Optimizing Injection Locations Relaxes Altitude‐Lifetime Trade‐Off for Stratospheric Aerosol Injection

• Published in: Geophysical research letters Vol. 50; no. 16
• Main Authors: Sun, Hongwei, Bourguet, Stephen, Eastham, Sebastian, Keith, David
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 28.08.2023; Wiley
• Subjects: Aerosols; Aircraft; Altitude; altitude‐lifetime trade‐off; Cerebral hemispheres; Climate change; Climate models; Cooling effects; Deployment; Environmental hazards; Environmental impact; Gases; Global aerosols; Global climate; Global climate models; Greenhouse effect; Greenhouse gases; Injection; injection locations; Latitude; Locations (working); Longitude; Mass flux; Microphysics; Optimization; Ozone; particle lifetime; Particle mass; Particle settling; Radiative forcing; Solar radiation; Stratosphere; stratospheric aerosol injection; Stratospheric winds; Sunlight; Winds; zonal asymmetry
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2023GL105371

Stratospheric Aerosol Injection (SAI) aims to offset some climate hazards by injecting aerosols into the stratosphere to reflect solar radiation. The lifetime of injected particles influences SAI's radiative efficacy—the ratio of radiative forcing to particle mass flux. We employ a Lagrangian trajectory model with particle sedimentation to simulate how background circulations influence the transport of passive particles (without microphysical growth) in the stratosphere and quantify sensitivities of particle lifetime to injection locations. At 20 km, optimizing injection locations can increase particle lifetime by >40%. Injection strategies can be constrained to maintain an interhemispheric balance of particle lifetime without significantly decreasing total lifetime. Generally, increasing injection altitude increases particle lifetime while also increasing costs and environmental impacts of deployment aircraft. Optimizing injection latitude and longitude can relax this altitude‐lifetime trade‐off by increasing lifetime without needing to increase altitude, which warrants further testing in global climate models with aerosol microphysics. Plain Language Summary Stratospheric Aerosol Injection (SAI) aims to reduce climate change by increasing the amount of aerosols in the stratosphere. These additional aerosols can reflect additional sunlight to partially offset the energy imbalance caused by greenhouse gases. The lifetime of injected particles in the stratosphere is one of the important factors that can influence the cooling effects of SAI, as particles that stay longer in the stratosphere can reflect more sunlight over their lifetime. We use observed stratospheric winds to simulate the transport of injected particles and then calculate the particle's lifetime in the stratosphere, aiming to understand how lifetime is related to the location and season at which the particles are injected. For particles injected at 20 km altitude, we can increase particle lifetime by >40% by optimally choosing injection locations. Increasing injection altitude can increase particle lifetime while also increasing the costs and environmental impacts of deployment aircraft. Our results suggest that optimizing injection latitude and longitude to increase lifetime can relax the trade‐off between altitude and lifetime by increasing particle lifetime without needing to increase injection altitude. Key Points Injection longitudes influence particle lifetime because of zonal asymmetry of poleward winds, especially in the lower stratosphere Optimizing injection latitude and longitude can increase stratospheric lifetime of injected particles without increasing injection altitude Injection strategies can be developed to maintain an interhemispheric balance of particle lifetime without compromising total lifetime