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Aerosols can scatter incoming sunlight, redirecting it in multiple directions.

Summary​

Aerosol particles scatter sunlight through various mechanisms, influencing the Earth's climate. Rayleigh scattering explains blue skies and red sunsets through wavelength-dependent interactions with atmospheric molecules. Mie scattering, relevant to solar geoengineering, occurs with larger particles and scatters all wavelengths with forward direction bias. Stratospheric aerosols also also absorb radiation, causing localized heating as observed after volcanic eruptions in the lower stratosphere. The scattering behavior depends on particle size, composition, and incoming light wavelength.

The physical mechanism that allows techniques such as SAI to cool the Earth is the scattering of light by aerosol particles. The scattering of light is a complex process based on electromagnetism theory. Scattering is the change in direction of a wave, such as light, when it interacts with a particle or a surface.

The degree to which light is scattered depends on both the size and composition of the particle (in this case, an aerosol suspended in the atmosphere), as well as the wavelength of the incoming light. Light scattering is always present in our daily lives and is responsible for phenomena such as the sky being blue and sunsets being red 1. The presence of light-scattering aerosols in the stratosphere can also result in localized heating of the stratosphere caused by the multidirectional light scattering and the absorption of radiation by the aerosols 2.

Rayleigh scattering​

The light scattering that gives the effect of blue skies and sunsets is known as Rayleigh scattering. Rayleigh scattering is the scattering of light by particles that are much smaller than the wavelength of the incoming light. The size of the particles must be much smaller than the wavelength of the light for Rayleigh scattering to occur.

The wavelength of visible blue light (around 520 – 430 nm) is shorter than that of visible red light (around 630 – 750 nm). When sunlight (containing a mix of wavelengths) passes through the atmosphere, it interacts with particles that have a similar size to the wavelength, such as oxygen and nitrogen molecules, which are some of the most common. As the wavelength of blue light is the shortest in the visible spectrum, it is scattered most efficiently by the atmosphere. The blue appearance of the sky is a result of this light scattering 1.

The scattering of light also explains the phenomenon of orange and red sunsets. As the sun is low on the horizon, the sunlight is hitting the Earth tangentially and must pass through more of the atmosphere to reach an observer on the ground. Shorter wavelengths (such as blue light) are scattered more efficiently than longer wavelengths (such as red light), so as the sunlight passes through more atmosphere than it would during the day, more of the blue light is scattered into space. This leave more longer wavelengths reaching our eyes, which is why sunsets are red and orange 1.

Mie scattering​

Mie scattering is the form of light scattering that is most relevant to SAI. Mie scattering is similar to Rayleigh scattering, but unlike Rayleigh scattering, Mie scattering occurs when light interacts with particles that are similar in size or larger than the wavelength of the light 1. This independence from wavelength is what makes Mie scattering unique and is why it is more applicable for SAI, as all wavelengths of light can be scattered to some degree 3.

Mie scattering also strongly exhibits a property known as directional bias which relates to the primary direction of the scattered light which tends to be in the forward direction i.e. in the direction that the light was initially traveling. The degree of this forward bias is proportional to the size of the particles. This means that the larger the particles are, the more forward scattering occurs 1.

Absorbed radiation​

When a particle or aerosol scatters or reflects radiation, a portion of that radiation is absorbed by the particle. As a consequence, there is local atmospheric heating in the altitudes where aerosols are located. This is a similar process to the greenhouse effect, where greenhouse gases absorb the radiation emitted by Earth and re-emit it in all directions, including back towards the surface. However, the key difference is that greenhouse gases are highly efficient at absorbing and re-emitting longwave radiation, whereas many atmospheric aerosols are more efficient at scattering shortwave radiation 4. After the eruption of Mt. Pinatubo in 1991, it was observed that within the lower tropical stratosphere (17 – 15 km), localized temperatures rose by approximately 4°C 5.

Sources​

Footnotes​

  1. Bohren, C. F., & Huffman, D. R. (2008). Absorption and scattering of light by small particles. John Wiley & Sons. https://staff.cs.manchester.ac.uk/~fumie/internal/scattering.pdf ↩ ↩2 ↩3 ↩4 ↩5

  2. Dykema, J., Keith, D., Anderson, J. G., & Weisenstein, D. (2014). Stratospheric controlled perturbation experiment (SCoPEx): A small-scale experiment to improve understanding of the risks of solar geoengineering. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 372(2031), 20140059. https://doi.org/10.1098/rsta.2014.0059 ↩

  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. Mitchell, J. F. B., Johns, T. C., Gregory, J. M., & Tett, S. F. B. (1995). Climate response to increasing levels of greenhouse gases and sulphate aerosols. Nature, 376(6540), 501-504. https://doi.org/10.1038/376501a0 ↩

  5. 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 ↩