Impact of the 2021 La Palma volcanic eruption on air quality: Insights from a multidisciplinary approach

The La Palma 2021 volcanic eruption was the first subaerial eruption in a 50-year period in the Canary Islands (Spain), emitting ~1.8 Tg of sulphur dioxide (SO₂) into the troposphere over nearly 3 months (19 September-13 December 2021), exceeding the total anthropogenic SO₂ emitted from the 27 European Union countries in 2019. We conducted a comprehensive evaluation of the impact of the 2021 volcanic eruption on air quality (SO₂, PM₁₀ and PM_2.5 concentrations) utilising a multidisciplinary approach, combining ground and satellite-based measurements with height-resolved aerosol and meteorological information. High concentrations of SO₂, PM₁₀ and PM_2.5 were observed in La Palma (hourly mean SO₂ up to ~2600 μg m^-3 and also sporadically at ~140 km distance on the island of Tenerife (> 7700 μg m^-3) in the free troposphere. PM₁₀ and PM_2.5 daily mean concentrations in La Palma peaked at ~380 and 60 μg m^-3. Volcanic aerosols and desert dust both impacted the lower troposphere in a similar height range (~ 0-6 km) during the eruption, providing a unique opportunity to study the combined effect of both natural phenomena. The impact of the 2021 volcanic eruption on SO₂ and PM concentrations was strongly influenced by the magnitude of the volcanic emissions, the injection height, the vertical stratification of the atmosphere and its seasonal dynamics. Mean daily SO₂ concentrations increased during the eruption, from 38 μg m^-3 (Phase I) to 92 μg m^-3 (Phase II), showing an opposite temporal trend to mean daily SO₂ emissions, which decreased from 34 kt (Phase I) to 7 kt (Phase II). The results of this study are relevant for emergency preparedness in all international areas at risk of volcanic eruptions; a multidisciplinary approach is key to understand the processes by which volcanic eruptions affect air quality and to mitigate and minimise impacts on the population.