Sulfate Geoengineering Cuts Global PM2.5 by 10-15% but Boosts Secondary Organics and Shifts Deposition Patterns

The GLENS project ran 20-member ensembles in CESM1 with continuous SO2 injection to offset RCP8.5 radiative forcing. Compared with the control, geoengineering increased soil moisture and leaf-area index over source regions, cutting dust emissions, while weaker surface winds reduced sea-salt aerosol. These changes produced a net global PM2.5 decline of roughly 0.5-1.0 µg m-3, yet excluding dust and sea salt revealed higher secondary organic aerosol mass because cooler temperatures favored gas-to-particle partitioning of prescribed precursors. Public-health implications arise from the altered composition rather than total mass. More sulfate and organic carbon in the fine fraction can penetrate deeper into lungs and alter oxidative potential, even if total PM2.5 falls. Spatial shifts in wet and dry deposition also move acid and nutrient loads away from current patterns, potentially affecting sensitive ecosystems and agricultural soils downwind of injection zones. Prior modeling studies such as the 2018 GLENS aerosol papers and the 2022 multi-model comparison in Atmospheric Chemistry and Physics focused on radiative and hydrological outcomes but omitted PM2.5 speciation and health metrics. The present work fills that gap yet still relies on fixed precursor emissions and a single model, limiting extrapolation to real-world deployment scenarios where emissions and chemistry would co-evolve. Next steps require coupling interactive chemistry, dynamic vegetation, and epidemiological exposure models within larger ensembles to quantify disability-adjusted life years and regional deposition risks before any field testing proceeds.