Insight into the conversion of SO2 to sulphate aerosols in volcanic plumes from the joint analysis of hyperspectral OMI and multi-angular polarimetric POLDER satellite observations

Volcanic sulfur dioxide (SO₂) is a gaseous precursor that is transformed into secondary sulfate aerosols (SO₄^2-) by several intricate chemical and physical atmospheric processes. It is currently unclear how quickly sulfate aerosols are produced in volcanic plumes, particularly in tropospheric plumes. We jointly analyze Aura/OMI SO₂ observations to constrain the sulfur-rich emissions and identify the volcanic plume dispersion pattern as well as multi-angle, multi-wavelength, and polarizing PARASOL/POLDER-3 observations that are particularly sensitive to fine mode particles to gain a better understanding of the lifecycle of volcanic sulfate aerosols. The GRASP/Component^[1] (Generalized retrieval of Aerosol and Surface Properties) algorithm gives us details about the soluble and insoluble aerosol components in both fine and coarse modes based on their complex refractive indices in addition to standard optical characteristics. In order to provide insight into SO₂ to particle conversion rate, we analyze the degassing of the Kilauea volcano (Hawaii, USA) between 2006 to 2012, which includes periods of passive and eruptive degassing.

We demonstrate that Kilauea SO₂-rich pixels from OMI measurements are broadly collocated with poorly-absorbing fine aerosol-rich pixels from POLDER measurements (fine AOD (440nm) ranging from 0.1 to 0.4, SSA (440nm) ranging from 0.95 to 1.0). We show that these volcanic particles also differ from long-distance transported man-made and natural fine-absorbing particles seen across the Kilauea domain from the Asian region in terms of their absorption characteristics. We, therefore attribute these fine mode particles to sulfate aerosols that result from the conversion of Kilauea SO₂ emissions.

In comparison to SO₂-rich plumes, Kilauea aerosol-rich plumes have a significantly wider spread and are characterized by an excess anomaly in fine AOD and high SSA values. Irrespective of the degassing strength, a pattern consistent with the oxidation of SO₂ to secondary sulfate aerosols is observed where the SO₂ concentration gradually drops with plume dispersion while the fine AOD gradually increases, peaking at a distance of around 800–3000 km from the Kilauea source. Depending on the intensity of volcanic activity, the season, and enduring local meteorological conditions, different time scales for oxidation of SO₂ and geographical dispersion of the Kilauea aerosol plumes are observed. We conducted additional analysis on the coarse AOD and coarse components to look for ash signals inside the plume. Furthermore, the complex refractive index of Kilauea particles, retrieved by the GRASP/Component algorithm, indicates an imaginary part (0.003-0.005) that is slightly higher than that of volcanic basaltic ash, as determined by laboratory experiments, while the real part (1.49-1.52) lies well in between pure sulfate (1.40-1.46) and basaltic ash (1.56-1.63). These refractive index values imply that Kilauea particles are not pure sulfate aerosols but instead contain some spectrally absorbing elements that may point to the existence of fine ash or sulfate-coated ash particles within the plume.

[1] Li, L., Dubovik, O., Derimian, Y., Schuster, G. L., Lapyonok, T., Litvinov, P., Ducos, F., Fuertes, D., Chen, C., Li, Z., Lopatin, A., Torres, B., and Che, H.: Retrieval of aerosol components directly from satellite and ground-based measurements, Atmos. Chem. Phys., 19, 13409–13443, https://doi.org/10.5194/acp-19- 13409-2019, 2019.