Air Pollution: Changing Our Sky And Hurting Our Planet

Air pollution is a pressing issue that affects the appearance of the sky in several ways. The sky's colour is determined by the scattering of sunlight by molecules in the atmosphere, a process called Rayleigh scattering. When there is an increase in air pollution, specifically the presence of aerosols, the sky's appearance changes. This is due to the increased scattering of sunlight, resulting in more colourful skies.

The impact of air pollution on the sky is twofold. Firstly, it can lead to a shift in the colour of the night sky towards red, with the glowing area of the night sky expanding as pollution concentrations increase. This phenomenon is known as skyglow and is caused by artificial light emitted on the ground, which interacts with aerosols in the atmosphere. Secondly, air pollution can cause haze and affect the transparency of the sky, reducing visibility and inhibiting radiational cooling at night, which may contribute to global warming.

The effects of air pollution on the sky are not limited to aesthetic concerns but also have implications for astronomical observation, human health, and the environment. Reducing air pollution is crucial to mitigate these impacts and promote a sustainable urban environment.

Air pollution can make sunsets more orange and red, especially when there are nearby forest fires or volcanic eruptions

Air pollution can have a significant impact on the appearance of the sky, and this is particularly evident during sunsets. When there is a high level of air pollution, sunsets tend to display more orange and red hues. This phenomenon is often observed in cities with high levels of pollution, but it is also common when there are nearby forest fires or volcanic eruptions.

The colours we see in the sky during sunsets are a result of Rayleigh scattering, where sunlight interacts with molecules and particles in the atmosphere. This scattering affects different colours of light to varying degrees, with shorter wavelengths such as blue and green scattering more easily than longer wavelengths like orange and red. As the sun sets, its light must travel through more of the atmosphere, and the shorter wavelengths are scattered away, leaving behind the warmer, longer wavelengths that create the vibrant orange and red hues of sunset.

Forest fires and volcanic eruptions release a large number of aerosols—small solid or liquid particles—into the atmosphere. These particles provide more opportunities for the sunlight to scatter, enhancing the appearance of the longer wavelengths. The particles from forest fires and volcanic eruptions can also travel great distances, leading to vibrant sunsets in locations far from the source of the pollution.

While the enhanced colours of a sunset can be visually stunning, they are a reminder of the presence of air pollution. Excessive pollution can also dampen the overall sunset experience, as too many large particles in the atmosphere can scatter all colours of light indiscriminately, washing out the vibrant hues. Therefore, while air pollution can lead to more orange and red sunsets, it is important to recognise the negative impact of pollution on the environment and public health.

It can also make the night sky brighter

Air pollution can make the night sky brighter in several ways. Firstly, artificial light at night (ALAN) interacts with the air, changing its chemistry and reducing air quality. This can make the night sky appear brighter and more colourful.

Secondly, air pollution can cause the night sky to shift towards red. This is due to the increased concentration of urban air pollutants, which can include particulate matter (PM), O3, SO2, NO2, and CO. The glowing area of the night sky expands with the increase in these pollutants.

Thirdly, air pollution can make the night sky brighter by increasing cloud cover. Clouds form in the presence of microscopic particles called aerosols, which are now mostly composed of black carbon and soot from cars, factories, and cookstoves. Air pollution can also make clouds appear more colourful, as more sunlight is scattered by the increased number of particles in the atmosphere.

Finally, air pollution can inhibit radiational cooling at night. Persistent contrails, which are caused by high humidity in the upper atmosphere, can cover much of the sky and reduce the amount of heat that escapes into space. This can lead to a minor increase in global temperatures.

Air pollution can negatively impact astronomical observation

Secondly, air pollution can alter the colour of the night sky. As the concentration of urban air pollutants increases, the night sky can shift towards red and orange hues. This is due to the scattering of light by pollutants, which affects the wavelengths of light that reach our eyes. While the enhanced colours at sunset may be visually appealing, they are a sign of poor air quality.

Thirdly, air pollution can contribute to light pollution, which affects astronomical observation. Artificial lighting from cities can illuminate the night sky, making it difficult to observe faint objects such as stars and galaxies. This is known as sky glow, and it is caused by light emitted from the ground reflecting off aerosols and other pollutants in the atmosphere.

Finally, air pollution can have indirect effects on astronomical observation by impacting the equipment used. For example, pollutants can accumulate on telescope lenses and mirrors, reducing their ability to collect light and degrading the quality of observations.

Overall, air pollution can have significant negative consequences for astronomy enthusiasts and professionals alike, limiting our ability to observe and understand the universe. Addressing air pollution is crucial not only for the health of people and the planet but also for the pursuit of scientific knowledge.

It can affect the colour of the night sky, shifting it towards red

Air pollution can affect the colour of the night sky, shifting it towards red. This phenomenon is known as "sky-glow" and is caused by the interplay of outdoor artificial light at night (ALAN) and atmospheric scattering. The more aerosol particles are present in the atmosphere, the more photons tend to scatter downwards, increasing the night sky brightness as seen from the ground. This effect is particularly noticeable near light sources.

A study in Hohhot, China, found that increasing urban air pollutant concentrations caused a shift in the night sky towards red. The glowing area of the night sky also expanded with higher levels of air pollution. Similar observations were made in Beijing, where the night sky turned red on polluted days.

The impact of air pollution on the colour of the night sky is influenced by the concentration and composition of aerosol particles. For example, smaller particles tend to interact with light more strongly than larger particles, and the effect is stronger for blue light compared to other colours.

Reducing air pollution, specifically aerosols, can decrease night sky brightness by tens of percent at relatively small distances from light sources. Cleaner air lowers aerosol optical depth, resulting in a darker night sky, particularly in directions towards light sources.

Air pollution can cause light pollution in the night sky

Light pollution is the adverse impact of artificial light at night. The most common form of light pollution is "sky glow", which is caused by artificial light being scattered by air molecules, moisture, and aerosols in the atmosphere. This gives the night sky a bright orange appearance.

The brightness of the night sky is strongly correlated with the concentration of particulate matter (PM) in the air. As the concentration of urban air pollutants increases, the glowing area of the night sky expands, and the sky shifts towards a redder colour.

Light pollution has several negative impacts on the environment, wildlife, and human health. It consumes a substantial amount of energy, and the light that is projected directly upwards from streetlights is about 30% of the total light emitted. This contributes to air pollution, as coal-fuelled power plants expel around 15 million tons of carbon dioxide pollution into the atmosphere each year to generate this amount of electricity.

Light pollution also affects human health. It disrupts the day-night cycle, suppressing melatonin levels in our brains and making it harder to fall asleep. It has been linked to sleep disorders, depression, hypertension, attention deficit disorder, obesity, diabetes, and heart disease.

Light pollution also affects wildlife. For example, sea turtle hatchlings instinctively head towards the brightest source of light, which used to be starlight reflecting off the ocean. Now, due to coastal cities flooding the night with bright lights, the hatchlings become confused and head towards the cities instead, falling prey to dehydration and predation.

To reduce light pollution, it is recommended to use fully shielded, light-efficient fixtures that prevent light from escaping and being wasted.

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