Every Earth system model contains a dedicated aerosol scheme, a smaller model that represents the life cycle of particles: how they form, how they grow as vapor condenses onto them and as they collide and merge with each other, how settling and transport remove them, and how they scatter and absorb radiation. Two examples are the Global Model of Aerosol Processes (GloMAP), which runs inside the UK Earth System Model, and the Modal Aerosol Module (MAM), which runs inside CESM2-WACCM.
Different research groups developed these aerosol schemes independently, each pursuing its own scientific questions and calibrating against its own body of observations such as volcanic eruptions, wildfire plumes, urban air quality, dust storms, and cloud formation. Each scheme therefore performs well in the regime its developers built it for rather than serving as a general-purpose representation of aerosol physics. The schemes consequently diverge, meaning that they produce different sizes of particles and concentrations with a given quantity of SO₂. This divergence is one of the principal sources of uncertainty in projections of SAI (Visioni et al., 2021) (Irvine et al., 2014).
Researchers have tested these schemes for SAI almost exclusively against volcanic eruptions, which are not a like-for-like analogue. Volcanic eruptions deliver very large quantities of SO₂ to the stratosphere in concentrated plumes that also carry ash and other co-emitted gases, and instruments have characterized that composition only approximately. SAI would instead involve smaller, more sustained releases from aircraft at locations, altitudes, and rates chosen deliberately. The eruption record therefore constrains a physical system that differs from SAI in emission rate, plume composition, and spatial scale alike.
No mechanism currently exists for feeding new observations back into microphysical code. If better measurements arrived tomorrow, no modelling group has a systematic procedure for translating them into a revised scheme. Calibration remains a one-off exercise, repeated separately by each group and rarely documented in a form others can reuse.
An aerosol microphysics experiment will measure how SO2 converts to aerosol particles under controlled, well-instrumented stratospheric conditions. Combined with the record from past volcanic eruptions, on-going stratospheric measurements, and from new observational campaigns, these measurements will constrain the microphysical schemes in the regime SAI would actually occupy. Building the pipeline that turns observations into model revisions is the essential first step, and will require Observing System Simulation Experiments (OSSEs), which test how a model's predictions respond to different scenarios and measurement designs.
