Exploring accumulation-mode Hsub2/subSOsub4/sub versus SOsub2/sub stratospheric sulfate geoengineering in a sectional aerosol–chemistry–climate model

摘要

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 sup?1/sup injected either in the form of accumulation-mode Hsub2/subSOsub4/sub droplets (AM Hsub2/subSOsub4/sub ), gas-phase SOsub2/sub 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 sup°/sup S–3.75 sup°/sup N to 30 sup°/sup S–30 sup°/sup N. In the SOsub2/sub emission scenarios, continuous production of tiny nucleation-mode particles results in increased coagulation, which together with gaseous Hsub2/subSOsub4/sub condensation, produces coarse-mode particles. These large particles are less effective for backscattering solar radiation and have a shorter stratospheric residence time than AM Hsub2/subSOsub4/sub particles. On average, the stratospheric aerosol burden and corresponding all-sky shortwave radiative forcing for the AM Hsub2/subSOsub4/sub scenarios are about 37?% larger than for the SOsub2/sub scenarios. The simulated stratospheric aerosol burdens show a weak dependence on the latitudinal spread of emissions. Emitting at 30 sup°/sup N–30 sup°/sup S instead of 10 sup°/sup N–10 sup°/sup 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 SOsub2/sub 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 Hsub2/subSOsub4/sub 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 Hsub2/subSOsub4/sub results in higher radiative forcing for the same sulfur equivalent mass injection strength than SOsub2/sub emissions, and that the sensitivity to different injection strategies varies for different forms of injected sulfur.