Major volcanic eruptions can inject large quantities of sulfur dioxide (SO₂) into the stratosphere, alongside ash, water, halogens and other species. These natural phenomena are our closest proxy for SAI, but our ability to capitalize on the opportunities they present to acquire new data is limited. For example, our knowledge of the features of the aerosol burden and its evolution after the 1991 eruption of Mount Pinatubo is an important constraint used to validate models’ representation of stratospheric sulfate aerosols (e.g. Timmreck et al., 2018; Quaglia et al., 2023). However, our observations of the Pinatubo eruption were incomplete. If another similarly sized eruption occurred today, there is no guarantee that sufficient observing capabilities could be deployed in time to make the most of the rare opportunity – a roughly 3% chance per year (NASA, 2018) – for developing our understanding of SAI.
Responding to such a large eruption would be invaluable, but smaller eruptions that cause measurable stratospheric aerosol perturbations happen far more frequently. Even these eruptions that have no measurable surface cooling would help constrain the fundamental aerosol processes relevant to SAI. The 2022 eruption of Hunga Tonga–Hunga Ha'apai injected less than one million tonnes of SO₂, albeit alongside large amounts of water, and spurred a collaborative campaign of observations which are now usefully informing model assessments (Bednarz et al., 2026). The threshold for which a rapid response would be justified is not yet clear, and defining such a threshold requires clarifying the trade-off between the available learning and the resources required for additional campaigns. This is the first component of the required research under this activity.
Satellites can detect an eruption, track the plume, and measure column aerosol optical depth and the aerosol vertical profile globally (e.g., Taylor et al., 2023). However, they have limitations which mean complementing them with aircraft- and balloon-borne instruments (e.g., Li et al., 2023), as well as ground-based ones, is necessary. The particle layer from a large eruption can be too thick for satellite instruments to see through, and satellites cannot directly sample a plume's chemistry or measure the size and composition of aerosol particles. Observing these features requires rapid in-situ measurement using balloon- and/or aircraft-borne instruments. Various processes for which eruption observations would add critical data – including plume dilution, conversion of SO₂ into sulfuric acid, and aerosol growth by nucleation and condensation – occur within the first month after the eruption. Data not collected during this narrow window cannot be recovered later. Balloon launches into the plume soon after eruption are likely to be central to this effort, supplemented with some aircraft flights. Maximizing the utility of the existing fixed network of ground-based LIDARs, surface radiation and surface atmospheric composition and aerosol observations would also be important, but is less relevant for the rapid response since these cannot generally be moved.
At present, there is no standing capability to collect this data quickly. The instruments, trained personnel, and launch sites needed for a fast response exist in principle but are scattered and not necessarily maintained in a ready-to-deploy state. This research activity is intended to close that gap: to build the capability that gives scientists and policymakers reliable, detailed observations of the volcanic cloud's composition and behavior during the critical early phase and throughout its lifetime.
