Trade-Offs between Sulfuric Acid Aerosol Precursors for Stratospheric Aerosol Geoengineering

Solar geoengineering with sulfuric acid aerosols in the lower stratosphere might be used to reduce the impacts of climate change. Almost all studies have assumed sulfur is transported aloft as sulfur dioxide; several other sulfur compounds are plausible. We evaluated elemental sulfur (S₈), hydrogen sulfide (H₂S), sulfur dioxide (SO₂), carbon disulfide (CS₂), sulfur trioxide (SO₃), and sulfuric acid (H₂SO₄), examining the mechanisms of dispersal in liquid form, safety, and costs for materials supply and deployment with a 20 km altitude aircraft. Levelized, combined costs for material production and lofting at 100 ktonne/year scale range from $1140 to $5150/tonne-S. We find that the cost to loft H₂S is about half that of SO₂, but the production cost of H₂S is more uncertain and could be high enough to change this conclusion. The additional oxidation step required for H₂S adds <20% to the aerosol formation time scale, so H₂S and SO₂ should produce similar aerosol size distributions if new particle formation is negligible. H₂S and SO₂ have high and roughly equivalent toxicity. H₂S presents a further flammability risk that SO₂ does not. S₈ would be much safer than H₂S or SO₂, but it requires airborne combustor technology that is not yet developed. CS₂ is not preferred on any metric. SO₃ and H₂SO₄ are ∼4 times more expensive than H₂S to loft but offer additional control over the aerosol size distribution that will likely reduce some side effects of sulfuric acid aerosol geoengineering.