Steered Stratospheric Aerosol Injection: Aircraft and Operation Design, Economic and Environmental Impact

Abstract

If global warming proceeds at high rates, it may become necessary to enact control measures to bridge the period required for mitigation measures to become effective. Solar radiation management via stratospheric aerosol injection is one approach [1, 2]. This study examines the design and impact of a steered SAI system using specialised aircraft. Four injection scenarios were considered. Three focus on 15 Mt/yr steered H2SO4 injection over a range of possible dispersion rates, one on 20 Mt/yr unsteered SO2 injection. Controlling dispersion rate, prescribed by initial aerosol concentration and engine plume diffusivity, allows control over early particle formation leading to favourable particle sizes [3, 4]. A coupled optimisation procedure was used for the design of the delivery system. Its economic and environmental impact analyses were performed using existing models and additional data from literature. The design procedure resulted in an unmanned aircraft with a large, slender, strut-braced wing and four custom turbofan engines. The aircraft carries high-temperature H2SO4 and evaporates it during injection into one outboard engine plume. H2SO4 dispersion rate has a strong impact on the scale of the operation in terms of fleet size and flights per day. Maximisation of dispersion rate within the range for favourable aerosol particle formation enables achieving the annual delivery rate using shorter and fewer flights. Fleet size is the largest contributor to economic impact, and fleet size and fuel consumption drive environmental impact, hence maximising dispersion rate minimises both impacts. The resources required and the impact of the most optimistic steered scenario are comparable to those of unsteered SO2 injection, assuming that SO2 injection requires twice as much annual S delivery. The results show that achieving high jet plume diffusivities for maximisation of dispersion rate is of significant benefit for the successful implementation of steered SAI. Yet, all the scenarios analysed are technologically and logistically attainable and the anticipated economic and environmental impact of developing and operating specialised aircraft can be considered to be manageable for all steered H2SO4 injection scenarios.