Air Pollution: Visible Signs And Their Impact

Air pollution is a pressing issue that poses a significant threat to both human health and the planet. It refers to the release of pollutants into the atmosphere, which can have detrimental effects on the natural characteristics of the air we breathe. One of the most visible indications of air pollution is haze, a white or brown veil that blurs our vision and reduces visibility. This haze is caused by particulate matter pollution, which includes tiny pollution particles that scatter sunlight, reducing the clarity and color of what we see. The effects of this pollution can be seen in many of our nation's scenic areas, where the visual range has been substantially reduced. Additionally, air pollution can also lead to smog, which forms when emissions from combusting fossil fuels react with sunlight, and soot, which is composed of tiny particles of chemicals, soil, smoke, dust, or allergens. These visible signs of air pollution are not just unsightly but also pose serious health risks, contributing to respiratory illnesses, decreased lung function, and even premature deaths worldwide.

Haze

To address the issue of haze, various interventions have been implemented, such as the Regional Haze Action Plan and the Agreement on Transboundary Haze Pollution in Southeast Asia, as well as visibility protection programs like IMPROVE in the United States, which aim to restore visibility in protected areas.

Smog

Primary pollutants are emitted directly from a source, such as sulfur dioxide from coal combustion, while secondary pollutants, such as ozone, are formed when primary pollutants undergo chemical reactions in the atmosphere. Photochemical smog, also known as "Los Angeles smog," occurs in urban areas with a high number of automobiles and does not require the presence of smoke or fog. It is caused by nitrogen oxide and hydrocarbon vapour emissions from automobiles and other sources, which undergo photochemical reactions in the lower atmosphere. The toxic gas ozone is produced from the reaction of nitrogen oxides with hydrocarbon vapours in the presence of sunlight.

The formation of smog is influenced by the season, with photochemical smog being more common in the summer due to warmer temperatures and increased sunlight. During the winter, there is an increase in coal and fossil fuel usage for heating, leading to higher emissions of combustion pollutants. The lack of pollutant dispersion under atmospheric inversions contributes to winter smog formation.

Efforts to reduce air pollution, such as implementing policies for sustainable land use, cleaner energy, and improved waste management, can help mitigate the formation of smog and its associated health and environmental consequences.

Soot

Policies and interventions to reduce soot pollution are crucial for protecting public health and mitigating the severe health impacts on vulnerable communities, including children, older adults, and people with pre-existing health conditions. Strengthening air quality standards and implementing cleaner technologies can effectively reduce soot pollution and improve health outcomes.

Sky colour

The sky's colour can be a visible indication of air pollution. Air pollution is the contamination of the indoor or outdoor environment by any chemical, physical, or biological agent that modifies the natural characteristics of the atmosphere. Haze, a common effect of air pollution, is caused when sunlight encounters tiny pollution particles in the air. Some light is absorbed by these particles, while other light is scattered before it reaches an observer. More pollutants mean more absorption and scattering of light, reducing the clarity and colour of what we see.

The colours we see in the sky come from sunlight scattered by molecules in the atmosphere. This process is called Rayleigh scattering. Nitrogen and oxygen constitute 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. Consequently, when more aerosols are in the atmosphere, more sunlight is scattered, resulting in more colourful skies.

Sunlight comprises all colours of light and appears white when all the colours are together. Different colours of light have different wavelengths: red light has long wavelengths, while blue light has short wavelengths. Light bounces off air molecules in the Earth's atmosphere, scattering in all directions. Blue light is scattered more than other colours because of its shorter, smaller waves.

Therefore, the presence of air pollutants can alter the sky's colour. For instance, a red sky at night, orange or yellow sky during the day, or fuzzy clouds could indicate high levels of air pollution. These colours result from the scattering of sunlight by pollutants in the atmosphere, creating a distinct haze that obscures the true colour of the sky.

Additionally, haze-causing particles can be directly emitted into the air or formed when gases create particles as they are carried by the wind over long distances from the pollution source. Sources of these particles include motor vehicles, industrial facilities, and forest fires.

Light scattering

The scattering of light by particles in the atmosphere is influenced by the size of the particles relative to the wavelength of light. When the particles are much smaller than the wavelength of light, as is the case with air molecules and small aerosols, Rayleigh scattering occurs. In this process, the electric field of the light wave interacts with the charges within the particles, causing them to become radiating dipoles that emit the scattered light. The intensity of Rayleigh scattering is inversely proportional to the fourth power of the wavelength, meaning shorter (blue) wavelengths are scattered more strongly than longer (red) wavelengths. This is why the sky appears blue due to the scattering of sunlight by tiny particles in the atmosphere.

On the other hand, when the particles are comparable in size or larger than the wavelength of light, the scattering is described by Mie theory. Mie scattering applies to spherical particles, such as cloud droplets, and results in the scattering of all wavelengths of visible light, creating a white appearance in clouds.

The presence of pollutants in the atmosphere, such as aerosols, can significantly impact light scattering. Aerosols are suspensions of liquid or solid particles in the air, excluding cloud droplets and precipitation. They can be formed through direct injection into the atmosphere, such as from human activities like burning fossil fuels, or through chemical reactions of gaseous materials. The concentration of aerosols is typically greater over continents than over oceans, and it is strongly influenced by the type of air mass, with higher concentrations in moist, tropical air masses.

The increase in aerosol particles in the atmosphere due to air pollution can lead to enhanced light scattering, contributing to the brightness of the night sky near cities, known as "skyglow." This phenomenon is a form of light pollution, which can have negative consequences for global biodiversity, nocturnal animal migration, and ground-based astronomy research.

Interventions to reduce light pollution, such as adopting modern lighting technologies and reducing aerosol pollution, can help mitigate the environmental impacts and restore the visibility of the night sky.

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