Stratospheric sulfate aerosols and their impact on climate: from volcanoes to proposed human interventions

Aerosols are fine solid, liquid or mixed-phase particles suspended in the air. Their chemical composition, concentration, size and shape is highly variable in the troposphere, while the stratosphere contains a persistent layer of predominantly sulfate particles, due to the different sources and transport mechanisms involved in the two layers of the atmosphere. Aerosols are a key component of the Earth's atmosphere because they affect air quality, meteorology and climate. They interact directly with solar and planetary radiation through its scattering and absorption, altering the Earth's energy balance and, in turn, affecting changes in temperature and precipitation. Indirect interaction with radiation occurs through modification of cloud reflectivity, as aerosols act as cloud condensation nuclei for cloud droplets and ice particles and thus affect the droplet size and number concentration. The aim of this thesis is to provide an overview of atmospheric aerosols covering main sources and sinks, the chemical and microphysical processes involved, and the optical properties which affect aerosol interaction with radiation. The main focus is on sulfate aerosols, which are dominant in the stratosphere, because of their implications in climate change. Sulfate aerosols scatter the incoming solar radiation and absorb the infrared radiation resulting in a cooling of the surface and heating of the region of the stratosphere where they resides, which in turn impact the atmospheric circulation, the stratospheric heterogeneous chemistry and the hydrological cycle. The main contributor to stratospheric aerosols under perturbed condition are volcanic eruptions, which can inject sulfate precursor gases directly into the stratosphere along with ash, water vapor and other compounds; therefore, due to the climatic impact of sulfate aerosols, volcanic eruptions are a natural cause of climate change. Here we study the evolution of the simulated stratospheric aerosol cloud following the well-observed June 1991 Mount Pinatubo event, comparing six interactive stratospheric aerosol microphysics models with different observations, and we show where there is agreement among the models and where there are differences in relation to differences among the dynamical, microphysical, and chemical modeling schemes. Simulating volcanic eruptions can be considered a test case for evaluating climate models and interest in assessing the reliability of climate models in simulating stratospheric sulfur injection has recently increased because of the similarity to geoengineering experiments, aimed to counteract global warming. Here we show the advances in Stratospheric Aerosol Injection (SAI) which is one of the geoengineering strategies and involves the continuous injection of sulfate precursor gases. We propose, among the SAI strategies proposed so far, the increased surface emission of carbonyl sulfide, a sulfate precursor gas in the stratosphere, evaluate its risks and benefits compared with the more commonly studied sulfur dioxide injection strategy. Finally, we assess the hydrological impact of large explosive volcanic eruptions occurring during SAI deployment, considering a medium and large case of large volcanic eruption and define the risks and whether an SAI strategy could be modified in order to limit them.