Grainger College of Engineering, All Events

SEA SALT AEROSOLS: HOW NATURAL AND INTENTIONAL PERTURBATIONS IMPACT CLIMATE AND ATMOSPHERIC CHEMISTRY

Advisor: Professor Hannah M. Horowitz

Sea salt aerosols (SSA) are a dominant natural aerosol species that influence both climate and atmospheric chemistry through radiative effects, cloud interactions, and atmospheric chemistry. Intentional perturbations of SSA through marine cloud brightening (MCB) have been proposed as a potential solar climate intervention strategy to offset anthropogenic greenhouse gas warming. This dissertation investigates how both natural and intentional modifications of SSA affect the Earth system, with a particular focus on polar processes, global climate, and tropospheric chemistry. Using a global chemical transport model (GEOS-Chem) and a fully coupled Earth system model (CESM2), this work quantifies the climate and chemical consequences of SSA perturbations.

First, the impacts of natural SSA are investigated by quantifying Arctic-wide SSA emissions from fractures in sea ice (open sea ice leads) during the cold season (November-April) from 2002-2008. Satellite-derived lead area fractions (the AMSR-E product) are combined with a wind- and temperature-dependent SSA emission parameterization in GEOS-Chem to estimate SSA emissions from large (>3 km) leads. Lead emissions increase total Arctic SSA emissions by 1.1% - 1.8% north of 60°N latitude and 5.6-7.5% north of 75°N latitude, with localized surface concentration increases exceeding 60% in regions of low background aerosol. While GEOS-Chem overestimates observed sodium concentrations at Arctic monitoring sites even without lead emissions, suggesting uncertainties in sinks or other sources, open lead-derived SSA increases multi-year mean surface bromine atom concentrations by 2.8% - 8.8% during the cold season. Ozone responses are negligible. These results indicate that although open leads contribute less than 10% of total Arctic SSA emissions during the study period, they exert chemical impacts in low-background concentration regions and may grow in importance as Arctic sea ice thins in the future.

Next, intentional SSA perturbations via MCB as a climate intervention strategy are explored. MCB aims to cool the planet by spraying SSA into marine clouds to enhance sunlight reflection. Using the Community Earth System Model version 2 (CESM2), the climatic effects of seasonally and hemispherically targeted midlatitude MCB are examined. The results demonstrate deployment season strongly modulates the global temperature and precipitation response of midlatitude MCB. Deploying MCB in the summer months (likely to induce stronger local cooling) of each hemisphere cools the globe non-uniformly and shifts precipitation patterns along the equator (the Intertropical Convergence Zone, or ITCZ). By deploying MCB in the fall and winter months in each hemisphere with year-round deployment, present-day conditions of surface temperature, precipitation, and the El Niño–Southern Oscillation (ENSO) is most effectively restored.

Building on this analysis, the polar climate response to different MCB deployment strategies is investigated. In addition to global temperature and precipitation, the location and season of MCB deployment shapes the Arctic and Antarctic surface temperature, sea ice area, and sea ice thickness responses. Hemispherically targeted MCB restores sea ice in the same hemisphere (i.e., Northern Hemisphere deployment for Arctic sea ice) but leads to less sea ice in the opposite hemisphere relative to the climate change scenario. However, by targeting MCB in the midlatitudes of both hemispheres, Arctic and Antarctic sea ice is restored. These findings underscore that MCB deployment location and season are critical for recovery of sea ice, and that there may be unintended consequences when targeting a single hemisphere.

Finally, this dissertation evaluates how MCB alters atmospheric chemistry using GEOS-Chem. The chemical response depends strongly on injection location, magnitude, and representation of nitrate aerosol photolysis within the model. MCB generally increases tropospheric oxidants when the nitrate aerosol photolysis mechanism is enabled, including hydroxyl radical (OH), possibly driven by changes in ozone and nitrogen oxides (NOx), while impacts on reactive halogen families vary depending on deployment location. These changes shorten the modeled methane lifetime, suggesting potential radiative effects purely derived from atmospheric chemistry impacts. However, the results demonstrate MCB impacts on atmospheric chemistry are not straightforward and vary nonlinearly with deployment strategy and chemical mechanism assumptions.

Overall, this dissertation demonstrates that both natural and intentional perturbations of SSA produce various climate and chemistry responses. Arctic lead SSA emissions modestly increase bromine atom concentrations, though impacts may increase in a changing climate. Furthermore, MCB deployment strategy critically determines global temperature, precipitation, cryosphere, and atmospheric chemistry outcomes. These findings strengthen the understanding of SSA-driven climate and chemical responses and highlight the importance of deployment design in evaluating marine cloud brightening as a potential climate intervention strategy.