Modelling impacts of aluminium from ablated space debris on atmospheric chemistry

The increasing quantities of anthropogenic objects in Low Earth Orbit (LEO) have led to concerns over space debris and collision risks in LEO, leading to the Federal Communications Commission’s introduction of the “5-year rule”, requiring deorbit of LEO spacecraft within 5 years of mission end. To mitigate the risk of debris impacting the surface and causing damage, spacecraft are increasingly designed to ablate in the atmosphere, with most of the mass being vapourised during re-entry [1]. This causes an influx of metals into the mesosphere, where they condense and settle into the winter polar stratosphere - around 10% of Junge layer sulphuric acid droplets have been measured to contain metals from ablated space debris. Some metals – Al, Li, Cu, Ni, Mn etc. – already exceed natural background levels from cosmic dust that has ablated in the mesopause region [2]. The effect of these metals on the stratosphere is not yet known, and space debris input has been projected to increase by more than an order of magnitude in the next 15 years [3]. It is therefore vitally important to determine the level of re-entering space debris that will cause significant changes to atmospheric aerosols and stratospheric chemistry, in particular to the ozone layer. 

We model the catalytic impact of ablated aluminium which recondenses in the atmosphere has “space debris particles” (SDPs), predicted to be composed mainly of aluminium hydroxide nano-particles. These particles are predicted to catalyse chlorine activation from gas phase HCl, ClONO₂, and HOCl by catalytic reactions on the SDP surface. Their impact on polar stratospheric cloud freezing and subsequent chlorine activation is also considered.

We present results of a modelling study using a sectional aerosol model within an Earth system model (Whole Atmosphere Community Climate Model with the Community Aerosol and Radiation Model for Atmospheres, WACCM-CARMA).  We simulate the transport of SDPs and meteoric smoke particles (MSPs) produced by condensation of Fe and Mg silicates from ablated cosmic dust. The particles grow by coagulation and deposition of sulphuric acid through 28 size bins (0.34 nm to 1.6 µm radius, where MSPs injected at 0.34 nm radius, SDPs at 10 nm radius, and sulphuric acid is allowed to condense both heterogeneous on the MSPS and SDPs, and homogeneously. The SDPs and MSPs are initially injected in concentrations consistent with current models and observations (7.9 t d^‑1 MSPs and 0.96 t d^-1 SDPs) to assess the transport and lifetimes of the particles in the atmosphere.

The effect of increasing the mass of SDPs in line with future increases in space travel is also simulated. The maximum possible impact of SDPs on stratospheric chemistry is then estimated from the available SDP surface area and assuming upper limits for unmeasured physico-chemical parameters. Condensation of sulphuric acid onto the particles during their descent through the stratosphere reduces the surface area available for catalytic chlorine activation, but this is highly sensitive to the shape parameter assumed for the SDPs and the fraction of the surface that is coated by the condensed sulphate.

The reaction rates and physical properties of SDPs adopted in this model have not been measured or observed. Taking reasonable upper limits for values indicates that the reactions have the potential to do significant damage to the stratospheric ozone layer due to increased chlorine activation in winter and spring. The precise morphology and composition of particles must be characterised by in situ sampling of the stratosphere, and laboratory measurements of the rate constants are also required to better constrain estimates.

References

[1] Kelley 2012, https://ntrs.nasa.gov/citations/20120002794

[2] Murphy et al. 2023, https://doi.org/10.1073/pnas.2313374120.

[3] Schulz & Glassmeier 2021, https://doi.org/10.1016/j.asr.2020.10.036