Balloon Observations Suggesting Sea Salt Injection into the Stratosphere from Hunga Tonga-Hunga Ha'apai

Abstract. Volcanic eruptions are crucial in the Earth's climate system, driving natural variability. Typically, sulfate aerosols generated by major volcanic events have persisted for years, cooling the Earth's surface while warming the stratosphere. The unprecedented submarine eruption of Hunga Tonga Hunga Ha'apai (HTHH) on January 15th, 2022, challenged this “paradigm” and offered a new perspective by injecting material up to the mesosphere. It led to a 10 % increase in the global stratospheric water vapor burden due to seawater injection, warming the Earth’s surface and competing with the sulfate-induced cooling. The resulting Stratospheric Aerosol Optical Depth appears to be much lower than that derived from satellite observations based on previous eruptions. This study shows a unique combination of balloon-borne and satellite-based measurements of the HTHH water-rich aerosol plume. We used an innovative balloon-based sampling technique and ion chromatographic analysis of the collected aerosol samples to suggest the presence of Na⁺, K⁺, NH₄⁺, Ca²⁺, Cl⁻, and traces of SO₄²⁻, 8 months after the eruption, indicating its greater complexity than previously assumed.

Based upon the chemical and optical balloon-borne and satellite observations, we suggest that marine aerosols played a role in accounting for the larger-than-expected aerosol burden compared to the modest 0.42 Tg SO₂ injected. These findings encourage the inclusion of sea salt in addition to sulfate in climate models to correctly simulate the climatic impact of the Hunga eruption.