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Recent studies have suggested that injection of solid particles such as alumina and calcite particles for stratospheric aerosol injection (SAI) instead of sulfur-based injections could reduce some of the adverse side effects of SAI such as ozone depletion and stratospheric heating. Here, we present a version of the global aerosol–chemistry–climate model SOCOL-AERv2 and the Earth system model (ESM) SOCOLv4 which incorporate a solid-particle microphysics scheme for assessment of SAI of solid particles. Microphysical interactions of the solid particle with the stratospheric sulfur cycle were interactively coupled to the heterogeneous chemistry scheme and the radiative transfer code (RTC) for the first time within an ESM. Therefore, the model allows simulation of heterogeneous chemistry at the particle surface as well as feedbacks between microphysics, chemistry, radiation and climate. We show that sulfur-based SAI results in a doubling of the stratospheric aerosol burden compared to the same mass injection rate of calcite and alumina particles with a radius of 240 nm. Most of the sulfuric acid aerosol mass resulting from SO2 injection does not need to be lifted to the stratosphere but is formed after in situ oxidation and subsequent water uptake in the stratosphere. Therefore, to achieve the same radiative forcing, larger injection rates are needed for calcite and alumina particle injection than for sulfur-based SAI. The stratospheric sulfur cycle would be significantly perturbed, with a reduction in stratospheric sulfuric acid burden by 53 %, when injecting 5 Mtyr-1 (megatons per year) of alumina or calcite particles of 240 nm radius. We show that alumina particles will acquire a sulfuric acid coating equivalent to about 10 nm thickness if the sulfuric acid is equally distributed over the whole available particle surface area in the lower stratosphere. However, due to the steep contact angle of sulfuric acid on alumina particles, the sulfuric acid coating would likely not cover the entire alumina surface, which would result in available surface for heterogeneous reactions other than the ones on sulfuric acid. When applying realistic uptake coefficients of 1.0, 10-5 and 10-4 for H2SO4 , HCl and HNO3 , respectively, the same scenario with injections of calcite particles results in 94 % of the particle mass remaining in the form of CaCO3. This likely keeps the optical properties of the calcite particles intact but could significantly alter the heterogeneous reactions occurring on the particle surfaces. The major process uncertainties of solid-particle SAI are (1) the solid-particle microphysics in the injection plume and degree of agglomeration of solid particles on the sub-ESM grid scale, (2) the scattering properties of the resulting agglomerates, (3) heterogeneous chemistry on the particle surface, and (4) aerosol–cloud interactions. These uncertainties can only be addressed with extensive, coordinated experimental and modelling research efforts. The model presented in this work offers a useful tool for sensitivity studies and incorporating new experimental results on SAI of solid particles. [ABSTRACT FROM AUTHOR]

Solar radiation modification by a sustained deliberate source of SO2 into the stratosphere (strat-SRM) has been proposed as an option for climate intervention. Global interactive aerosol–chemistry–climate models are often used to investigate the potential cooling efficiencies and associated side effects of hypothesized strat-SRM scenarios. A recent model intercomparison study for composition–climate models with interactive stratospheric aerosol suggests that the modeled climate response to a particular assumed injection strategy depends on the type of aerosol microphysical scheme used (e.g., modal or sectional representation) alongside host model resolution and transport. Compared to short-duration volcanic SO2 emissions, the continuous SO2 injections in strat-SRM scenarios may pose a greater challenge to the numerical implementation of microphysical processes such as nucleation, condensation, and coagulation. This study explores how changing the time steps and sequencing of microphysical processes in the sectional aerosol–chemistry–climate model SOCOL-AERv2 (40 mass bins) affects model-predicted climate and ozone layer impacts considering strat-SRM by SO2 injections of 5 and 25 Tg(S) yr −1 at 20 km altitude between 30° S and 30° N. The model experiments consider the year 2040 to be the boundary conditions for ozone-depleting substances and greenhouse gases (GHGs). We focus on the length of the microphysical time step and the call sequence of nucleation and condensation, the two competing sink processes for gaseous H2SO4. Under stratospheric background conditions, we find no effect of the microphysical setup on the simulated aerosol properties. However, at the high sulfur loadings reached in the scenarios injecting 25 Tg(S) yr −1 of SO2 with a default microphysical time step of 6 min, changing the call sequence from the default "condensation first" to "nucleation first" leads to a massive increase in the number densities of particles in the nucleation mode (R<0.01 µm) and a small decrease in coarse-mode particles (R>1 µm). As expected, the influence of the call sequence becomes negligible when the microphysical time step is reduced to a few seconds, with the model solutions converging to a size distribution with a pronounced nucleation mode. While the main features and spatial patterns of climate forcing by SO 2 injections are not strongly affected by the microphysical configuration, the absolute numbers vary considerably. For the extreme injection with 25 Tg(S) yr −1 , the simulated net global radiative forcing ranges from - 2.3 to - 5.3 Wm-2 , depending on the microphysical configuration. Nucleation first shifts the size distribution towards radii better suited for solar scattering (0.3 µm

Stratospheric aerosols play important roles 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 (SO 2) into the stratosphere. Beginning on 9 April 2021, La Soufrière erupted, injecting SO 2 into the tropical upper troposphere and lower stratosphere, yielding a peak SO 2 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. From June to August 2021 and May to July 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 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, as observed in regular ER-2 profiles over Salina, Kansas. A similar reduction in aerosol effective diameter was not observed in ER-2 profiles over Palmdale, California, possibly due to the values that were already smaller in that region during the limited sampling period in 2022. Additionally, we modeled the eruption with the SOCOL-AERv2 aerosol–chemistry–climate model. The modeled aerosol enhancement aligned well with DCOTSS observations, although the direct comparison was complicated by issues related to the model's background aerosol burden. This study indicates that the La Soufrière eruption contributed approximately 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 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. [ABSTRACT FROM AUTHOR]

Solar radiation management as a sustained deliberate source of SO2 into the stratosphere (strat-SRM) has been proposed as an option for climate intervention. Global interactive aerosol-chemistry-climate models are often used to investigate the potential cooling efficiencies and side effects of hypothesised strat-SRM scenarios. A recent strat-SRM model intercompar-ison study for composition-climate models with interactive stratospheric aerosol suggests that the modelled climate response 5 to a particular assumed injection strategy, depends on the type of aerosol microphysical scheme used (e.g., modal or sectional representation), alongside also host model resolution and transport. Compared to short-duration volcanic SO2 emission, the continuous SO2 injections in strat-SRM scenarios may pose a greater challenge to the numerical implementation of of microphysical processes such as nucleation, condensation, and coagulation. This study explores how changing the timesteps and sequencing of microphysical processes in the sectional aerosol-chemistry-climate model SOCOL-AERv2 (40 size bins) 10 affect model predicted climate and ozone layer impacts considering strat-SRM SO2 injections of of 5 and 25 Tg(S) yr-1 at 20 km altitude between 30°S and 30°N. The model experiments consider year 2040 boundary conditions for ozone depleting substances and green house gases. We focus on the length of the microphysical timestep and the call sequence of nucleation and condensation, the two competing sink processes for gaseous H2SO4. Under stratospheric background conditions, we find no effect of the microphysical setup on the simulated aerosol properties. However, at the high sulfur loadings reached in the 15 scenarios injecting 25 Mt/yr of sulfur with a default microphysical timesetp of 6 min, changing the call sequence from the default "condensation first" to "nucleation first" leads to a massive increase in the number densities of particles in the nucle-ation mode (R < 0.01 µm) and a small decrease in coarse mode particles (R > 1 µm). As expected, the influence of the call sequence becomes negligible when the microphysical timestep is reduced to a few seconds, with the model solutions converging to a size distribution with a pronounced nucleation mode. While the main features and spatial patterns of climate forcing 20 by SO2 injections are not strongly affected by the microphysical configuration, the absolute numbers vary considerably. For the extreme injection with 25 Tg(S) yr-1, the simulated net global radiative forcing ranges from -2.3 W m-2 to -5.3 W m-2, depending on the microphysical configuration. "Nucleation first" shifts the size distribution towards radii better suited for solar scattering (0.3 µm < R < 0.4 µm), enhancing the intervention efficiency. The size-distribution shift however generates more ultra-fine aerosol particles, increasing the surface area density, resulting in 10 DU less ozone (about 3% of total column) in 25 the northern midlatitudes and 20 DU less ozone (6%) over the polar caps, compared to the "condensation first" approach. Our results suggest that a reasonably short microphysical time step of 2 minutes or less must be applied to accurately capture the magnitude of the H2SO2 supersaturation resulting from SO2 injection scenarios or volcanic eruptions. Taken together these results underscore how structural aspects of model representation of aerosol microphysical processes become important under conditions of elevated stratospheric sulfur in determining atmospheric chemistry and climate impacts. [ABSTRACT FROM AUTHOR]

Stratospheric aerosols play important roles 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. Beginning on April 9th, 2021, La Soufrière erupted injecting SO2 into the tropical upper troposphere and lower stratosphere, yielding a peak SO2 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. From June to August 2021 and May to July 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 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, as observed in regular ER-2 profiles over Salina, Kansas. A similar reduction in aerosol effective diameter was not observed in ER-2 profiles over Palmdale, California, possibly due to the already smaller values in that region during the limited sampling period in 2022. The La Soufrière eruption was modeled with the SOCOL-AERv2 aerosol-chemistry-climate model. The modeled aerosol enhancement aligned well with DCOTSS observations, although the direct comparison was complicated by issues related to the model's background aerosol burden. This study indicates 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 top-of-atmosphere one-year global average radiative forcing was -0.08 W/m2 clear-sky and -0.04 W/m2 all-sky. The radiative effects were concentrated in the tropics and NH midlatitudes and diminished to near-baseline levels after one year. [ABSTRACT FROM AUTHOR]

Stratospheric sulfate geoengineering (SSG) could contribute to avoiding some of the adverse impacts of climate change. We used the global 3D-aerosol-chemistry-climate model, SOCOL-AER, to investigate 21 different SSG scenarios, each with 1.83MtSyr−1 injected either in the form of accumulation-mode-H2SO4 droplets (AM-H2SO4), gas-phase SO2, or as combinations of both. For most scenarios, the sulfur was continuously emitted at 50hPa (≈20km) altitude in the tropics and subtropics, zonally and latitudinally symmetric about the equator (ranging from ±3.75° to ±30°). In the SO2 emission scenarios, continuous production of tiny nucleation mode particles results in increased coagulation, which together with condensation produces larger coarse mode particles. These larger particles are less effective for backscattering solar radiation and sedimentation out of the stratosphere is faster. On average, AM-H2SO4 injection increases stratospheric aerosol residence times by 32% and stratospheric aerosol burdens 37–41% when comparing to SO2 injection. The modelled all-sky (clear-sky) short-wave radiative forcing for AM-H2SO4 injection scenarios is up to 17–70% (44%–57%) larger than is the case for SO2. Aerosol burdens have a surprisingly week dependence on the latitudinal spread of emissions with emission in the stratospheric surf zone (>15°N–15°S) decreasing burdens by only about 10%. This is because the faster removal through stratosphere-to-troposphere transport via tropopause folds found when injection is spread farther from the equator is roughly balanced by a decrease in coagulation. Increasing injection altitude is also surprisingly ineffective because the increase in burden is compensated by an increase in large aerosols due to increased condensation. Increasing the local SO2 flux in the injection region by pulse or point emissions reduces the total global annual nucleation. Coagulation is also reduced due to the interruption of the continuous flow of freshly formed particles. The net effect of pulse or point emission of SO2 is to increase stratospheric aerosol residence time and radiative forcing. Pulse or point emissions of AM-H2SO4 has the opposite effect—decreasing stratospheric aerosol burden and radiative forcing by increasing coagulation. In summary, this study corroborates previous studies with uncoupled aerosol and radiation modules, suggesting that, compared to SO2 injection, the direct emission of AM-H2SO4 results in more radiative forcing for the same sulfur equivalent mass injection strength and that sensitivities to different injection strategies may vary for different forms of injected sulfur. [ABSTRACT FROM AUTHOR]