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 (S 8 ), hydrogen sulfide (H 2 S), sulfur dioxide (SO 2 ), carbon disulfide (CS 2 ), sulfur trioxide (SO 3 ), and sulfuric acid (H 2 SO 4 ), 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 H 2 S is about half that of SO 2, but the production cost of H 2 S is more uncertain and could be high enough to change this conclusion. The additional oxidation step required for H 2 S adds <20% to the aerosol formation time scale, so H 2 S and SO 2 should produce similar aerosol size distributions if new particle formation is negligible. H 2 S and SO 2 have high and roughly equivalent toxicity. H 2 S presents a further flammability risk that SO 2 does not. S 8 would be much safer than H 2 S or SO 2, but it requires airborne combustor technology that is not yet developed. CS 2 is not preferred on any metric. SO 3 and H 2 SO 4 are ∼4 times more expensive than H 2 S to loft but offer additional control over the aerosol size distribution that will likely reduce some side effects of sulfuric acid aerosol geoengineering.