Early Evolution of the Stratospheric Aerosol Plume Following the 2022 Hunga Tonga-Hunga Ha'apai Eruption: Lidar Observations From Reunion (21°S, 55°E)

The exceptionally violent eruption of the Hunga Tonga-Hunga Ha'apai volcano (HTHH) of 15 January 2022, in the South Pacific, was associated with a powerful blast that injected gases, steam and aerosol to unprecedentedly high altitudes. This article details unique observations of the young volcanic plume from ground-based lidars at Reunion (21 degrees S, 55 degrees E). Two lidars, operating at wavelengths of 355 and 532 nm, recorded the plume overhead from 19 January until 28 January providing the vertical structure and the optical properties of the plume. A series of thick stratospheric plumes between 36 and 18 km altitude have been characterized along time, with aerosol optical depth as high as 0.84 at 532 nm and negative Angstrom exponents for the main layers down to -0.8 +/- 0.8. The diversity of plumes properties is explained by the injection heights of the volcanic material as well as stratospheric dynamics and chemistry. Plain Language Summary In January 2022, the Hunga Tonga-Hunga Ha'apai (HTHH) underwater volcano exploded in the southern Pacific (20.5 degrees S, 175.4 degrees W). Eruption metrics of this outbreak is to be compared to historic climate impacting volcanic events of the past century (e.g., Mount Pinatubo). Based on laser remote-sensing observations of the HTHH plume, its early structural and optical properties were assessed during its passage over Reunion (21 degrees S, 55 degrees E). Our results show record-breaking optical characteristics for such high altitudes, deep in the stratosphere between 18 and 36 km. In particular, peak values of aerosol optical thickness which represents the opacity of the atmosphere, were never recorded as high. Moreover, although this property is expected to decrease with increasing wavelength of the light spectrum, the thickest aerosol layers we recorded show a different optical behavior. They are opaquer in the visible spectrum around 532 nm than in the UV at 355 nm. This is likely to be link to the specific size distribution of these volcanic particles, driven by an unusual stratospheric chemistry resulting from the presence of large amount of water vapor. These findings are original and essential observations to question our understanding of such atmospheric processes and to help improve global climate models.