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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 PM2.5 concentrations) utilising a multidisciplinary approach, combining ground and satellite-based measurements with height-resolved aerosol and meteorological information. High concentrations of SO2, PM10 and PM2.5 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 PM2.5 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., The authors gratefully acknowledge the extraordinary effort carried out by the AEMET staff (both in La Palma and in support of the activities in La Palma) during the volcanic eruption, from the Izaña Atmospheric Research Center and the Delegation of AEMET in the Canary Islands. We thank the CSIC deployment service during the eruption and its coordination by Manuel Nogales. We also gratefully acknowledge the dedication and information provided by the PEVOLCA Scientific Committee and all the support received from the insular and local governments (Cabildo Insular de La Palma and the Ayuntamientos de Tazacorte, Los Llanos de Aridane and El Paso). The authors would also like to thank the Canary Islands Government for data from their Air Quality Monitoring Network and the MPL Network (MPLNET) for their support to the lidar measurements used in this paper. The authors also acknowledge the support from ACTRIS and ACTRIS-Spain, the Spanish Ministry of Science and Innovation and the support from the European Union H2020 program through the following projects (PID2019-104205GB-C21/AEI/10.13039/501100011033, EQC2018-004686-P, PID2019-103886RB-I00/AEI/10.13039/501100011033 and PID2020- 521-118793GA-I00) and programs (GA No. 654109, 778349, 871115, 101008004 and 101086690). Research activities of the CSIC staff during the eruption were funded by CSIC through the CSIC-PIE project with ID numbers PIE20223PAL009 and PIE20223PAL013 (Real Decreto 1078/2021, de 7 de diciembre). Part of this study was performed within the framework of the project AERO-EXTREME (PID2021-125669NBI00) funded by the Spanish State Research Agency (AEI) and ERDF funds.

The MODDAS-2D model (MOdel of Dispersion and Deposition of Ammonia over the Short-range in two dimensions) is presented. This stationary model couples a two-dimensional Lagrangian stochastic model for short-range dispersion, with a leaf-scale bi-directional exchange model for ammonia (NH3), which includes cuticular uptake and a stomatal compensation point. The coupling is obtained by splitting the upward and downward components of the flux, which can be generalized for any trace gas, and hence provides a way of simply incorporating bi-directional exchanges in existing deposition velocity models. The leaf boundary-layer resistance is parametrized to account for mixed convection in the canopy, and the model incorporates a stability correction for the Lagrangian time-scale for vertical velocity, which tends to increase the Lagrangian time-scale in very stable conditions compared with usual parametrizations. The model is validated against three datasets, where concentrations of atmospheric NH3 were measured at several distances from a line source. Two datasets are over grassland and one is over maize, giving a range of canopy structure. The model correctly simulates the concentration in one situation, but consistently overestimates it at further distances or underestimates it at small distances in the two other situations. It is argued that these discrepancies are mainly due to the lack of length of one of the line sources and non-aligned winds. Analysis shows that the surface exchange parameters and the turbulent mixing at the source level are the predominant factors controlling short-range deposition of NH3. Copyright © 2006 Royal Meteorological Society