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Other aerosols besides sulfates have been proposed for SAI such as: diamond dust, alumina, calcium carbonate, and black carbon

Summary​

Various non-sulfate aerosols have been proposed for stratospheric aerosol injection (SAI), each with distinct properties and challenges. Diamond dust offers high light-scattering efficiency but faces cost barriers, while alumina benefits from existing production infrastructure. Calcium carbonate shows potential for minimal ozone impact but shares dispersion challenges with other solid particles. Black carbon's extreme efficiency carries risks of catastrophic ozone loss through stratospheric heating. Titanium dioxide requires novel delivery methods despite its refractive advantages. All alternatives face hurdles in dispersion technology and lack natural analogs compared to sulfate aerosols.

Sulfate aerosols are the most commonly proposed for stratospheric aerosol injection (SAI) due to their inspiration from natural phenomena, high albedo effect, established scientific understanding, and their ability to be formed from the precursor gas sulfur dioxide (SO₂). However, other aerosols have been proposed for SAI for a number of reasons, including their desirable optical properties and minimization of ozone depletion. The main optical property that dictates how effective a particle is at scattering light is its refractive index, followed by the particle's size and shape. For reference, the typical refractive index at a 0.55 μm light wavelength is 1.5 for sulfate aerosols 1.

General considerations for non-sulfate aerosols​

When considering non-sulfate aerosols for SAI, there are a number of challenges that almost all of them face: dispersion and observable natural analogs. One of the largest benefits of sulfate aerosols is that they can be produced via a reaction between sulfur dioxide (SO₂) and naturally present particles in the atmosphere. This is an enormous benefit as gases are significantly easier to disperse into the atmosphere than solid particles. When using solid particles, consideration must be given to ensure that the particles achieve a uniform dispersion and do not clump together, and technology that is capable of achieving this does not exist yet 2.

Sulfate aerosols also have a unique benefit in the fact that we know with a high degree of understanding what happens when large amounts of sulfur dioxide (SO₂) are injected into the stratosphere because of volcanic eruptions such as Mt. Pinatubo in 1991. This cannot be said for almost every other potential aerosol type. Even naturally occurring materials such as Black Carbon (BC) and Calcium Carbonate (CaCO₃) rarely make it to the high altitudes of the stratosphere.

Black carbon (BC) aka soot​

Black carbon or soot has also been proposed for SAI due to its light absorption properties and its high natural occurrence. The suitability of black carbon as a potential aerosol is not from its light-scattering ability, but rather its ability to absorb incoming sunlight. This should not be confused with the greenhouse gas effect. Black carbon is not technically a greenhouse gas, being considered a short-lived climate forcing particulate matter. Other greenhouse gases such as carbon dioxide (CO₂) predominantly trap radiation emitted by the Earth from escaping; they do not block incoming visible sunlight from entering. Black carbon is such an efficient absorber of incoming sunlight that it is estimated that less than 2% of the mass of sulfur would be required to be injected into the stratosphere to achieve the same albedo effect as sulfate aerosols 3. It is for this reason that the use of BC as an SAI aerosol has been described as a mini "nuclear winter." The major trade-off with BC as an atmospheric aerosol is that the high degree of incoming solar radiation absorption leads to tremendous levels of localized heating in the stratosphere greater than 50°C 4 5. High temperatures in the atmosphere lead to faster catalytic reactions with ozone and reactive chemicals such as chlorine (Cl) and bromine (Br) and greater levels of thermally driven convection, which can alter the distribution of ozone and reactive material 6. The ozone loss predicted from this situation is considered catastrophic, making the use of BC as an SAI aerosol undesirable.

Titanium dioxide (TiO₂)​

Titanium dioxide has a high refractive index of 2.5, and due to this, the mass of TiO₂ required to reach the same albedo effect as sulfate aerosols would be approximately three times less 1. However, this is complicated by the fact that TiO₂ would require a carrier gas to be injected into the stratosphere as it cannot be delivered like sulfate aerosols which use a precursor gas 1. As titanium dioxide is not naturally produced in large quantities as with sulfate aerosols and volcanic eruptions, the effect its dispersion would have on nature is not well understood. Another key concern with TiO₂ is its high degree of solar UV radiation absorption, which leads to more localized heating within the stratosphere compared to sulfate aerosols 4.

Alumina (Al₂O₃)​

Alumina has a high refractive index of 1.77, making it slightly more effective at scattering light than sulfate aerosols for certain wavelengths 1. Alumina is a precursor for aluminum production and is already produced in large quantities, with the International Aluminium Institute estimating global production in 2023 of 143 Mt, making it a relatively practical material for SAI usage 7. An advantage of alumina is that it is possible to control the size of the particles, which is not the case for sulfate aerosols. Alumina, like sulfate aerosols, provides surfaces for reactive chemicals such as chlorine (Cl) and bromine (Br) to attach to and react with ozone (O₃), which can lead to high degrees of ozone depletion 1 4. The magnitude of the ozone depletion is related to the size of the alumina particles, and controlling particle's size may make it possible to minimize ozone depletion compared to sulfate aerosols 4. There is also a high degree of experience with the usage and manufacture of alumina as it is a commonly used nanoparticle in a range of industries. As with many SAI particle alternatives, the influence on the environment and agriculture is not well understood.

Calcium carbonate (CaCO₃) aka limestone​

Calcium carbonate is a naturally occurring mineral that is found in limestone used for cement production that has a refractive index slightly greater than sulfate aerosols 1 8. CaCO₃ shows potential as an SAI aerosol due to its ubiquity in nature and very low cost 4. The main advantages of Calcium carbonate are that modeling and lab experimentation have shown that it has a rather minimal effect on ozone depletion compared to most other aerosol types, and it has a low degree of localized stratospheric heating 4. As with other solid particles, the main challenge with Calcium carbonate is its dispersion method.

Diamond dust (C)​

Diamond dust has been proposed as an SAI aerosol due to its high efficiency at scattering light relative to its mass, with a refractive index at 0.55 μm light wavelength of around 2.42, meaning that only a third of the mass of sulfate aerosols is needed to achieve the same albedo effect 1 4. The main benefit of diamond dust is that diamonds naturally have a very low degree of radiation absorption, meaning that they would have a minimal localized stratospheric heating effect compared to other aerosol types, which would carry a number of advantages such as reduced ozone depletion 4. The obvious downside to diamond dust is that diamonds are extremely expensive to manufacture, making it the most expensive aerosol type. Another drawback to diamond dust is that there is a severe lack of research and information pertaining to its behavior in the atmosphere, which makes it difficult to assess its potential risks and benefits 4.

Sources​

Footnotes​

  1. Pope, F., Braesicke, P., Grainger, R., Kalberer, M., Watson, I., Davidson, P., & Telford, P. (2012). Stratospheric aerosol particles and solar-radiation management. Nature Climate Change, 2(9), 713–719. https://doi.org/10.1038/nclimate1528 ↩ ↩2 ↩3 ↩4 ↩5 ↩6 ↩7

  2. National Academies of Sciences, Engineering, and Medicine. (2021). Reflecting sunlight: Recommendations for solar geoengineering research and research governance. National Academies Press. https://doi.org/10.17226/25762 ↩

  3. Crutzen, P. J. (2006). Albedo enhancement by stratospheric sulfur injections: A contribution to resolve a policy dilemma? Climatic Change, 77(3-4), 211–220. https://doi.org/10.1007/s10584-006-9101-y ↩

  4. Huynh, K. A., & McNeill, V. F. (2024). The potential environmental and climate impacts of stratospheric aerosol injection: A review. Environmental Science: Atmospheres, 4(1), 114–143. https://doi.org/10.1039/D3EA00134B ↩ ↩2 ↩3 ↩4 ↩5 ↩6 ↩7 ↩8 ↩9

  5. Kravitz, B., Robock, A., Shindell, D. T., & Miller, M. A. (2012). Sensitivity of stratospheric geoengineering with black carbon to aerosol size and altitude of injection. Journal of Geophysical Research, 117, D09203. https://doi.org/10.1029/2011JD017341 ↩

  6. Mills, M. J., Toon, O. B., Turco, R. P., Kinnison, D. E., & Garcia, R. R. (2008). Massive global ozone loss predicted following regional nuclear conflict. Proceedings of the National Academy of Sciences of the United States of America, 105(14), 5307–5312. https://doi.org/10.1073/pnas.0710058105 ↩

  7. International Aluminium Institute. (2024). Alumina production statistics. https://international-aluminium.org/statistics/alumina-production/ ↩

  8. Dykema, J. A., Keith, D. W., & Keutsch, F. N. (2016). Improved aerosol radiative properties as a foundation for solar geoengineering risk assessment. Geophysical Research Letters, 43(15), 7758–7766. https://doi.org/10.1002/2016GL069258 ↩