroscopy (DOAS).

Chlorine dioxide (OClO) is a by-product of the ozone depleting halogen chemistry in the stratosphere. Although being rapidly photolysed at low solar zenith angles (SZAs) it plays an important role as an indicator of the chlorine activation in polar regions during polar winter and spring at twilight conditions because of the nearly linear dependence of its formation to chlorine oxide (ClO).

Here we present a new retrieval algorithm of the slant column densities (SCDs) of chlorine dioxide (OClO) by DOAS. To achieve a substantially improved accuracy, which is especially important for OClO observations, accounting for absorber and pseudo absorber structures in optical depth even of the order of 10−4 is important. Therefore in comparison to existing retrievals, we include several additional fit parameters accounting for spectral effects like the temperature dependency of the Ring effect and Ring absorption effects, higher order term for the OClO SCD dependency on wavelength and account for the BrO absorption.

We investigate the performance of different retrieval settings by an error analysis with respect to random variations, large scale systematic variations as function of solar zenith angle and also more localised systematic variations by a novel application of an autocorrelation analysis.

The retrieved TROPOMI OClO SCDs show a very good agreement with ground based zenith sky measurements and are correlated well with preliminary data of the opeartional TROPOMI OClO retrieval algorithm currently being developed as part of ESA's S5p+I project.

Jānis Puķīte et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2021-178', Anonymous Referee #2, 28 Jul 2021
  • RC2: 'Comment on amt-2021-178', Anonymous Referee #1, 28 Jul 2021
  • RC3: 'Comment on amt-2021-178', Anonymous Referee #3, 18 Aug 2021

Jānis Puķīte et al.

Jānis Puķīte et al.


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Short summary
Chlorine dioxide (OClO) is used as an indicator for chlorine activation. We present a new DOAS retrieval algorithm for OClO from measurements of TROPOMI on the Sentinel-5P satellite. To achieve a substantially improved accuracy for the weak absober OClO we consider several additional fit parameters accounting for various higher order spectral effects. The retrieved OClO slant column densities are compared with ground based zenith sky measurements.