Evolution of optical parameters of volcanic and wildfire plumes in the stratosphere from CALIOP and ATLID observations

Volcanic eruptions and extreme wildfires produce stratospheric aerosol plumes with distinct optical properties and lifetimes. Here we analyze the evolution of volcanic and wildfire aerosols using Level-2 aerosol layer products from the CALIOP (CALIPSO) and ATLID (EarthCARE) spaceborne lidars.

The analysis is based on a layer approach, in which aerosol properties are binned and analyzed at the Level-2 aerosol layer scale rather than along individual vertical profiles, allowing a consistent comparison between events and throughout plume ageing. Aerosol layers are characterized using observations at 532 and 1064 nm (CALIOP) and 355 nm (ATLID) in terms of scattering ratio, depolarization ratios (volume depolarization, VDR, and particulate linear depolarization, PLDR), and color ratio. The scattering ratio constrains aerosol concentration, depolarization ratios provide insight into particle shape and type, whereas the color ratio scales with particle size, with coarse particles preferentially removed by gravitational sedimentation.

Distinct optical fingerprints are found for volcanic and wildfire aerosols. Volcanic eruptions such as Puyehue–Cordón Caulle (2011), Calbuco (2015) and Raikoke (2019) eruptions exhibit strong ash signatures at early stages, characterized by high PLDR and elevated color ratios, indicative of coarse, non-spherical particles. In contrast, Kasatochi (2008) and Sarychev (2009) eruptions shows intermediate PLDR values, consistent with a mixed aerosol composition combining volcanic ash and sulfate particles. Hunga eruption (2022) is dominated by sulfate aerosols and shows low depolarization but relatively high color ratios in the young plume, which rapidly decrease as the largest ash particles are efficiently removed by gravitational sedimentation.

ATLID Level-2 product is  used to document the temporal, vertical, and zonal evolution of stratospheric smoke after the Panboreal wildfire outbreak in May 2025. Very high value of scattering ratio and aerosol optical depth (AOD) are observed shortly after the largest pyroCb  injection on 29 May, followed by a progressive decrease associated with plume dilution and redistribution during vertical ascent and long-range transport. PLDR values remain moderate throughout the plume evolution, indicating the presence of non-spherical particle components in stratospheric smoke. A slight increase in PLDR with plume ageing is observed for most events, possibly related to particle aggregation or microphysical processing. These consistent PLDR patterns across different events provide insight into the ageing processes of stratospheric smoke.
The lidar ratio exhibits coherent values within individual layers throughout plume evolution, providing a stable constraint on aerosol optical properties despite decreasing aerosol loading.

First ATLID observations of the Canadian wildfires in May 2025 demonstrate the added value of the HSRL (High Spectral Resolution LiDAR) technique. ATLID exploits Rayleigh and Mie backscatter separation to provide direct measurements of aerosol extinction and lidar ratio. These observations offer new constraints on aerosol type and ageing of smoke aerosols in the stratosphere while extending the CALIOP-based statistics of stratospheric aerosol optical properties.