Enhanced TROPOMI SO2 height and density retrievals applied to eruptions in 2023-2024

Sulfur dioxide (SO₂) emissions from volcanic activity have significant impacts on human health, society, aviation, and atmospheric composition in general. While nadir-viewing satellites have delivered decades of valuable information on the SO₂ Vertical Column Density (VCD), accurate retrieval of its Layer Height (LH) remains a major challenge, yet critical to further understand volcanic events and refine estimates of SO₂ emissions, which play a key role on climate. Indeed, current UV retrieval techniques often face limitations in sensitivity and computational efficiency, particularly in aerosol-rich conditions.

Here, we present an enhanced SO₂ height and column density retrieval algorithm developed from the high-resolution TROPOspheric Monitoring Instrument (TROPOMI). Our approach focuses on the second UV spectral band (BD2), which benefits from a strong SO₂ absorption, rather than the third band (BD3) commonly used for sulfur dioxide retrievals.

Sensitivity analyses were first carried out on a set of synthetic spectra representative of TROPOMI observations with the Look-Up Table Covariance-Based Retrieval Algorithm (LUT-COBRA) [1, 2]. The impact of atmospheric, spectroscopic and observation conditions on the retrieval quality has been thoroughly studied, highlighting the considerable effect of ozone. Furthermore, results indicate that BD2 retrievals provide more accurate SO₂ LHs and VCDs, with much lower retrieval errors, especially in the Upper Troposphere/Lower Stratosphere (UTLS).

The algorithm was then applied to real TROPOMI observations of different volcanic eruptions (e.g., Ruang, Etna). In comparison to BD3 retrievals, our method leads to a better sensitivity, with less noise and a detection limit as low as 2.0 DU, outperforming the current operational TROPOMI SO₂ product. Moreover, our plume height estimates align well with independent measurements from IASI, CALIOP, and OMPS-LP, confirming the reliability of our results. 

 

[1] N. Theys et al., Atmospheric Chemistry and Physics, 21(22):16727–16744, 2021.

[2] N. Theys et al., Atmospheric Measurement Techniques, 15(16):4801–4817, 2022.