Potential detection and quantification of aluminum oxide aerosols from space debris via infrared limb-emission sounding

The planned deployment of satellite mega-constellations will substantially increase the flux of anthropogenic space debris re-entering Earth’s atmosphere. A large fraction of this material is composed of aluminum, which will ablate during re-entry and form aluminum oxide (Al₂O₃) containing aerosols in the mesosphere and lower thermosphere. These particles represent a new, human-made metal aerosol source that may interact with natural meteoric smoke and potentially impact upper-and middle- atmospheric chemistry, radiative balance, polar mesospheric cloud, polar stratospheric cloud as well as stratospheric aerosol formation. However, observational constraints on the abundance and vertical distribution of such aluminum-bearing aerosols are currently very limited.

Aluminum oxide exhibits characteristic spectral features in the mid-infrared, allowing detection via remote sensing spectroscopic measurements. In contrast to techniques based on scattering in the visible wavelength range, mid-infrared spectroscopic detection is independent of particle size as long as the particle radius remains small compared to the wavelength. This makes it particularly suited to constraining nanometer- to sub-micrometer-sized aluminum oxide aerosols expected from debris ablation. Moreover, spectrally resolved infrared limb measurements enable the quantification of total aerosol volume (and thus mass) profiles, providing a direct link between observed aerosol burdens and modeled debris input fluxes.

In this work, we quantitatively assess the capability of a space-borne infrared limb-imaging instrument to detect and characterize aluminum oxide aerosols from re-entering space debris. We perform end-to-end simulations of atmospheric radiances and instrument response in the mid-infrared, incorporating realistic Al₂O₃ optical properties and assumed vertical profiles derived from debris model scenarios associated with upcoming mega-constellations. Radiative transfer calculations are used to compute infrared limb-emission spectra and sensitivities, which are then passed through an instrument simulator system representative of the CAIRT (Changing-Atmosphere Infra-Red Tomography) limb-imaging mission concept, studied as an EE11 candidate for ESA’s Earth Explorer program.

We demonstrate that the characteristic mid-infrared absorption features of aluminum oxide remain detectable at realistic noise levels for CAIRT-like performance, over a range of plausible aerosol loads. Sensitivity analyses show that vertical profiles of total Al₂O₃ aerosol volume can be retrieved, even when particle sizes and shapes are not well constrained. Our results indicate that a CAIRT-type infrared limb-sounding mission could provide the first global, vertically resolved observational constraints on aluminum oxide aerosols from space debris.