How Pollution Turns Our Skies Grey

The colour of the sky is determined by Rayleigh scattering, which is influenced by the size of particles in the atmosphere. Blue light is scattered more than other colours because of its shorter, smaller waves, which is why the sky typically appears blue. However, when there are more particles in the air, more light is scattered, and the colours are enhanced. Pollution is made up of particles that can interact with and destroy ozone particles in the atmosphere. These particles can change the colour of the sky, particularly during sunrise and sunset when the sun is close to the horizon and light must travel through more of the atmosphere. The most heavily polluted cities tend to have more orange and red sunsets due to an abundance of human-made aerosols, which are solid or liquid particles suspended in the air.

Particulate matter and Mie scattering

The presence of particulate matter in the atmosphere can alter the colour of the sky through a process known as Mie scattering. Mie scattering occurs when particles in the atmosphere have dimensions similar to the wavelength of the incident radiation, which is typically within the lower 4,500 metres (15,000 feet) of the atmosphere. This phenomenon is named after German physicist Gustav Mie, who developed the Mie solution to describe the behaviour of light scattering off spherical particles.

Mie scattering is particularly relevant in situations where the size of the scattering particles is comparable to the wavelength of light. In this context, the scattering of light by particulate matter in the atmosphere can lead to a change in the colour of the sky. The intensity of Mie-scattered radiation is given by the summation of an infinite series of terms, and it increases with particle size.

The effect of Mie scattering on the colour of the sky is frequency-dependent, meaning that different wavelengths of light are bent by varying amounts. For example, shorter wavelengths of light, such as blue, tend to be scattered more than longer wavelengths like red. As a result, during sunrise or sunset when the sun is low on the horizon, more red and orange light is scattered, creating vibrant sunsets that are often observed in polluted skies.

The impact of particulate matter on the colour of the sky is not limited to Mie scattering. Some pollutants inherently possess colour due to their absorption and reflection spectra. For instance, certain pollutants may exhibit brown or orange hues, contributing to the overall colour of the sky.

It is worth noting that the measurement of particulate matter concentration has been refined to meet stricter environmental standards. Techniques such as the light scattering method and the use of optical sensors have been employed to optimise the detection angle and enhance the accuracy of measurements, particularly in industries like coal-fired power plants.

More particles, more light scattered

The colour of the sky is determined by Rayleigh scattering, which is dependent on the size of the particles in the atmosphere. When the sky is blue, it is because blue light is scattered more than other colours of light due to its shorter, smaller waves. During sunrise or sunset, when the sun is close to the horizon, light must travel through more of the atmosphere, resulting in more scattering of light, including longer wavelengths such as yellow, orange, and red.

The particles that make up pollution can interact with and destroy ozone particles in the air. When there are more particles in the air, more light is scattered, and the colours are enhanced. This is why sunsets in heavily polluted cities tend to be more orange and red, as a result of an abundance of human-made aerosols.

Aerosols are solid or liquid particles suspended in the air that originate from both natural processes and human activity. Natural aerosols come from sources such as forest fires, mineral dust kicked up by sandstorms, sea spray, and volcanic eruptions. Human-made aerosols, on the other hand, contribute to air pollution and can lead to colourful skies.

While pollution can enhance the colour of sunsets, it is important to note that excess pollution will also dampen the overall experience. Large numbers of big particles in the atmosphere can scatter all colours indiscriminately, increasing the overall brightness of the sky but reducing colour contrast.

Therefore, the presence of more particles in the atmosphere due to pollution can indeed change the colour of the sky, particularly during sunrise and sunset when more light is scattered, resulting in enhanced colours.

Blue light and Rayleigh scattering

The colour of the sky is determined by the Earth's atmosphere and a process known as Rayleigh scattering. This scattering of light is caused by molecules in the atmosphere. The molecules of nitrogen and oxygen that make up most of the Earth's atmosphere scatter rays of sunlight into separate wavelengths of light.

The process of Rayleigh scattering is named after 19th-century British physicist Lord Rayleigh (John William Strutt), who discovered it. It specifically applies to particles that are small compared to the wavelengths of light and are optically "soft", meaning they have a refractive index close to 1.

The scattering of light by particles is influenced by their size relative to the wavelength of the light. When the particles are much smaller than the wavelength, as is the case with air molecules and blue light, the amount of scattering is greater for shorter wavelengths. This results in blue light being scattered more strongly than longer wavelengths like red light.

The scattering of sunlight in the Earth's atmosphere by small particles causes diffuse sky radiation, resulting in the blue colour of the daytime and twilight sky. When the sun is low in the sky during sunrise and sunset, the blue light is scattered away, and we observe the longer-wavelength red and orange light that reaches us, creating colourful sunsets.

Natural vs human-made aerosols

The colour of the sky is determined by the scattering of sunlight by molecules and particles in the atmosphere. This is called Rayleigh scattering. Nitrogen and oxygen make up most of the molecules in our atmosphere, but any gas or aerosol suspended in the air will scatter rays of sunlight into separate wavelengths of light.

Aerosols are small particles, either solid or liquid, in the atmosphere. They can come from both natural sources and human activities. Natural aerosols include dust, ash, sea salt, smoke from forest fires, and volcanic emissions. Human-made aerosols are generated from activities such as fossil fuel combustion, the burning of plants for land clearing, and the disturbance of surface soils.

Natural aerosols make up about 90% of the total amount of aerosols in our atmosphere, but human-made aerosols can still have a significant impact, especially in urban and industrial areas. The chemical composition, size, and ability to reflect or absorb solar radiation are all factors that determine the effect of an aerosol.

The presence of more particles in the air, whether natural or human-made, leads to more scattering of light and more colourful skies. This is why sunsets are often more vibrant in heavily polluted cities, with orange and red hues resulting from an abundance of human-made aerosols.

While the colourful skies resulting from increased aerosols can be spectacular to observe, they are also an indication of increased air pollution. Intense red sunsets, for example, can be visible when forest fires are burning nearby or during volcanic eruptions, both of which release large amounts of natural aerosols into the atmosphere.

Excess pollution dampens colours

The colour of the sky is determined by the scattering of sunlight by molecules in the atmosphere. This process, known as Rayleigh scattering, results in the blue colour typically associated with the sky due to blue light having shorter wavelengths that are scattered more than other colours.

However, when there is an increase in particulate matter and pollutants in the atmosphere, the colour of the sky can be altered. This is because pollution is made up of particles that can interact with and destroy ozone particles, changing the composition of the atmosphere. The presence of these particles can lead to Mie scattering, where different wavelengths of light are bent by varying amounts, resulting in vivid red and orange skies during sunsets or sunrises.

While pollution can enhance the colours of sunsets, excess pollution can also dampen the overall colour contrast of the sky. Large particles, such as those found in high concentrations during periods of intense air pollution, can scatter all colours of light indiscriminately. This increases the overall brightness of the sky while reducing the purity and intensity of colours.

For example, during large-scale events such as forest fires, volcanic eruptions, or industrial disasters like the burning of oil wells, the sky can take on unusual colours. In such cases, the excessive release of pollutants and particles into the atmosphere can saturate the sky, reducing the visibility of the sun and muting the colours usually associated with sunrise or sunset.

Therefore, while pollution can indeed alter the colours of the sky, excessive pollution can also dampen colours, reducing the contrast and intensity typically seen in a clear sky or during vibrant sunsets.

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