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

If global warming proceeds at rates leading to unacceptable consequences [1], it may become necessary to enact temporary control measures to bridge the period required for greenhouse gas emission reductions to become effective. Solar radiation management (SRM) via stratospheric aerosol injection (SAI) has been identified as one of the most feasible approaches for this purpose [2, 3]. An aerosol known to be effective for SAI is H2SO4. It can be introduced by injecting SO2 and letting it evolve to H2SO4 over a period of weeks [4], or in a steered process in which H2SO4 is injected directly at prescribed dispersion rates. The latter potentially allows more precise control over particle size and effectiveness, which offers the potential benefit of providing more effective negative radiative forcing per unit mass of injected sulfur [5, 6, 7].

The development of a delivery system for SAI presents a significant technical challenge. Among the options proposed to date, systems employing specialised aircraft seem to be the most feasible [8, 9]. However, research with higher fidelity models than considered so far is required to address uncertainties concerning their feasibility and design details, as well as their economic and environmental impact. To reduce these uncertainties, this study examines in some detail the design and impact of a steered SAI system employing specialised aircraft. We outline a set of injection scenarios, propose a feasible aircraft design and quantify the economic and environmental impact of the resulting delivery system at a level of detail which exceeds that of previous studies.

Four injection scenarios were considered. Three of these focus on 15 Mt/yr steered H2SO4 injection over a range of possible dispersion rates. The dispersion rate is prescribed by a combination of initial aerosol concentration and engine plume diffusivity anticipated to elicit early particle formation processes, leading to favourable particle sizes [6]. The steered scenarios differ in these two parameters, ranging from a conservative scenario with low dispersion rate to an optimistic scenario with high dispersion rate. A 20 Mt/yr unsteered SO2 scenario is also considered for comparison. To accommodate the extreme requirements associated with the delivery of large payloads to stratospheric altitudes, a coupled optimisation procedure employing several state of the art aerospace conceptual design tools (such as [10] and [11]) was used for the design of the delivery system. Its economic and environmental impact analyses are separated into estimations of initial and operational components. The estimation of economic impact is based on Raymer's adjustment of Rand corporation's DAPCA model [8, 12], augmented by additional data. Environmental impact is expressed in terms of equivalent CO2 (CO2eq) and is estimated from existing data and the computed fuel consumption for each of the scenarios. The latter estimates include an equivalent weighting factor to also account for non-CO2 aircraft combustion products at altitude.

The coupled optimisation procedure resulted in an unmanned aircraft configuration featuring a large, slender, strut-braced wing and four custom turbofan engines. The custom engines were shown to provide substantial benefits over the considered off-the-shelf alternatives. The aircraft carries high-temperature H2SO4 which is evaporated during injection into a single outboard engine plume. Different optimised flight profiles were produced for each injection scenario, all involving an initial climb to an outgoing dispersion leg at 20 km altitude, followed by a return dispersion leg at a higher altitude. The change in H2SO4 dispersion rates between the steered scenarios is shown to have a strong impact on the scale of the operation, expressed 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 single largest contributor to economic impact, and fleet size and fuel consumption drive environmental impact, implying that maximising dispersion rate minimises both impacts.

Fleet sizes range between 2360 and 233 for the most conservative and the most optimistic steered H2SO4 injection scenarios. Flights per day range between 6040 and 1420. For the most conservative scenario, the impacts are relatively high, 412 and 153 B USD for initial and annual operating economic impact, and 13.3 and 366 Mt CO2eq for initial and annual operating environmental impact. For the most optimistic scenario, the impacts become quite low: 80.1 and 20.3 B USD with 10.3 and 25.2 CO2eq. The resources required for implementation as well as the impact of this latter steered H2SO4 scenario are comparable to the impact associated with unsteered SO2 injection, under the assumption that SO2 injection requires twice as much annual S delivery [5, 7]. An analysis of realistic value ranges for uncertain input parameters, including aircraft operational empty weight, engine specific fuel consumption, fuel prices and aerosol prices, indicates that the above conclusions are robust. The sensitivity of the design to delivery altitude, however, was found to be high, precluding practical operation at altitudes substantially above 20 km.

The results demonstrate that achieving high jet plume diffusivities to allow 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 a fleet of specialised aircraft for all three steered H2SO4 injection scenarios can be considered to be manageable.

(For bibliography, see image)