Stratospheric Aerosol Property retrievals using polarimetric observations above clouds

Aerosols injected into the stratosphere can persist for months to years and influence the radiative properties, stratospheric chemistry and large scale atmospheric dynamics. Despite their importance, accurate retrieval of their optical and microphysical properties using satellite observations, remains challenging. On one hand, while active remote sensing provides valuable information on stratospheric aerosol vertical distribution, retrievals of the full suite of particle properties are often limited by instrumental constraints and the need for strong a priori assumptions. On the other hand, passive remote sensing relying on intensity-only measurements, cannot differentiate between different aerosol types coincidentally present in the atmospheric column. Furthermore, passive aerosol retrieval algorithms are mainly designed for clear-sky conditions, limiting their applicability in the presence of clouds.

Multi-angle polarimetric observations offer extended capabilities for deriving the optical and microphysical properties of aerosols, also in presence of underlying liquid clouds. Previous studies [Wanquet et al., 2009; 2013, Hasekamp, 2010] have demonstrated the potential of retrieving aerosol properties above clouds, with best performance reported for fine-mode absorbing aerosols located above liquid clouds, while the retrievals for coarse mode non-spherical aerosols presents larger uncertainties.

Stratospheric particles represent a particular category of ‘aerosols above clouds’, since in the presence of tropospheric clouds they are always situated above them. In this context, we present a first attempt to extend the polarimetric above-cloud retrieval techniques for stratospheric aerosols originating from a moderate volcanic eruption. To this end, we exploit synergistic observations from EarthCARE and PACE satellite missions following the eruption of Ruang volcano in Indonesia in April 2024.

Approximately one month after Ruang eruption, sulfate-rich stratospheric aerosol layers were detected by the EarthCARE/ATLID high-spectral-resolution lidar at altitudes between 20 and 25 km. These observations provide information on layer vertical distribution and aerosol optical depth (AOD) at 0.355 um, while collocated PACE/HARP2 hyper-angular polarimetric measurements, in combination with optical modeling [Gasteiger and Wiegner, 2018] and radiative transfer simulations [Mayer and Kylling, 2005] are used to infer particle size and AOD at 0.67 um. It is worth mentioning that our methodology, while building on previous studies, needs to adapt them to non-simultaneous satellite overpasses.

First results demonstrate the feasibility of retrieving stratospheric AOD and particle size (i.e. effective radius) using the collocated EarthCARE and PACE observations, highlighting the potential of their synergy towards advanced stratospheric aerosol characterization.

This study contributes to the ACtIon4Cooling (Aerosol Cloud Interactions for Cooling) project, which investigates key mechanisms that have the potential to efficiently modify the Earth’s solar radiation budget using natural analogues, including volcanic eruptions as a proxy for stratospheric aerosol injection.