Retrieval of Sulfur Dioxide in Volcanic Region from EMI-Ⅱ

Yuanyuan Qian; Yuhan Luo; Haijin Zhou; Zhen Chang; Taiping Yang; Liang Xi; Fuying Tang; Fuqi Si

doi:10.3788/AOS221398

Abstract

Results and Discussions To validate the retrieved SO₂ VCD results, we compare the SO₂ VCD from EMI-Ⅱ with that from TROPOMI at the La Palma volcano on 27, 29, and 31 October 2021. The SO₂ VCD from EMI-Ⅱ showes similar spatial distributions to those of the SO₂ VCD from TROPOMI (Fig. 4) with R of 0.891, 0.901, and 0.915 (Fig. 5), respectively. In addition, the SO₂ VCD from EMI-Ⅱ is also compared with that from TROPOMI in the region of the Tonga submarine volcano from 14 to 18 January 2022 (Fig. 6), and the SO₂ VCD from EMI-Ⅱ is found to have similar spatial distributions to those of the SO₂ VCD from TROPOMI. The SO₂ plume transported from Tonga to Australia, which corresponds to the wind field results from a hybrid single-particle Lagrangian integrated trajectory (HYSPLIT) model (Fig. 7). However, the SO₂ VCD from EMI-Ⅱ is lower than that from TROPOMI in the grid with a high SO₂ SCD, which is mainly because the a priori profile of TROPOMI is different from that of EMI-Ⅱ in the radiative transfer model. According to the calculated SO₂ VCD and wind field data, the fluxes of SO₂ on 14 and 15 January 2022 in the region of Tonga submarine volcano are 345.83 and 504.85 t/s, respectively.Objective

The SO₂ slant column density (SCD) of environmental trace gases monitoring instrument Ⅱ (EMI-Ⅱ) from China is firstly retrieved using differential optical absorption spectroscopy (DOAS). The air mass factor (AMF) look-up table of SO₂ is established using the SCIATRAN radiative transfer model. The vertical column density (VCD) is obtained after destriping. With the La Palma volcano at the end of October 2021 as an example, the SO₂ VCD obtained by retrieval from EMI-Ⅱ data is consistent with that from TROPOspheric monitoring instrument (TROPOMI) with the correlation coefficients (R) of 0.89, 0.90, and 0.92. In addition, the retrieved SO₂ VCD in the region of Tonga submarine volcano is also compared with that from TROPOMI. The EMI?Ⅱ results show similar spatial distributions to those of the TROPOMI results, and the transmission process (from the east to the west) of the SO₂ plume is monitored. With the wind field data, this paper calculates the fluxes of SO₂ generated from the eruption of the Tonga submarine volcano on 14 and 15 January 2022. The results of the paper show that EMI-Ⅱ can yield reliable SO₂ VCD in volcanic regions via retrieval and realize the early warning of global volcanic eruptions.

SO₂ not only affects human health (e.g., respiratory diseases) but also is closely related to climate and environment (e.g., acid rain). Its oxidation may lead to the formation of aerosols and photochemical smog. SO₂ is an important indicator of air quality and is closely associated with volcanic eruptions. The SO₂ VCD can provide a data basis for tracing the SO₂ pollution caused by industrial emissions and early warning signals for volcanic eruptions around the world. Therefore, it is extremely important to obtain the daily global SO₂ VCD. In this study, we report the SO₂ VCD results in volcanic regions from EMI-Ⅱ and validate the retrieved results with those from TROPOMI. In addition, the fluxes of SO₂ from the eruption of the Tonga submarine volcano are calculated, which may help make clear the dynamics of magma degassing. We hope that our results can contribute to the development and global validation of the EMI-Ⅱ SO₂ VCD.

Methods

The SO₂ SCD is calculated using the QDOAS software with DOAS method. DOAS retrieves the concentrations of trace gases depending on their characteristic absorption and the measured optical intensity, which is based on the Lambert-Beer's law. Then, the corresponding SO₂ AMF of the EMI-Ⅱ is calculated using the established AMF look-up table, which is simulated in the SCIATRAN radiative transfer model. The SO₂ VCD is then obtained from SCD and AMF. We use spatial filtering following the Fourier transform method to remove obvious stripes caused by the irradiance calibration error when retrieving the SO₂ VCD from EMI?Ⅱ. The fluxes of SO₂ from satellite-based measurements can be calculated using the above method. For the Tonga submarine volcano, the effect of distance can be ignored for the long lifetime of the stratospheric SO₂ plume.

Conclusions

In this paper, the SO₂ VCD is retrieved from EMI-Ⅱ and validated in volcanic regions. With the La Palma volcano and the Tonga submarine volcano as examples, the SO₂ VCD from EMI-Ⅱ presents similar spatial distributions to those of the SO₂ VCD from TROPOMI. In addition, the transmission process of SO₂ plume in a volcanic region can be monitored using the retrieved SO₂ VCD from EMI-Ⅱ. The results of this study confirm that EMI-Ⅱ can monitor SO₂ in volcanic regions and realize the early warning of global volcanic eruptions. This paper is of great importance for the development and global validation of SO₂ VCD from EMI-Ⅱ.