SO2 or H2S needs to be compressed to a liquid; calculations suggest that H2S would rapidly vaporize but SO2 might fall significantly if liquid droplet size was too large. Given that the aircraft wake will result in significant mixing, it is likely (but not confirmed) that allowing droplets to fall out of the wake before evaporating would be unacceptable, requiring release as a gas.
Metric
Time to design and verify the technology to quickly disperse the gaseous precursor (stored on board as a liquid), and ensure it is entrained into aircraft wake, is more than 5 years from first funding.
Uncertainty
No design effort to date. This is very unlikely to be a limiting step; the physics is sufficiently well understood and there are sufficiently similar engineering systems.
Decision relevance
If this turns out to be much harder than assumed, it could make SAI infeasible with these injection materials. However, we assume that this is highly unlikely and that any unexpected difficulties would only delay the development time, so we list consequence level as medium.