How Pollution Affects The Ground's Color

While pollution does not directly change the colour of the ground, it does alter the colour of the sky and sunsets. The ground is illuminated by the light that passes through the atmosphere, so the colour of the ground is influenced by the colour of the sky. Particulate matter in the air, such as aerosols, can scatter light of different wavelengths, changing the colour of the sky and the quality of light that reaches the ground. This is known as Rayleigh scattering. The size of the particles in the air is a determining factor in the colour of the sky, with larger particles tending to mute sunset colours. While some believe that pollution enhances the beauty of sunsets, others argue that excess pollution dampens the overall sunset experience.

Particulate matter and Mie scattering

The phenomenon of particulate matter in the atmosphere influencing the colour of the sky is known as Mie scattering. Mie scattering, also referred to as non-molecular or aerosol particle scattering, occurs in the lower 4,500 metres of the atmosphere, where many essentially spherical particles with diameters approximately equal to the wavelength of the incident ray may be present.

Mie scattering is particularly relevant when the size of the scattering particles is comparable to the wavelength of light, rather than much smaller or larger. In this range of particle sizes, the scattering differs from Rayleigh scattering in that it is roughly independent of wavelength and larger in the forward direction than in the reverse direction. The greater the particle size, the more light is scattered in the forward direction.

Aerosols, which are solid or liquid particles suspended in the air, are responsible for the red hues often seen at sunset in polluted skies. These particles can originate from both natural processes and human activity, with the latter contributing to air pollution. Examples of natural sources of aerosols include forest fires, mineral dust from sandstorms, sea spray, and volcanic eruptions.

The accuracy of particulate matter emission measurements in coal-fired power plants has been a focus of research, with the goal of meeting stricter environmental standards. Mie scattering theory has been applied to study the effects of particle size, refractive index, and scattering angle on the distribution of scattered light intensity. The results have helped to optimise the detection angle for real-time mass concentration measurements of particulates.

Nitrogen and oxygen molecules

The colour of the sky during sunrise and sunset is influenced by the scattering of light by nitrogen and oxygen molecules in the air. This phenomenon is known as Rayleigh scattering. While nitrogen and oxygen molecules can explain orange and reddish hues during sunset, other factors are at play for more vibrant red sunsets.

Nitrogen oxides, particularly nitric oxide (NO) and nitrogen dioxide (NO2), are significant contributors to air pollution. These gases are formed during high-temperature combustion when oxygen combines with nitrogen. Once NO is released into the atmosphere, it quickly reacts with oxygen to form NO2. Nitrogen dioxide, with its distinct red-orange colour, lends its hue to the reddish-brown haze of smog.

In addition to their role in smog formation, nitrogen oxides also contribute to acid rain and the depletion of the ozone layer. They can be produced naturally by lightning strikes or through human activities, such as the use of nitrogen-based fertilisers in agriculture. While nitrogen oxides have gained a negative reputation as pollutants, they also find beneficial applications in industrial processes. For instance, nitric oxide is manufactured on a large scale for the production of nitric acid (HNO3), which is used in creating chemical fertilisers, explosives, and other useful substances.

Aerosols and Rayleigh scattering

The colour of the sky is the result of sunlight interacting with molecules in the air, primarily nitrogen and oxygen, which causes it to be deflected in all directions, a phenomenon called Rayleigh scattering. This scattering of light by molecules or particles in the atmosphere is also influenced by the presence of aerosols, which are solid or liquid particles suspended in the air.

Aerosols can originate from both natural processes and human activity. Natural aerosols can be the result of forest fires, mineral dust from sandstorms, sea spray, and volcanic eruptions. Human activities, such as car emissions and industrial processes, also contribute to aerosol formation through the release of pollutants like soot, dust, and sea salt particles.

The scattering of light by aerosols is known as Mie scattering, which is a type of light scattering that occurs when particles have dimensions comparable to the wavelength of light. Mie scattering theory explains how larger particles, including aerosols, can reflect or refract radiation in all directions, resulting in phenomena like coronae, haloes, and rainbows.

The presence of aerosols in the atmosphere can have a significant impact on the appearance of the sky, especially during sunsets and sunrises. While Rayleigh scattering by nitrogen and oxygen molecules can explain the orange and reddish hues of sunsets, the addition of aerosols can intensify the red colours. This is because aerosols tend to scatter all colours of light, increasing the overall brightness of the sky while dampening colour contrast.

However, it is important to note that while pollution from aerosols can enhance the redness of sunsets, excessive pollution can also diminish the overall aesthetic experience. The accumulation of large particles in the atmosphere can make the sky appear brighter but washed out, reducing the purity of colours. Therefore, while pollution can indeed change the colour of the sky, the overall visual appeal may be negatively impacted when pollution levels become too high.

Volcanoes and forest fires

Volcanoes

Volcanic eruptions inject vast amounts of volcanic gas, aerosol droplets, and ash into the stratosphere, which can affect the climate. While the injected ash generally falls back to the stratosphere within days to weeks and has a minimal impact on climate change, volcanic gases can have a more lasting effect. Sulfur dioxide, for instance, can lead to global cooling, while carbon dioxide (CO2), a greenhouse gas, has the potential to cause global warming. Studies indicate that present-day volcanic CO2 emissions are far lower than those caused by human activities, with all estimates falling below 1% of human-induced CO2 emissions. However, it is believed that intense volcanic CO2 release in the distant past may have contributed to global warming and even mass extinctions.

Forest Fires

Forest fires, or wildfires, also have a significant impact on the ground and the atmosphere. They release a range of pollutants, including black carbon, carbon monoxide, nitrogen oxides, and particulate matter. These pollutants can combine with existing air pollution, exacerbating their harmful effects. Wildfires are often fueled by high temperatures, winds, and drought conditions, and their frequency and intensity are influenced by climate change. As forests burn, they release stored CO2 back into the atmosphere, contributing to the greenhouse effect. Additionally, the loss of forests reduces the Earth's capacity to absorb and sequester carbon dioxide.

Forest fires also leave behind long-lasting changes on the ground. Emissions from decomposing dead wood can persist for decades after a large fire. The regrowth process further influences the reflectivity of the land, known as albedo. Initially, lighter-colored grasses and shrubs reflect more solar radiation, creating a cooling effect. Over time, as the vegetation thickens and darkens, this effect diminishes.

Both volcanoes and forest fires contribute to pollution and climate change, impacting the ground and the atmosphere. While volcanic eruptions can have immediate and short-term effects, forest fires often lead to long-lasting changes in the environment, affecting ecosystems, air quality, and global climate patterns.

Human-made smog

Smog, also known as ground-level ozone, is a type of air pollution that poses serious risks to public health and the environment. It is caused by the combustion of fossil fuels, such as coal, gasoline, and natural gas, and is often associated with industrial activities, traffic congestion, and urban areas. Smog is a visible form of air pollution that can appear as a thick haze, giving the air a brownish tint due to high concentrations of nitrogen oxides.

The term "smog" was first used in London during the early 1900s to describe the combination of smoke and fog that frequently enveloped the city. Today, smog remains a significant issue in many urban areas, with at least half of the smog precursors coming from cars, buses, trucks, and boats in typical cities. Major smog events are often linked to heavy motor vehicle traffic, high temperatures, sunshine, and calm winds. The presence of smog can reduce visibility, leading to road accidents and flight cancellations.

The effects of smog on human health are extensive and range from short-term irritation of the eyes and trachea to long-term genetic mutations and respiratory, cardiovascular, and neurological disorders. Smog can also aggravate asthma, emphysema, chronic bronchitis, and other respiratory problems. Children, pregnant women, and senior citizens are particularly vulnerable to the harmful effects of smog. Additionally, the ozone in smog inhibits plant growth and can cause widespread damage to crops and forests.

The sources of smog pollutants are diverse and include automobile exhaust, power plants, factories, and consumer products such as paint, hairspray, charcoal starter fluid, chemical solvents, and foam plastic products. The formation of smog involves complex photochemical reactions between volatile organic compounds (VOCs), nitrogen oxides, and sunlight, resulting in ground-level ozone. While ozone in the stratosphere protects us from harmful ultraviolet radiation, ground-level ozone is harmful to human health and the environment.

The impact of smog has been observed globally, affecting both developing and under-developing countries. The Air Quality Index (AQI) is used to assess the air quality of a region, with higher AQI values indicating increased pollution levels. Environment protection agencies work towards improving air quality by implementing standards and regulations to monitor and control pollutant concentrations, especially during the COVID-19 pandemic when air pollution and the coronavirus had a combined negative impact on human health.

Frequently asked questions