Fig. 4.
Key scientific and technical considerations and challenges for stratospheric aerosol injection (SAI). A wide range of scientific and technical factors would need to be considered in choosing which particle composition or combination of particle types to employ in possible implementation of SAI: a A high degree of control would be desired over the resulting aerosol particle size distribution, which influences the aerosol layer optical properties (for both solar and terrestrial radiation), the residence time, and the dispersion and transport of the aerosol layer. Such control would be more straightforward with manufactured particles such as TiO2, ZrO2, and Al2O3, than for H2SO4 or gaseous precursor injections (SO2, etc.). b Particle types that have limited effects on the stratospheric ozone layer would be preferable, which is a particular disadvantage of sulfate particles151,152. c Limited heating of the lower stratosphere would be preferable. Heating would depend on particle composition86, with some particle types, especially soot88 and small Al2O3 particles92, possibly heating the polar stratosphere by 10 °C or more, with significant but poorly understood impacts on stratospheric water vapour and dynamics90,92,93,153,154, including the possibility of increased stratospheric particle lifetime due to lofting95. d Particles with a high radiative forcing efficiency per unit mass would be preferable, as this would reduce the particle or precursor mass that needs to be transported to the stratosphere. e SAI would affect the ratio of direct and diffuse solar radiation, which would in turn impact photosynthesis, and thus crop yields155 and global net primary productivity156. Little is known yet about how this varies with particle type and size, or about other possible effects on ecosystems, as well as on solar energy production. f Human safety and environmental impact issues are of concern for several particle compositions, e.g., H2SO4 is a powerful acid, while aluminium and several other proposed particle components are well-known environmental contaminants, though their effective toxicity depends on their specific chemical forms; this is generally less of a concern for most proposed gaseous precursors like SO2