Source apportionment of processes contributing to volcanic PM10 aerosols during the 2021 eruption of Tajogaite.

The 2021-eruption of Tajogaite (La Palma, Canary Islands) was associated with the formation of large amounts of respirable PM10 aerosols (smaller than 10 μm) that triggered air quality crisis and lockdowns for ∼35,000 persons. This study aims to quantify the contribution of the aerosol formation mechanisms to the volcanic PM10 concentrations. During the eruption and post-eruption, we monitored trace gases (SO2, HF, HCl and NO2), and the size distribution and chemical composition of falling-tephra and PM10 aerosols. We also applied a stoichiometric apportionment methodology to investigate the atmospheric chemistry involved in the formation of secondary aerosols. During tephra-fallouts, giant particles (mm-to-cm size) settled rapidly, while gas-phase species condensed onto smaller particles. This resulted in the formation of (i) primary volcanic aerosols composed of micron-sized quenched lava fragments (volcanic-glass and rock-forming minerals) with low compositional variability in Si (18 wt%), Al (6.7 %), Fe (6.75 %), Ca (6 %), Na (4 %) and Mg (3 %), and (ii) secondary aerosols (gas-phase condensation/reaction) associated with sulphate, chloride and fluoride volcanic salts. These include Na2SO4 (33 % of total SO42-), NH4HSO4 (16 %), K2SO4 (11 %), MgSO4 (6 %), (NH4)2SO4 (2.2 %), and 30 % as excess SO4= relative to measured cations. Na2SO4 and MgSO4 were formed via quick interactions between SO2 and volcanic-glass in the eruption plume, while NH4HSO4 and excess sulphate persisted during post-eruption degassing. Cl- occurred predominantly as NaCl (98 %), and F- as CaF2 (94 %). Lava flows enhanced ammonia emissions from agricultural-soils; as result, 87 % of nitrate was present as NH4NO3, stabilized by the NH4+-NO3- chemical bond under the high humidity linked to volcanic water emissions. Primary and secondary aerosols accounted for 61 % and 26 % of PM10 as average during the eruption, respectively. These results highlight the importance of atmospheric chemistry after the fresh volcanic emissions, that may even result in the interactions between volcanic and non-volcanic species.