In Situ Measurements of Perturbations to Stratospheric Aerosol and Modeled Ozone and Radiative Impacts Following the 2021 La Soufrière Eruption

Stratospheric aerosols play important roles in Earth’s radiative budget and heterogeneous chemistry. Volcanic eruptions modulate the stratospheric aerosol layer by injecting particles and particle precursors like sulfur dioxide (SO₂) into the stratosphere. La Soufrière (13˚ N, 61˚ W) eruption in April 2021 injected SO₂ into the tropical upper troposphere and lower stratosphere, yielding a peak loading of 0.3-0.4 Tg. The resulting volcanic aerosol plumes dispersed predominately over the northern hemisphere (NH), as indicated by the CALIOP/CALIPSO satellite observations and model simulations. In summer 2021 and 2022, the NASA ER-2 high-altitude aircraft extensively sampled the stratospheric aerosol layer over the continental United States during the Dynamics and Chemistry of the Summer Stratosphere (DCOTSS) mission. These in situ aerosol measurements provide detailed insights into the number concentration, size distribution, and spatiotemporal variations of particles within volcanic plumes.

Notably, aerosol surface area and number densities in 2021 were 2-4 times higher between 380-500 K potential temperature compared to the 2022 DCOTSS observations, which were minimally influenced by volcanic activity. Within the volcanic plume, the observed aerosol number density exhibited significant meridional and zonal variations while the mode and shape of aerosol size distributions did not vary. Intriguingly, La Soufrière eruption resulted in a smaller aerosol effective diameter in the midlatitude lower stratosphere, due to the emergence of a notable quantity of small particles (<400 nm). The eruption was simulated with the SOCOL-AERv2 aerosol-chemistry-climate model. The modeled aerosol enhancement aligned well with DCOTSS observations in NH midlatitudes, although the model missed the tropical upper plume which exhibited restricted poleward transport. The tropical upper plume potentially influenced midlatitudes transiently, as indicated by DCOTSS balloon-borne measurements showcasing a transient aerosol layer around 21.5 km in late August 2021. These measurements also implied that particles within the upper plume were larger than those present in the lower plume, likely due to an extended process time within the tropical reservoir. Model simulations indicate that the La Soufrière eruption contributed at most 0.6% to Arctic and Antarctic ozone column depletion in both 2021 and 2022, which is well within the range of natural variability. The modeled global radiative forcing effect was modest, mainly affecting tropical and NH midlatitude areas.