Develop SO₂ Hardware

For stratospheric aerosol injection to be effective, the released material must form sulfate aerosol particles small enough both to be efficient at scattering light, and to not quickly fall out of the stratosphere. Any material capable of producing reflective aerosols must also be stored and lofted to the stratosphere safely, within aircraft operational requirements.

Sulfur dioxide (SO₂) must be stored as a liquid — even pressurized, as a gas it would take up far too much volume or require too heavy a pressure vessel. Storing as a liquid means either holding it at or above its vapor pressure in a pressure vessel, or keeping it chilled and liquid through thermal controls. Pressure vessels add weight and therefore cost, while thermal controls add complexity and fuel requirements. Depending on the aircraft platform chosen, a unique tank location would also increase the structural design effort.

For release, nozzle designs must ensure proper atomization — breaking the liquid into fine droplets — at the desired mass flow rate. Initial calculations show that 500 µm (half a millimeter) is the theoretical upper size limit for liquid SO₂ droplets to disperse and evaporate quickly enough  (Franke et al., 2026), but even that may be too large. For the aerosol microphysics experiment, high dispersal rates are particularly valuable so that observing instruments can detect the SO₂. For deployment, dispersal must be fast enough that the aircraft payload can be released in a reasonable timeframe to keep costs down.

Designing a tank and nozzle system that stores and atomizes SO₂ to the required droplet size, at the required mass flow rate, while meeting the stringent safety and regulatory requirements of aircraft operation, is a novel engineering challenge.

This activity encompasses engineering and laboratory testing to design an SO₂ storage tank, plumbing, and dispersal nozzle system meeting the following critical specifications: storing multi-tonne quantities of SO₂ as a liquid, atomizing liquid SO₂ into droplets small enough to evaporate before falling (much smaller than 500 µm), releasing liquid SO₂ at rates on the order of multiple tonnes per minute, and doing so safely within the constraints of aircraft operation. Laboratory testing will validate the most promising approaches by characterizing droplet size distributions, confirming mass flow rates, and resolving thermal and safety concerns. The output will be a validated system design — a prerequisite for the prototype development and aircraft integration work that follows.