The January 15, 2022 phreatomagmatic eruption of the submarine Hunga Tonga-Hunga Ha’apai (HTHH) volcano (Tonga) generated an explosion of historic magnitude, and was preceded by ~1 month of Surtseyan eruptive activity and two precursory explosive eruptions. We present an analysis of ultraviolet (UV) satellite measurements of volcanic sulfur dioxide (SO2) between December 2021 and the climactic January 15, 2022 eruption, comprising an unprecedented record of Surtseyan eruptive emissions. UV measurements from the Ozone Monitoring Instrument (OMI) on NASA’s Aura satellite, the Ozone Mapping and Profiler Suite (OMPS) on Suomi-NPP, the Tropospheric Monitoring Instrument (TROPOMI) on ESA’s Sentinel-5P, and the Earth Polychromatic Imaging Camera (EPIC) aboard the Deep Space Climate Observatory (DSCOVR) are combined to yield a consistent multi-sensor record of SO2 emissions during the eruptive sequence. We estimate SO2 emissions during the key phases of the eruption: the initial December 19, 2021 eruption (~0.01 Tg SO2); continuous SO2 emissions from December 20, 2021-early January 2022 (~0.12 Tg SO2); the January 13, 2022 stratospheric eruption (0.06 Tg SO2); and the paroxysmal January 15, 2022 eruption (~0.4-0.5 Tg SO2); yielding a total SO2 emission of ~0.6-0.7 Tg SO2 for the entire eruptive episode. We interpret the vigorous SO2 emissions observed prior to the January 2022 eruptions, which were significantly higher than measured in the 2009 and 2014 HTHH eruptions, as strong evidence for a rejuvenated magmatic system. High cadence DSCOVR/EPIC SO2 imagery permits the first UV-based analysis of umbrella cloud spreading and volume flux in the January 13, 2022 eruption, and also tracks early dispersion of the stratospheric SO2 cloud injected by the January 15 eruption. The ~0.4-0.5 Tg SO2 discharged by the paroxysmal January 15, 2022 HTHH eruption is low relative to other eruptions of similar magnitude, and a review of previous submarine eruptions of the satellite era indicates that such modest SO2 yield may be characteristic of these events, with the emissions and atmospheric impacts likely dominated by water vapor (WV). The origin of the low SO2 loading awaits further investigation but scrubbing of SO2 in the water-rich eruption plumes and rapid conversion to sulfate aerosol are highly plausible, given the exceptional WV emission measured in the January 15, 2022 HTHH eruption.

Determining the height of a volcanic SO2 cloud after a volcanic eruption is a challenging task in UV satellite retrievals. The height is nevertheless the most important yet uncertain parameter required to forecast the movement of the volcanic cloud and to determine the total SO2 column and ejected SO2 mass, especially for local authorities and aviation safety applications. Retrieval algorithms developed so far use direct fitting and optimal estimation techniques to determine the height information, which is hidden in the spectral signature. They are computationally very expensive and time consuming and therefore are not practical in near-real time operational retrievals, especially for current and future satellite UV instruments with high resolution and related high data amount. We have therefore developed the ‘Full-Physics Inverse Learning Machine’ (FP_ILM) retrieval algorithm that combines principle component analysis and neural network , which performs an extremely fast (3 ms per TROPOMI pixel) yet accurate (<2km average accuracy) SO2 LH retrieval based on UV satellite measurements. The algorithm was first applied to GOME-2 and introduced by Efremenko et al. (2017) . Hedelt et al. (2019) further improved the algorithm and applied it to Sentinel-5p/TROPOMI data. The algorithm was optimized and validated in the framework of ESA’s Sentinel-5p Innovations project (S5P+I) and is already performing SO2 LH retrievals in a semi-operational near-real time environment. Recently, Fedkin et al. (2021) applied the FP_ILM algorithm to OMI and OMPS data. Application to future UV LEO (Sentinel-5) and GEO (Sentinel-4, GEMS, TEMPO) satellite missions will follow. We present here SO2 LH results based on GOME-2, OMI/OMPS, and TROPOMI measurements of the Raikoke (Kuril Islands) volcanic eruption in June-July 2019 and La Soufriere St. Vincent volcanic eruption in April 2021 and make intercomparisons between the UV sensors as well as other (IR) independent measurements.