This study attempts to quantify the radiative impact over Reunion Island (21°S, 55°E) in the southern tropical Indian Ocean of the aerosols and water vapor injected in the stratosphere by the eruption on 15 January 2022 in the South Pacific of the Hunga Tonga-Hunga Ha’apai underwater volcano. Ground-based lidar and satellite passive instruments are used to parametrize a state-of-the-art radiative transfer model for the first thirteen months after the volcano eruption. The descending rate of the aerosol volcanic plume is -0.008 km day-1. At this rate, aerosols are expected to be present in the stratosphere until the first half of 2025. The overall aerosol and water vapor impact on the Earth’s radiation budget for the whole period is positive (warming, +0.06 ± 0.45 W m-2) and dominated by the aerosol impact. However, the decreasing rate with time of the aerosol warming effect is larger than that of the water vapor cooling effect, so that, in the long run, the impact on the Earth’s radiation budget might reduce to quasi-neutral, or even become slightly negative. At the Earth’s surface, aerosols are the main driver and produce a negative (cooling, -0.91 ± 0.61 W m-2) radiative impact with also a decreasing tendency with time. Heating/cooling rate profiles show a clear vertical difference in the stratosphere between the aerosol warming impact (17 to 25 km) and the water vapor cooling one (25 to 40 km).

The highly-explosive eruption of the Hunga-Tonga Hunga Ha’apai volcano (HTHH), that occurred on 15 Jan. in the South Pacific, was associated with a powerful blast that injected gases, steam and aerosols to unprecedentedly high altitudes. Here we present unique observations of the young volcanic aerosol plumes by ground-based lidars at La Reunion island (21°S, 55°E), located directly downwind of the eruption. Two lidars, operating at 355 nm and 532 nm, sampled the overflying plume every nights from 19 Jan. until 28 Jan. We assess both the vertical structure and the optical properties. A wide plume altitude range from 36 km down to 18 km has been highlighted along time, with heterogeneous aerosol optical depth that reached 0.84 at 532 nm and Angström exponents from-0.8 to 1.2. Such temporal evolution is related to both the injection heights of the volcanic material and the stratospheric dynamic and chemistry.