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 (∆F) and efficiency (∆F E f f) from solar radiation measurements at the Izaña Observatory (IZO) located on the island of Tenerife (∼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 (Sentinel5P 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 ∆F of the volcanic aerosols being greater than that of the dust events (associated with the larger aerosol load present), the ∆F E f f was found to be lower. The spectral ∆F E f f 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. The authors also acknowledge the support of ACTRIS, Ministerio de Ciencia e Innovación of Spain, through the projects SYNERA: PID2020-118793GA-I00 and RT2018- 097864-B-I00, and Junta de Castilla y León grant N◦. VA227P20.