Observational requirements for the stratosphere

Responsible SAI deployment requires a feedback loop: monitor what is happening in the stratosphere, compare observations to predictions, and adjust injection strategy if they diverge. However, it remains unclear what observational capabilities are necessary to enable these adjustments. Researchers do not yet know what precision, spatial resolution, and measurement frequency are required to confidently detect deviations from expected stratospheric aerosol behavior and make informed decisions about deployment adjustments. If observational systems are inadequate, decision makers will lack the information needed to respond to unexpected outcomes.

Determining observational requirements in advance of deployment is therefore critical — it ensures that monitoring systems can be built and validated before deployment begins, and that an adequate baseline record can be collected to provide context for interpreting future observations. This activity uses modeling to answer the fundamental question: given realistic measurement constraints, what observations would be sufficient to effectively manage SAI deployment?

This activity develops and tests a control algorithm — a decision-making framework that uses observations of stratospheric aerosol optical depth and particle size distribution to adjust SAI injection strategy (location, magnitude, or timing) in response to observed deviations from predictions. The work includes:

1. Developing the control algorithm using one Earth system model as "truth" and testing whether perfect observational knowledge of aerosol properties would enable researchers to adjust an independent model to match the truth model
2. Implementing realistic observational characteristics (instrumental precision, spatial resolution, measurement frequency) based on existing or proposed satellite and in situ observations to create synthetic degraded observational data, and testing how measurement limitations affect the ability to adjust strategy effectively
3. Identifying the minimum observational requirements needed to follow this algorithm during deployment.

The activity will quantify observational needs separately for different parameters (aerosol optical depth, particle size distribution, and potentially ozone or temperature if relevant to strategy adjustment). Key deliverables include papers describing the control algorithm framework, demonstrating how observational degradation affects adjustment capability, and inventorying existing satellite and in situ instruments suitable for deployment monitoring — including their technology readiness levels, anticipated launch dates, and estimated costs. The findings will directly inform decisions about which observational systems must be developed or procured before an aerosol evolution and transport experiment or deployment begins.