16 May 2022
16 May 2022
Status: this preprint is currently under review for the journal AMT.

Improved retrieval of SO2 plume height from TROPOMI using an iterative Covariance-Based Retrieval Algorithm

Nicolas Theys1, Christophe Lerot1, Hugues Brenot1, Jeroen van Gent1, Isabelle De Smedt1, Lieven Clarisse2, Mike Burton3, Matthew Varnam3, and Michel Van Roozendael1 Nicolas Theys et al.
  • 1Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Brussels, Belgium
  • 2Université libre de Bruxelles (ULB), Spectroscopy, Quantum Chemistry and Atmospheric Remote Sensing (SQUARES), C. P. 160/09, Brussels, Belgium
  • 3School of Earth and Environmental Sciences, University of Manchester, Oxford Road, Manchester, M139PL, UK

Abstract. Knowledge of sulfur dioxide layer height (SO2 LH) is important to understand volcanic eruption processes, the climate impact of SO2 emissions and to mitigate volcanic risk for civil aviation. However, the estimation of SO2 LH from ground-based instruments is challenging in particular for rapidly evolving and sustained eruptions. Satellite wide-swath nadir observations have the advantage to cover large-scale plumes and the potential to provide key information on SO2 LH. In the ultraviolet, SO2 LH retrievals leverage the fact that, for large SO2 columns, the light path and its associated air mass factor (AMF) depends on the SO2 absorption (and therefore on the vertical distribution of SO2), and SO2 LH information can be obtained from the analysis of measured back-scattered radiances coupled with radiative transfer simulations. However, existing algorithms are mainly sensitive to SO2 LH for SO2 vertical columns of at least 20 DU. Here we develop a new SO2 LH algorithm and apply it to observations from the high spatial resolution TROPOspheric Monitoring Instrument (TROPOMI). It is based on an SO2 optical depth look-up-table and an iterative approach. The strength of this scheme lies in the fact that it is a Covariance-Based Retrieval Algorithm (COBRA; Theys et al., 2021). This means that the SO2-free contribution of the measured optical depth is treated in an optimal way, resulting in an improvement of the SO2 LH sensitivity to SO2 columns as low as 5 DU, with a precision better than 2 km. We demonstrate the value of this new data through a number of examples and comparison with satellite plume height estimates (from IASI and CALIOP), and back trajectory analyses. The comparisons indicates an SO2 LH accuracy of 1–2 km, expect for some difficult observation conditions.

Nicolas Theys et al.

Status: open (until 15 Jul 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2022-148', Anonymous Referee #1, 29 Jun 2022 reply
  • RC2: 'Comment on amt-2022-148', Anonymous Referee #1, 29 Jun 2022 reply
  • RC3: 'Review Comment on amt-2022-148', Anonymous Referee #2, 01 Jul 2022 reply
  • RC4: 'Comment on amt-2022-148', Anonymous Referee #3, 01 Jul 2022 reply

Nicolas Theys et al.

Nicolas Theys et al.


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Latest update: 05 Jul 2022
Short summary
Sulfur dioxide plume height after a volcanic eruption is an important information for many different scientific studies and applications. Satellite UV retrievals are useful in this respect but available algorithms have shown so far limited sensitivity to SO2 height. Here we present a new technique to improve the retrieval of SO2 plume height for SO2 columns as low as 5 DU. We demonstrate the algorithm using TROPOMI measurements and compare with other height estimates.