Unexpectedly rapid aerosol formation in the Hunga Tonga plume

Significance

Large volcanic eruptions play an important role in Earth’s radiative balance through stratospheric injections of sulfur dioxide that form sulfate aerosol. Here, we show that in situ observations are critical to constrain the injection mass of stratospheric sulfur and the stratospheric lifetime of sulfur dioxide. Such information is needed to better represent aerosol microphysics and improve predictions of the impacts of natural (or potentially anthropogenic) sulfur dioxide injections. Measurements in the fresh volcanic Hunga Tonga–Hunga Ha’apai plume in January 2022 revealed that stratospheric aerosol formation ended approximately three times faster than is typical in the presence of a large amount of water vapor, resulting in a high signal in aerosol extinction from an abundance of large particles.

Abstract

The Hunga Tonga–Hunga Ha’apai (HT-HH) volcanic eruptions on January 13 and 15, 2022, produced a plume with the highest signal in stratospheric aerosol optical depth observed since the eruption of Mt. Pinatubo in 1991. Suites of balloon-borne instruments on a series of launches from Réunion Island intercepted the HT-HH plume between 7 and 10 d of the eruptions, yielding observations of the aerosol number and size distribution and sulfur dioxide (SO ₂) and water vapor (H ₂O) concentrations. The measurements reveal an unexpected abundance of large particles in the plume, constrain the total sulfur injected to approximately 0.2 Tg, provide information on the altitude of the injection, and indicate that the formation of sulfuric acid aerosol was complete within 3 wk. Large H ₂O enhancements contributed as much as ~30% to ambient aerosol surface area and likely accelerated SO ₂ oxidation and aerosol formation rates in the plume to approximately three times faster than under normal stratospheric conditions.