Stratospheric Aerosol Injection and Growth in Aircraft Engine Plumes

Abstract

Stratospheric Aerosol Injection (SAI) is a geoengineering method to mitigate the effects of increased greenhouse gas concentrations in the Earth’s atmosphere, and to prevent further global warming. SAI does not reverse climate change, it merely counteracts its symptoms by offsetting the radiative forcing of greenhouse gases, and it should be accompanied by aggressive programmes of Carbon Dioxide Removal to cool down the climate. Operational studies suggest that the injection of condensable vapor from specialized high-altitude aircraft would be a reasonable option to form the desired aerosol particles. Yet, remarkably little is known about the growth evolution of aerosol inside a jet engine wake, especially in light of SAI where high initial concentrations of condensable vapor are injected into the exhaust stream. The lack of resolution in the flow field obscures a lot of the intricacies of the aerosol formation process, and raises serious doubts on the steerability of SAI, which is the capacity to spatially and temporally control its effects both in nature and scale. Without reassurance that the steerability requirement can be satisfied, the potential risks to the global ecosystem would be too high. In this respect, the study investigates the mechanisms within the near-field of a jet engine wake that lead to the creation of aerosols and subsequent growth, when accounting for local variations in temperature and relative humidity. This is done through a decoupled plume dispersion model that includes a flow solver which resolves the average velocity and turbulent intensity of the flow, a sectional chemistry module which includes all the relevant microphysical processes that affect aerosol growth, and a diffusion-advection model which calculates the displacement of aerosol, vapor, and chemiions in the flow field. In addition, several modeling errors are identified in classical thermodynamic approaches to high-density aerosol formation. Results demonstrate that the onset of particle formation in the plume is complex, and is heavily dependent on the injection concentration of sulfuric acid vapor and the mixing rate of the plume. Different aerosol particle sizes are formed depending on the location in the plume, leading to a nonuniform distribution of the volume-mean radius across the plume’s cross-sectional area. There is also a
strong indication that core particles experience preferential growth, which implies that the final particle distribution at the end of the plume’s lifetime might not be as uni-modal as self-limited theory predicts. The study concludes with a list of recommendations to solve the challenges that remain in order to truly
understand the early growth evolution of sulfate aerosol in a jet engine wake.