Tropospheric sulfate from Cumbre Vieja volcano at Las Palmas, transported towards Cabo Verde – lidar measurements of aerosol extinction, backscatter and depolarization at 355, 532 and 1064 nm

Abstract. From 19 September to 13 December 2021, volcanic eruptions took place at the Cumbre Vieja ridge, Las Palmas, Canary Islands. Thereby, fine ash and volatiles, like sulfur dioxide (SO₂), were emitted and transported over hundreds to thousands of kilometers away from the island. Continuous lidar observations with the multiwavelength-Raman-polarization lidar PollyXT were performed at the Ocean Science Center at Mindelo, Cabo Verde, in the framework of the Joint Aeolus-Tropical Atlantic Campaign (JATAC) 2021/2022 enabling the characterization of the atmospheric state above Mindelo during the eruption period. A special feature of the system operated at Mindelo is, that measurements of the particle extinction coefficient, the particle extinction-to-backscatter ratio (lidar ratio) and the particle linear depolarization ratio are available at all three wavelengths (355, 532 and 1064 nm). The typical aerosol conditions over Mindelo are a clean marine planetary boundary layer (PBL) up to approx. 1 km and above a Saharan dust layer (SAL, up to 6 km) during northern hemispheric summer and fall. A particle extinction coefficient smaller than 200 Mm⁻¹, a lidar ratio smaller than 30 sr and a particle linear depolarization ratio close to 0 % have been typically observed within the planetary boundary layer, while a lidar ratio between 40 and 60 sr and a linear depolarization ratio between 20 and 30 % are characteristic for the SAL above. In contrast, during the time of the volcanic eruptions, a strongly polluted PBL was observed on specific days beginning on the 23 September 2021, whereby the particle extinction coefficient and the lidar ratio increased up to 800 Mm⁻¹ and 80 sr (at 355 nm), respectively. On 24 September, the aerosol optical depth, determined by an AERONET (Aerosol Robotic Network) sun photometer, was as high as 0.9 and 1.1 (daily averages at 500 and 340 nm). HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory) trajectories indicate air mass transport from Canary Islands to Mindelo at heights below 2 km. The observed pollution in the PBL over Mindelo is attributed to sulfate aerosol from the volcanic eruption at Las Palmas as the particle linear depolarization ratio was low (≤ 3 %) and, thus, does not indicate non-spherical particles, such as Saharan dust or volcanic ash. We thus conclude that sulfate aerosol formed from gaseous precursors during the transport (2–3 days for a distance of 1500 km) from Las Palmas towards Cabo Verde. No indications of volcanic ash over Mindelo were found in the SAL. This finding is supported by the HYSPLIT trajectories, which show that air masses in higher altitudes originate from the African continent and not from the Canary Islands.