Spectral Aerosol Radiative Forcing and Efficiency of the La Palma Volcanic Plume over the Izaña Observatory

By injecting aerosols and gases into the atmosphere, volcanoes significantly affect global climate, force changes in atmospheric dynamics, and influence many distinct cycles such as hydrological, carbon, and biogeochemical cycles. However, the irregular temporal and spatial distributions of volcanic processes and their effects are still poorly characterised. The volcanic eruption on La Palma (Canary Islands, Spain), which occurred in the autumn of 2021, presented an outstanding opportunity to improve the current understanding of these natural phenomena. The special conditions at the Izaña Observatory (IZO, Tenerife) and its proximity to La Palma (∼140 km) make it a strategic site for the comprehensive study of the almost unperturbed volcanic plume including the climate effects.

In this context, the present work deals with the experimental estimation of the solar spectral direct radiative forcing (ΔF) and efficiency (ΔF^Eff) during the volcanic eruption based on radiation measurements performed with an EKO MS-711 grating spectroradiometer during three events characterised by the presence of different types of aerosols: fresh volcanic aerosols, Saharan mineral dust, and a mixture of volcanic and Saharan dust aerosols. Three case studies were identified using ground-based (lidar) data, satellite-based (Sentinel-5P Tropospheric Monitoring Instrument, TROPOMI) data, reanalysis data (Modern-Era Retrospective Analysis for Research and Applications, version 2, MERRA-2), and backward trajectories (Flexible Trajectories, FLEXTRA), and subsequently characterised in terms of optical and micro-physical properties using ground-based sun-photometry measurements. Despite the ΔF of the volcanic aerosols being greater than that of the dust events (associated with the larger aerosol load present), the ΔF^Eff was found to be lower. The spectral ΔF^Eff values at 440 nm ranged between −1.9 and −2.6 Wm⁻²nm⁻¹AOD⁻¹ for the mineral dust and mixed volcanic and dust particles, and between −1.6 and −3.3 Wm⁻²nm⁻¹AOD⁻¹ for the volcanic aerosols, considering solar zenith angles between 30^∘ and 70^∘, respectively.