Exploring accumulation-mode H2SO4 versus SO2 stratospheric sulfate geoengineering in a sectional aerosol–chemistry–climate model

Abstract:

Stratospheric sulfate geoengineering (SSG) could contribute to avoiding some of the adverse impacts of climate change. We used the SOCOL-AER global aerosol–chemistry–climate model to investigate 21 different SSG scenarios, each with 1.83 Mt S yr⁻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 an altitude of 50 hPa (≈20 km) in the tropics and subtropics. We assumed emissions to be zonally and latitudinally symmetric around the Equator. The spread of emissions ranged from 3.75^∘ S–3.75^∘ N to 30^∘ S–30^∘ N. In the SO₂ emission scenarios, continuous production of tiny nucleation-mode particles results in increased coagulation, which together with gaseous H₂SO₄ condensation, produces coarse-mode particles. These large particles are less effective for backscattering solar radiation and have a shorter stratospheric residence time than AM H₂SO₄ particles. On average, the stratospheric aerosol burden and corresponding all-sky shortwave radiative forcing for the AM H₂SO₄ scenarios are about 37 % larger than for the SO₂ scenarios. The simulated stratospheric aerosol burdens show a weak dependence on the latitudinal spread of emissions. Emitting at 30^∘ N–30^∘ S instead of 10^∘ N–10^∘ S only decreases stratospheric burdens by about 10 %. This is because a decrease in coagulation and the resulting smaller particle size is roughly balanced by faster removal through stratosphere-to-troposphere transport via tropopause folds. Increasing the injection altitude is also ineffective, although it generates a larger stratospheric burden, because enhanced condensation and/or coagulation leads to larger particles, which are less effective scatterers. In the case of gaseous SO₂ emissions, limiting the sulfur injections spatially and temporally in the form of point and pulsed emissions reduces the total global annual nucleation, leading to less coagulation and thus smaller particles with increased stratospheric residence times. Pulse or point emissions of AM H₂SO₄ have the opposite effect: they decrease the stratospheric aerosol burden by increasing coagulation and only slightly decrease clear-sky radiative forcing. This study shows that direct emission of AM H₂SO₄ results in higher radiative forcing for the same sulfur equivalent mass injection strength than SO₂ emissions, and that the sensitivity to different injection strategies varies for different forms of injected sulfur.

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