SAI Research Roadmap

PHASEFoundational knowledgeWhat can existing evidence tell us, and where does it fall short?Time2–3 yrsCost~$30M–$75M

PHASEAerosol microphysicsDoes data from small outdoor experiments validate the processes in our models?Time4–8 yrsCost~$70M–$150MEst. uncertainty reduction (cooling efficacy)~25%

PHASEStratospheric distributionDo sulfate aerosols mix and distribute, globally, in the way our models predict?Time4–11 yrsCost~$270M–$1.14BEst. uncertainty reduction (cooling efficacy)~66%

PHASEMonitored coolingAs cooling begins, do observed effects match predictions? Are any unexpected effects emerging?TimeOngoingCostTBD

ImpactsClimate response, impacts, and risks

AerosolsMicrophysics, chemistry, transport

ObservationsMonitoring systems, instruments, and data

EngineeringAircraft R&D, materials handling, operations

Aerosols Observations

Southern Hemisphere in-situ monitoring

Expand Southern Hemisphere in situ monitoring to establish a pre-experiment baseline

Est. direct cost: $600K–$1.5M annually
Duration

Why it’s on the roadmap

To assess whether SAI has produced detectable changes, scientists must know what the natural baseline of stratospheric aerosols looks like. Without this baseline, it is impossible to distinguish between normal variability and signals from a field experiment or deployment. Currently, understanding of global stratospheric aerosol variability is incomplete: good data exist for some latitudes, but significant gaps remain, particularly in the tropics, where stratospheric dynamics are most active, and the Southern Hemisphere, where data coverage is lower.

Establishing a comprehensive baseline observational record is essential before a larger-scale aerosol microphysics experiment begins. The aerosol transport experiment will release SO₂ daily and track how it transforms into aerosols and how it spreads. But to determine whether observed changes are meaningful or just part of normal noise, researchers need to know the typical range of aerosol concentration and size at different latitudes and seasons.

Without this knowledge, even a well-designed experiment may produce ambiguous results: did the measured aerosols come from the SO₂ release, or are they part of the background variability? Detailed characterization of the stratospheric background is also essential for understanding whether changes from an eventual deployment would be detectable.

The Balloon Baseline Stratospheric Aerosol Profiles (B2SAP) program, a NOAA Earth's Radiation Budget Initiative project, provides direct, in situ measurements of stratospheric aerosol number and size using balloon-borne instruments (Asher et al., 2024). It characterizes both the average background state and its natural variability across the globe, directly informing what constitutes a detectable change. The expanded network — particularly new tropical and Southern Hemisphere sites — captures aerosol dynamics in the low-coverage regions most relevant for SAI deployment. Building this baseline record now ensures that results from an aerosol microphysics experiment can be interpreted with high confidence.

Scope of work

This research activity expands the B2SAP observational network to establish a robust baseline of stratospheric aerosol variability in regions critical both for understanding natural perturbations and for potential SAI research.

A minimal version of this research expands the B2SAP network through quarterly balloon launches at one tropical site and one Southern Hemisphere site over approximately three years. Each launch carries instruments to measure stratospheric aerosol number and size distribution at high vertical resolution (nominally 5 to 30 kilometers altitude), as well as coincident water vapor and ozone measurements.

Researchers will strengthen operations at the tropical site by training collaborators on instrument handling, launch procedures, data collection, and quality assurance protocols. Data from all launches will be processed, quality-controlled, and integrated into the B2SAP database alongside existing observations from the Boulder, Colorado baseline site and other network locations.

The key output is a systematic characterization of aerosol variability at these locations over the annual cycle, capturing both seasonal patterns and episodic changes. This dataset will establish detection thresholds — what constitutes a significant aerosol change above natural variability — which are essential for interpreting aerosol experiment results.

The extended program doubles the geographic coverage through quarterly launches at two tropical sites and two Southern Hemisphere sites (Lauder and La Réunion) indefinitely through all phases of research, providing denser spatial sampling in regions where air masses mix across latitudes, and better characterization of variability.

Additionally, the extended version includes targeted improvements to payload design and recovery methods. Enhanced instruments will extend the detectable size range of aerosol particles and improve measurement accuracy. Recovery capability improvements—such as refined parachute systems or coordination with local recovery services—will increase the fraction of instruments successfully recovered, enabling more frequent reuse of instruments and reducing overall program costs per profile.

The extended dataset will enable more detailed analysis of how aerosol properties vary with latitude and season, and how stratospheric dynamics transport aerosols across the global network. This richer spatial and temporal coverage provides higher confidence in baseline variability estimates and better constrains what would be detectable as an anomalous signal in aerosol microphysics and aerosol evolution and transport experiment experiments.

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