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Dykema, John

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Dykema

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Dykema, John
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    Production of Sulfates Onboard an Aircraft: Implications for the Cost and Feasibility of Stratospheric Solar Geoengineering
    (Wiley, 2018-04) Smith, Jordan P.; Dykema, John; Keith, David
    Injection of sulfate aerosols into the stratosphere, a form of solar geoengineering, has been proposed as a means to reduce some climatic changes by decreasing net anthropogenic radiative forcing. The cost and technical feasibility of forming aerosols with the appropriate size distribution are uncertain. We examine the possibility of producing the relevant sulfur species, SO2 or SO3, by in situ conversion from elemental sulfur onboard an aircraft. We provide a first-order engineering analysis of an open cycle chemical plant for in situ sulfur to sulfate conversion using a Brayton cycle combustor and a catalytic converter. We find that such a plant could have sufficiently low mass that the overall requirement for mass transport to the lower stratosphere may be reduced by roughly a factor of 2. All else equal, this suggests that—for a given radiative forcing—the cost of delivering sulfate aerosols may be nearly halved. Beyond reducing cost, the use of elemental sulfur reduces operational health and safety risks and should therefore reduce environmental side effects associated with delivery. Reduction in cost is not necessarily beneficial as it reduces practical barriers to deployment, increasing the urgency of questions concerning the efficacy, risks, and governance of solar geoengineering.
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    Stratospheric controlled perturbation experiment: a small-scale experiment to improve understanding of the risks of solar geoengineering
    (The Royal Society Publishing, 2014) Dykema, John; Keith, David; Anderson, James; Weisenstein, Debra
    Although solar radiation management (SRM) through stratospheric aerosol methods has the potential to mitigate impacts of climate change, our current knowledge of stratospheric processes suggests that these methods may entail significant risks. In addition to the risks associated with current knowledge, the possibility of ‘unknown unknowns’ exists that could significantly alter the risk assessment relative to our current understanding. While laboratory experimentation can improve the current state of knowledge and atmospheric models can assess large-scale climate response, they cannot capture possible unknown chemistry or represent the full range of interactive atmospheric chemical physics. Small-scale, in situ experimentation under well-regulated circumstances can begin to remove some of these uncertainties. This experiment—provisionally titled the stratospheric controlled perturbation experiment—is under development and will only proceed with transparent and predominantly governmental funding and independent risk assessment. We describe the scientific and technical foundation for performing, under external oversight, small-scale experiments to quantify the risks posed by SRM to activation of halogen species and subsequent erosion of stratospheric ozone. The paper's scope includes selection of the measurement platform, relevant aspects of stratospheric meteorology, operational considerations and instrument design and engineering.
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    Solar geoengineering using solid aerosol in the stratosphere
    (Copernicus GmbH, 2015-04-21) Weisenstein, Debra; Keith, David; Dykema, John
    Solid aerosol particles have long been proposed as an alternative to sulfate aerosols for solar geoengineering. Any solid aerosol introduced into the stratosphere would be subject to coagulation with itself, producing fractal aggregates, and with the natural sulfate aerosol, producing liquid-coated solids. Solid aerosols that are coated with sulfate and/or have formed aggregates may have very different scattering properties and chemical behavior than do uncoated non-aggregated monomers. We use a twodimensional chemical transport model to capture the dynamics of interacting solid and liquid aerosols in the stratosphere. As an example, we apply the model to the possible use of alumina and diamond particles for solar geoengineering. For 240 nm radius alumina particles, for example, an injection rate of 4 Mt yr−1 produces a global-average radiative forcing of 1.3 Wm−2 and minimal self-coagulation of alumina yet almost all alumina outside the tropics is coated with sulfate. For the same radiative forcing, these solid aerosols can produce less ozone loss, less stratospheric heating, and less forward scattering than do sulfate aerosols. Our results suggest that appropriately sized alumina, diamond or similar high-index particles may have less severe technologyspecific risks than do sulfate aerosols. These results, particularly the ozone response, are subject to large uncertainties due the limited data on the rate constants of reactions on the dry surfaces.