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 SO₄^2-), 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.