Skip to main content

Potential risks of Stratospheric Aerosol Injection (SAI): acid rain

A potential risk associated with the deployment of stratospheric aerosol injection (SAI), especially with the use of sulfate aerosols, is the creation of acid rain. Acid rain occurs when acidic compounds dissolve in water located in the atmosphere, lowering the water's pH. Acid rain can impact ecosystems by reducing the pH of waterways, lowering the overall quality of the water, and damaging the health of the animals that rely on it 1. Acid rain also impacts human-made structures, leading to faster degradation of materials such as concrete and steel 2.

The method in which acid rain is formed via SAI is very similar to how it is formed in volcanic eruptions. In both cases, sulfur dioxide (SO₂) is injected into the upper stratosphere where it reacts to form sulfuric acid (H₂SO₄), which is the aerosol that is desirable in SAI for its ability to scatter incoming light. When the sulfuric acid remains in the stratosphere, it does not lead to the formation of acid rain as it lies above the altitude where rain is formed. However, if it is moved to a lower altitude via convection or other means into the troposphere, acid rain can develop 3.

The aerosols used in SAI are expected to remain in the stratosphere for between one to three years, after which the majority are expected to return down to Earth 4. Acid rain is a well-understood phenomenon that has been present since the 19th century, but its formation via SAI is a relatively new area of research, and the lack of real-world experimentation means that the long-term effects of SAI on acid rain are not fully understood. Recent research has found that the use of sulfate aerosols in SAI may lead to slight ocean acidification; however, the scale of this will be lower than contemporary ocean acidification caused by high atmospheric concentrations of carbon dioxide (CO₂) 4. The influence over land will be less than the current acid rain caused by the burning of fossil fuels 4. However, total covering of the Earth's atmosphere with aerosols required for SAI to work could lead to acid rain forming in pristine environments that currently do not experience it.

Sources​

Footnotes​

  1. Tracy, S. M., Moch, J. M., Eastham, S. D., & Buonocore, J. J. (2022). Stratospheric aerosol injection may impact global systems and human health outcomes. Elementa: Science of the Anthropocene, 10(1), 00047. https://doi.org/10.1525/elementa.2022.00047 ↩

  2. Hung, Y.-T., & Sicurezza, J. (2020). The causes, history and effects of acid rain. In Handbook of environment and waste management (Chapter 2, pp. 27-57). World Scientific. https://doi.org/10.1142/9789811207136_0002 ↩

  3. Visioni, D., Slessarev, E., MacMartin, D. G., Mahowald, N. M., Goodale, C. L., & Xia, L. (2020). What goes up must come down: Impacts of deposition in a sulfate geoengineering scenario. Environmental Research Letters, 15(9), 094063. https://doi.org/10.1088/1748-9326/ab94eb ↩

  4. National Academies of Sciences, Engineering, and Medicine. (2021). Reflecting Sunlight: Recommendations for Solar Geoengineering Research and Research Governance. The National Academies Press. https://doi.org/10.17226/25762 ↩ ↩2 ↩3