Small-scale outdoor field experiments are necessary to advance understanding of SAI by improving how models represent the aerosol size distribution (the mix of particle sizes, which determines reflectivity and lifetime) from an injection of sulfur dioxide (SO₂). Several processes critical to SAI outcomes — SO₂ mixing in aircraft wakes, large-scale plume dispersion over weeks, nucleation of new aerosol particles (the formation of new particles), and coagulation (particles colliding and merging into larger ones) — carry significant uncertainties that cannot be resolved through laboratory work or volcanic observations alone (Linz et al., 2026).
Aircraft-released SO₂ differs fundamentally from volcanic emissions: it occurs in narrow plumes, where gravitational settling could matter significantly for the evolution of the aerosol size distribution. These conditions have no direct analogue in laboratory studies or volcanic observations. Existing climate models – calibrated against observations from volcanic eruptions like Mt. Pinatubo – already fail to reliably reproduce even those imperfect observations, meaning that without additional observational data, consequential deployment decisions would rest on demonstrably inadequate models.
The interdependencies between processes make an integrated field experiment essential. While individual processes have been studied in isolation, the coupling between mixing, oxidation, nucleation, condensation, and coagulation fundamentally shapes the resulting aerosol size distribution and efficacy. Modeling studies show that a 10,000-fold uncertainty in nucleation rates translates to only a factor-of-two difference in particles at climate-relevant sizes, due to competition with condensation and coagulation — yet even modest coagulation uncertainties produce effects as large as enormous nucleation uncertainties. These feedbacks cannot be fully captured without observing how SO₂ transforms in a plume from an airplane under real stratospheric conditions.
Crucially, the experiments required to close these gaps would be small in scale, with effects too small to detect even with the most precise instruments. A proposed release of just 10 tonnes of SO₂ — less than 2% of what global aviation emits daily — would perturb temperature by no more than a hundred-thousandth of a degree Celsius, acidity by less than one millionth of a pH unit, and ozone 30,000 times less than the effects observed after the La Soufrière eruption. A responsible aerosol microphysics experiment would sample the evolving plume to directly observe how SO₂ mixes, disperses, nucleates, and coagulates under deployment-relevant conditions. This empirical foundation would validate process-level models and improve the mathematical representations of particle behavior that sit at the foundation of all current SAI modeling.
