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

On 19 September 2021, a volcanic eruption began on the island of La Palma (Canary Islands, Spain). The eruption has allowed the assessment of an unprecedented multidisciplinary study on the effects of the volcanic plume. This work presents the estimation of the spectral direct radiative forcing (Delta F) and efficiency (Delta F-Eff ) from solar radiation measurements at the Izana Observatory (IZO) located on the island of Tenerife (similar to 140 km from the volcano). During the eruption, the IZO was affected by different types of aerosols: volcanic, Saharan mineral dust, and a mixture of volcanic and 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 Delta F of the volcanic aerosols being greater than that of the dust events (associated with the larger aerosol load present), the Delta F-Eff was found to be lower. The spectral Delta F-Eff values at 440 nm ranged between -1.9 and - 2.6 Wm(-2)nm(-1)AOD(-1) for the mineral dust and mixed volcanic and dust particles, and between - 1.6 and -3.3 Wm(-2)nm(-1)AOD(-1) for the volcanic aerosols, considering solar zenith angles between 30 degrees and 70 degrees, respectively.