Air Pollution: Reddish-Brown Skies And Their Causes

The reddish-brown haze that hangs over many cities is a result of poor air quality and pollution from smog. Smog is a type of pollution that reduces visibility and is often a result of car exhaust, coal power plants, factory emissions, and other sources. The reddish-brown colour is caused by the absorption and scattering of solar radiation by black carbon, fly ash, soil dust particles, and nitrogen dioxide. While some believe that air pollution enhances the beauty of a sunset, others argue that excess pollution will dampen the overall sunset experience.

Particles in the air scatter light, removing blues and enhancing reds

The reddish-brown haze that appears over cities is known as smog, a type of pollution that reduces visibility. Smog is produced from car exhaust, coal power plants, factory emissions, and other sources. The reddish-brown colour of the smog is caused by the scattering of light by particles in the air.

This phenomenon is known as Rayleigh scattering, where small particles in the air scatter light, causing optical phenomena such as the blue colour of the sky. However, when the air is polluted, the particles scatter light differently, removing blues and enhancing reds.

The particles in polluted air are often larger than the particles in clean air, including aerosols, black carbon, fly ash, soil dust particles, and nitrogen dioxide. These larger particles scatter light differently than smaller particles, removing the shorter blue wavelengths and enhancing the longer red wavelengths. This is why polluted air often appears reddish-brown.

Additionally, the larger particles in polluted air can scatter all colours of light indiscriminately, increasing the overall brightness of the sky while dampening colour contrast. This can make the sky appear washed out, especially when there is a high concentration of large particles.

The scattering of light by particles in polluted air can also be influenced by the angle of the sun. During sunrise and sunset, when the sun is lower on the horizon, more of the shorter blue wavelengths are scattered, leaving behind more of the longer red wavelengths. This is why sunsets in polluted cities often appear more crimson or reddish.

Nitrogen oxides and toluene increase the light-absorbing properties of brown carbon particles

The reddish-brown haze of smog that often lingers over cities is linked to climate issues. The thousands of chemicals involved act as a warming blanket, absorbing sunlight and trapping heat. While smog is a type of pollution that reduces visibility, it can also make sunsets appear more vibrant. This is because air pollutants soften sky colours and reduce the total amount of light that reaches the ground, intensifying the colours of the sunset.

Brown carbon particles, a type of particle in smog, are composed of organic molecules that absorb and retain heat. These particles are formed by the photo-oxidation of toluene, a common pollutant. Toluene is a clear, colourless liquid with a smell similar to paint thinners. It is a common solvent in many chemical and manufacturing processes.

Recent research from the Pacific Northwest National Laboratory has revealed that a mixture of nitrogen oxides and toluene increases the light-absorbing properties of brown carbon particles. Nitrogen oxides are highly reactive air pollutants that play a significant role in the formation of smog and acid rain. They are primarily produced from vehicle emissions and the burning of fossil fuels.

The study conducted by Dr. John Shilling and Dr. Jiumeng Liu synthesised brown carbon particles by mixing nitrogen oxides with toluene inside an environmental chamber. They then exposed the resulting gas to light, mimicking the natural aging process that occurs in the atmosphere. This process, known as photo-oxidation, is critical in understanding the evolution and behaviour of brown carbon particles.

The findings provide new insights into the behaviour of brown carbon particles and can inform computational models to assess energy choices and their influence on the climate. Additionally, a better understanding of these particles can lead to the development of better predictive models to address air quality issues and minimise future problems.

Black carbon, fly ash, soil dust particles, and nitrogen dioxide scatter and absorb solar radiation

The reddish-brown haze that appears over cities is known as smog, a type of pollution that reduces visibility. Smog is produced from car exhaust, coal power plants, factory emissions, and other sources. The reddish-brown colour of the clouds is caused by the absorption and scattering of solar radiation by black carbon, fly ash, soil dust particles, and nitrogen dioxide.

Black carbon, a component of soot, is a significant contributor to the reddish-brown colour of polluted air. Black carbon aerosols absorb sunlight rather than reflecting it, warming the layer of the atmosphere in which they are present. They are produced by the combustion of fossil fuels and biomass. In the second half of the 20th century, black carbon emissions increased by a factor of five in China.

Fly ash is another contributor to the reddish-brown colour of polluted air. Fly ash, like black carbon, absorbs and scatters solar radiation. It is also a product of the combustion of fossil fuels and biomass.

Soil dust particles also play a role in the reddish-brown colour of polluted air. These particles can absorb and scatter solar radiation, depending on their composition and the minerals they contain. Soil dust particles are often coated with black or brown carbon, enhancing their ability to absorb sunlight.

Nitrogen dioxide is a gas that turns into nitrogen dioxide aerosols, which scatter and absorb solar radiation. These aerosols contribute to the reddish-brown colour of polluted air and are produced by the combustion of fossil fuels.

The presence of these pollutants in the atmosphere reduces the amount of solar radiation reaching the Earth's surface, resulting in a phenomenon known as "dimming". This reduction in solar radiation can have significant regional climatic effects, posing risks to human health and food security.

Photochemical smog is caused by a reaction of sunlight with nitrogen oxides and volatile organic compounds

Smog is a type of pollution that reduces visibility, and it can be used as an indicator of poor air quality. There are two main types of smog: photochemical and sulphurous. Photochemical smog is caused by a reaction of sunlight with nitrogen oxides and volatile organic compounds (VOCs). This type of smog is common in urban areas and is produced from car exhaust, coal power plants, factory emissions, and other sources.

Nitrogen oxides are emitted into the air as pollutants mainly from internal combustion engines. They absorb the visible or ultraviolet energy of sunlight, forming nitric oxide (NO), which then combines with molecular oxygen (O2) to form ozone (O3). Ozone is a major component of smog and a reactive gas that causes inflammation. It has serious health risks, including triggering asthma, breathing problems, reduced lung function, and lung diseases.

In the presence of hydrocarbons, certain other organic compounds, and sunlight, various chemical reactions take place to form photochemical smog. These reactions result in the formation of ground-level ozone, which is a secondary pollutant. The formation of ozone depends on two key ingredients: nitrogen oxides and volatile organic compounds. These compounds are emitted from the tailpipes of cars, power plants, petroleum refineries, and other sources.

The presence of brown clouds of pollution over urban areas has been a concern for decades. These atmospheric brown clouds are caused by emissions associated with the combustion of fossil fuels and biomass, and they are particularly prevalent in tropical regions. The brown colour results from the absorption and scattering of solar radiation by black carbon, fly ash, soil dust particles, and nitrogen dioxide.

While pollution can indeed change the appearance of sunsets, making them appear reddish or crimson, excessive pollution will dampen the overall experience. Large particles in the troposphere can scatter all colours of light, increasing the overall brightness of the sky but reducing colour contrast.

High levels of air pollution can dampen the overall sunset experience

While it is true that air pollution can alter the appearance of sunsets, high levels of air pollution can dampen the overall sunset experience. The reddish-brown haze that often hangs over cities is a tell-tale sign of poor air quality caused by smog. Smog is a type of air pollution that reduces visibility and can be classified into two main types: photochemical and sulphurous. Photochemical smog, prevalent in urban areas, is produced by the reaction of sunlight with nitrogen oxides and volatile organic compounds from sources such as car exhaust, coal power plants, and factory emissions. Sulphurous smog, on the other hand, results from high concentrations of sulfur oxides in the atmosphere, primarily from the burning of fossil fuels.

The presence of reddish-brown clouds, also known as atmospheric brown clouds, is a global phenomenon linked to human-generated air pollution. These clouds are particularly prevalent in tropical regions due to elevated pollutant emissions and prolonged dry seasons that prevent the removal of aerosols from the atmosphere through precipitation. The brown colour of these clouds is a result of the absorption and scattering of solar radiation by particles such as black carbon, fly ash, soil dust, and nitrogen dioxide.

While it is commonly believed that air pollution enhances the beauty of sunsets, the impact of pollution on sunset appearances is subjective and depends on the level of pollution. At high concentrations, air pollution can saturate the sky, making it difficult to see the sun clearly. This is because large particles in the troposphere, the layer closest to the ground, scatter all colours of light indiscriminately, increasing overall brightness but reducing colour contrast.

Additionally, aerosols, which are produced by the burning of fossil fuels, scatter radiation and remove cooler colours like violet and blue while enhancing reds. While this can contribute to crimson sunsets in polluted cities, excessive pollution can diminish the sunset experience by reducing the colour contrast and purity of the sky. Therefore, while air pollution can alter sunset colours, high levels of pollution may detract from the overall aesthetic experience.

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