The Hunga Tonga-Hunga Ha'apai volcano eruption on January 15, 2022, was exceptionally powerful, estimated at a Volcanic Explosivity Index (VEI) of 5.8. Its impact was globally felt, generating a Lamb wave comparable only to the 1883 Krakatoa eruption. The eruption plume reached an unprecedented altitude of 58 km, directly injecting volcanic gases, vaporized seawater, and entrained tropospheric moisture into the stratosphere. The stratosphere's dryness is typically maintained by the cold tropical tropopause, limiting water vapor to a few ppmv. However, changes in stratospheric water vapor content significantly impact the atmospheric radiation budget, causing radiative forcing and altering stratospheric ozone chemistry. Even small increases in stratospheric water vapor, as projected by climate models due to global warming, can amplify surface warming through positive climate feedbacks. Previous studies suggested that major historical eruptions like Tambora (1815) and Krakatoa (1883) may have caused stratospheric hydration, with water vapor comprising a significant portion of the erupted material. However, observational evidence for substantial volcano-driven stratospheric hydration was limited until the Hunga eruption. The Hunga eruption provided a unique natural experiment to investigate the stratospheric impacts of massive water vapor injection, with the potential for long-lasting effects on stratospheric ozone, radiative balance, and dynamics. This study investigates the formation and evolution of the stratospheric moisture and sulfate aerosol plume using a combination of satellite and ground-based observations and transport modeling, providing the first comprehensive assessment of the annual-scale stratospheric aftermath.

Early studies on volcanic columns hypothesized that major eruptions could lead to stratospheric hydration due to the significant water vapor content in erupted gases. However, quantitative estimates for previous eruptions, like Pinatubo (1991), primarily relied on modeling, lacking direct observational evidence. The Hunga eruption provided the first opportunity for extensive observational analysis of substantial volcano-driven stratospheric hydration, particularly due to the volcano's submarine location. Existing literature highlighted the potential climatic effects of such hydration, including impacts on stratospheric ozone, radiative balance, and atmospheric dynamics. The study's novelty lies in quantifying these effects using a comprehensive suite of observations and modeling.

This study employed a multi-faceted approach combining various satellite and ground-based observations with transport modeling to quantify the stratospheric impacts of the Hunga eruption. Specific methods included: 1. **Stereoscopic Cloud Top Height (CTH) Retrieval:** High-resolution infrared imagery from GOES-17 and Himawari-8 geostationary satellites was used with a stereoscopic algorithm to determine the eruption plume's height and temporal evolution, achieving sub-pixel accuracy. 2. **COSMIC-2 Water Vapor Retrieval:** Global Navigation Satellite System (GNSS) radio occultation soundings from COSMIC-2 were used to retrieve water vapor profiles within the plume, accounting for uncertainties in temperature and pressure profiles from ECMWF data. 3. **Aura MLS Data Analysis:** Microwave Limb Sounder (MLS) data from the Aura satellite provided vertical profiles of water vapor and geopotential height, allowing assessment of the plume's spatial and temporal evolution. Both versions 4 and 5 of the MLS data were used, with considerations for data quality issues in the presence of high humidity. 4. **OMPS-LP Aerosol Observations:** Ozone Monitoring and Profiling Suite Limb Profiler (OMPS-LP) data provided observations of aerosol extinction and optical depth, along with tomographic retrievals for estimating aerosol sedimentation rates. 5. **CALIPSO CALIOP Data:** Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) data provided measurements of aerosol backscatter and depolarization ratios, characterizing the aerosol properties and plume structure. 6. **SCISAT ACE-FTS Analysis:** Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) data allowed for the determination of changes in the stratospheric water isotopic composition. 7. **ISS SAGE III Data:** Stratospheric Aerosol and Gas Experiment (SAGE) III data from the International Space Station provided extinction profiles for aerosol size retrievals. 8. **ALADIN/Aeolus Data:** Doppler wind lidar data from Aeolus provided additional information on the aerosol distribution and sedimentation. 9. **Meteorological Radiosoundings:** Data from various radiosounding stations in the Southern tropics were utilized to independently validate the satellite-derived observations. 10. **Ground-Based Lidars:** Multiple ground-based lidar stations provided data on aerosol layer dispersion in both hemispheres. 11. **GloSSAC Merged Satellite Aerosol Climatology:** GloSSAC data were combined with OMPS-LP and SAGE III data to establish the Hunga eruption's stratospheric aerosol load within a historical context. 12. **CLaMS Chemistry-Transport Model Simulation:** The Chemical Lagrangian Model of the Stratosphere (CLaMS) was used to simulate the transport and evolution of the water vapor plume, using ECMWF data for driving the model.

The Hunga eruption resulted in unprecedented changes to the stratosphere: 1. **Unprecedented Stratospheric Hydration:** The eruption caused a 13% increase in global stratospheric water mass, nearly double the annual cycle's amplitude, and reaching ~24% in the Southern Hemisphere and ~11% in the Northern Hemisphere. Estimates of the injected water mass ranged from 70-150 Tg, consistent with various independent methods. This perturbation is unprecedented in the satellite record dating back to 1985. 2. **Massive Aerosol Increase:** The eruption led to a 4-5-fold increase in stratospheric aerosol optical depth (SAOD), significantly exceeding previous volcanic or wildfire events in the last three decades. The peak SAOD was at least a factor of 6 smaller than the 1991 Pinatubo eruption, and a factor of 3 smaller than the 1982 El Chichon eruption. 3. **Rapid Global Dispersal:** The high altitude of injection resulted in exceptionally rapid circumglobal transport of the plume. The uppermost plume circumnavigated the Earth in a week, while the middle and lower plumes completed circumnavigation in 9 days and 2 weeks, respectively. This rapid transport contrasts with previous eruptions like El Chichon (1982) and Pinatubo (1991). 4. **Expedited Aerosol Formation:** The abundance of water vapor in the Hunga plume likely expedited the conversion of sulfur dioxide (SO2) to sulfate aerosols, as indicated by the early and intense aerosol plume detected at La Réunion. This is further highlighted by comparison with the El Chichon eruption where maximum aerosol scattering was observed much later. 5. **Complex Plume Evolution:** The plume's evolution involved complex interactions of wind shear, sedimentation, and sublimation, resulting in a multitude of moist and aerosol-rich layers throughout the stratosphere. The CLaMS model simulations accurately captured the plume's evolution, including its cross-Pacific extent and circumglobal transport. 6. **Meridional Dispersion:** The plume dispersed nearly pole-to-pole in about three months, with transport pathways consistent with the stratospheric Brewer-Dobson circulation. The transport was faster towards the South Pole in the lower stratosphere than towards the North Pole in the upper stratosphere. Ground-based lidar data support the fast and slower branches of meridional transport. 7. **Vertical Decoupling of Water and Aerosols:** The bulk of gaseous water rose in the tropical upwelling, while the bulk of aerosols settled, with a sedimentation rate estimated around 0.26 mm/s. Both the water and aerosol layers exhibited similar meridional dispersion patterns. 8. **Isotopic Evidence for Seawater Source:** The significant increase in the HDO/H2O ratio, nearing Standard Mean Ocean Water (SMOW) levels, indicated that seawater was the primary source of the injected moisture. This contrasts with previous stratospheric hydration events like the 2019/20 Australian wildfires that did not show a similar isotopic shift. 9. **Large Aerosol Particle Size:** The effective radius of sulfate particles increased to 400-500 nm, larger than observed at any point in the SAGE III/ISS record. Analysis suggested complex interplay of sedimentation, condensation, and coagulation processes. The settling rate was consistent with estimates from CALIOP and OMPS-LP data.

The findings demonstrate the unprecedented magnitude and rapidity of the stratospheric perturbation caused by the Hunga eruption, surpassing previous events in both water vapor and aerosol loading. The rapid global dispersal highlights the potential for swift climatic effects from high-altitude eruptions. The expedited aerosol formation and large particle sizes indicate a complex interplay between water vapor and aerosol microphysics. The isotopic analysis confirms seawater as the primary source of injected water. The substantial changes in stratospheric composition will likely have lasting consequences for atmospheric dynamics, radiative balance, and ozone chemistry, particularly in the Southern Hemisphere where the majority of the volcanic material remained. Further research is needed to accurately model the long-term effects and refine the understanding of the interactions between water vapor and aerosol particles in the stratosphere. The results emphasize the need to continue monitoring stratospheric composition changes to better anticipate and assess the potential impacts of future volcanic eruptions.

The Hunga eruption represents a unique and significant event in modern climatology, marking a substantial change in stratospheric composition. The study's findings highlight the unprecedented increase in both stratospheric water vapor and aerosol burden, their rapid global dispersion, and the long-term implications for climate. Further research is needed to fully assess the long-term impacts of this event on atmospheric chemistry, dynamics, and radiative balance. The combined use of advanced satellite observations and modeling techniques offers a powerful means to enhance understanding of stratospheric processes and improve climate prediction capabilities.

The study's analysis is limited to the first nine months following the eruption. While the data indicate long-term effects, the full extent of the impacts on climate and atmospheric processes remains to be seen over longer timescales. Uncertainties associated with data retrieval and model simulations, particularly those related to the complex interaction between water vapor and aerosol microphysics, may affect the precision of certain estimates. Additional research incorporating longer-term observations and more sophisticated models is necessary for a more complete understanding of the Hunga eruption's consequences.