Exploring accumulation-mode-H<sub>2</sub>SO<sub>4</sub> versus SO<sub>2</sub> stratospheric sulfate geoengineering in a sectional aerosol-chemistry-climate model

Abstract

<p><strong>Abstract.</strong> 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 1.83 Mt S yr<sup>−1</sup> injected either in the form of accumulation-mode-H<sub>2</sub>SO<sub>4</sub> droplets (AM-H<sub>2</sub>SO<sub>4</sub>), gas-phase SO<sub>2</sub>, or as combinations of both. For most scenarios, the sulfur was continuously emitted at 50 hPa (≈ 20 km) altitude in the tropics and subtropics, zonally and latitudinally symmetric about the equator (ranging from ±3.75° to ±30°). In the SO<sub>2</sub> 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-H<sub>2</sub>SO<sub>4</sub> injection increases stratospheric aerosol residence times by 32 % and stratospheric aerosol burdens 37–41 % when comparing to SO<sub>2</sub> injection. The modelled all-sky (clear-sky) short-wave radiative forcing for AM-H<sub>2</sub>SO<sub>4</sub> injection scenarios is up to 17–70 % (44 %–57 %) larger than is the case for SO<sub>2</sub>. 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 SO<sub>2</sub> 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 SO<sub>2</sub> is to increase stratospheric aerosol residence time and radiative forcing. Pulse or point emissions of AM-H<sub>2</sub>SO<sub>4</sub> 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 SO<sub>2</sub> injection, the direct emission of AM-H<sub>2</sub>SO<sub>4</sub> 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.</p>

Is Turning Down the Sun a Good Proxy for Stratospheric Sulfate Geoengineering?

“…In the case of some surface variables, this happens because of dynamical changes in the circulation (Figure ). Previous works have shown that stratospheric chemistry would also be impacted by the sulfate aerosols (Tilmes, Richter, Mills, et al., 2018; Vattioni et al., 2019; Visioni, Pitari, Aquila, Tilmes, et al., 2017) but in most cases, these changes (such as in the concentration of N 2 O and CH 4 ) are also due to modifications of stratospheric dynamics. The effects of SS on stratospheric ozone may however vary due to different causes other than dynamical changes (Pitari et al., 2014; Tilmes et al., 2008; Tilmes, Richter, Mills, et al., 2018), for instance by the direct increase in Surface Area Density (SAD) resulting in changes in heterogeneous chemistry (Richter et al., 2017), both in the tropics and at higher altitudes.…”

Reduced Poleward Transport Due to Stratospheric Heating Under Stratospheric Aerosols Geoengineering

“…Larger sulfate aerosols are less effective at backscattering insolation and have reduced particle lifetimes (Pierce et al, 2010), reducing the SO 4 burden and resulting global aerosol optical depth (AOD) that results from a given SO 2 injection rate. Some studies (Benduhn et al, 2016; Pierce et al, 2010) have proposed that the injection of H 2 SO 4 droplets instead of SO 2 might produce an aerosol size distribution with smaller particles, capable of reflecting more solar radiation per unit mass (Dykema et al, 2016), thus potentially reducing some of the side effects related to the heating produced by the aerosols (Vattioni et al, 2019); other aerosol choices might result in less, or even no, stratospheric heating (Keith et al, 2016).…”

Application of Tropospheric Sulfate Aerosol Emissions to Mitigate Meteorological Phenomena with Extremely High Daily Temperatures

“…In addition, Pierce et al found that the H2SO4 injection doubled the sulfate loading compared to the SO2 injection used by Heckendorn et al [50]. Vattioni et al [53] corroborated previous study with uncoupled aerosol and radiation modules, suggesting that, compared to SO2 injection, the direct emission of Accumulation-mode-H2SO4 droplet results in more radiative forcing for the same sulfur equivalent mass injection strength. English et al [54] compared the efficiency of injecting three different kinds of sulfates: SO2, H2SO4 and SO4 2− .…”