Variations in stratospheric aerosol layer and aerosol microphysical processes following the 2021 La Soufrière eruption: insights from in situ and satellite observations

Stratospheric aerosol plays an important role in Earth's radiative budget and in heterogeneous chemistry. Volcanic eruptions modulate the stratospheric aerosol layer by injecting particles and particle precursors like sulfur dioxide (SO₂) into the stratosphere. The eruption of La Soufrière in April 2021 resulted in two distinct enhanced aerosol layers in the tropical lower stratosphere. These layers emerged approximately 3–4 weeks after the eruption, specifically at altitudes of 18 km (∼400 K) and 21 km (∼490 K), as observed through CALIOP/CALIPSO measurements. The lower plumes dispersed to higher latitudes in the Northern Hemisphere, while the upper plume exhibited restricted poleward transport. From June to August 2021 and May to July 2022, the NASA ER-2 high-altitude aircraft and balloon-borne instruments 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 density and number density in 2021 were enhanced by a factor of 2–4 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. The La Soufrière eruption led to an increase in the number concentration of small particles (<400 nm), resulting in a smaller aerosol effective diameter during the summer of 2021 compared to the baseline conditions in the summer of 2022. Balloon-borne measurements also implied that particles within the upper plume were larger than those present in the lower plume, likely due to an extended processing time within the tropical reservoir. The variance in volcanic aerosol microphysical processes between the tropical reservoir and the midlatitude lower stratosphere, along with their consequent impact on changes in aerosol size, will be further discussed. We modeled the eruption with the SOCOL-AERv2 aerosol–chemistry–climate model. The modeled aerosol enhancement aligned well with DCOTSS observations. The modeled top-of-atmosphere 1-year global average radiative forcing was −0.08 W m⁻² clear-sky and −0.04 W m⁻² all-sky. The radiative effects were concentrated in the tropics and NH midlatitudes and diminished to near-baseline levels after 1 year.