Air Pollution: Why Does It Turn Yellow Or Brown?

The colour of the sky can indicate the level of air pollution in a given area. For instance, an orange or yellow sky during the day or a red sky at night can be signs of high levels of air pollution. Atmospheric brown clouds, which are a result of elevated pollutant emissions, are another indication of poor air quality. These clouds are caused by emissions associated with the combustion of fossil fuels and biomass, and the brown colour results from the absorption and scattering of solar radiation by black carbon, fly ash, soil dust particles, and nitrogen dioxide.

The reddish-brown haze of smog

Smog is a type of air pollution that reduces visibility and gives a reddish-brown haze of colour over our cities. It is produced from car exhausts, coal power plants, factory emissions, and other sources. The reddish-brown haze of smog is caused by the absorption and scattering of solar radiation by black carbon, fly ash, soil dust particles, and nitrogen dioxide. These particles are emitted from the combustion of fossil fuels and biomass.

The presence of brown clouds of pollution over urban areas has been a concern for decades. Urban brown clouds are influenced by thermal inversions in the atmosphere and occur over several cities. Atmospheric brown clouds are a more widespread, regional phenomenon. They are caused by elevated pollutant emissions and a lengthy dry season that prevents aerosols from being removed from the atmosphere through precipitation.

In addition to the colour of the sky, air pollution can also be detected by smell. Ground-level ozone (O3) is a major component of smog with a pale blue colour and a pungent smell. O3 is formed from photochemical reactions of existing air pollutants like carbon monoxide (CO), NOx, and VOCs. Human sources of O3 include vehicles, fossil fuel power plants, oil and chemical refineries, and the agricultural sector.

Black carbon and organic carbon

The brown colour of atmospheric clouds is caused by the absorption and scattering of solar radiation by black carbon, fly ash, soil dust particles, and nitrogen dioxide. Black carbon is a form of ultrafine particulate matter, which when released into the air, causes premature human mortality and disability. It is produced during the incomplete combustion of carbon-containing fuels, such as fossil fuels, wood, and other biomass fuels, as well as waste. Black carbon emissions have increased in developing countries, with the largest sources being Asia, Latin America, and Africa. Solid fuels and kerosene used for cooking, lighting, and heating contribute significantly to global black carbon emissions.

Black carbon is a component of fine particulate matter (PM2.5) air pollution, the leading environmental cause of poor health and premature deaths. These particles are extremely small, measuring less than 1 micrometer in diameter, and can penetrate deep into the lungs, facilitating the transport of toxic compounds into the bloodstream. Long-term exposure to PM2.5 air pollution has been linked to various health issues, including lung and heart diseases, strokes, chronic respiratory diseases, and aggravated asthma.

Black carbon emissions can be reduced through the implementation of clean technologies and the adoption of alternative fuels. For example, the use of clean diesel, clean coal technologies, and second-generation technologies can help decrease black carbon emissions. Additionally, simple technologies such as clean cookstoves can significantly reduce indoor air pollution and improve the health and well-being of vulnerable communities.

Organic carbon, released alongside black carbon during incomplete combustion, contributes to air pollution. It forms part of the complex mixture of organic compounds that make up soot. Organic carbon emissions, along with black carbon, have increased due to rapid economic development, particularly in developing countries.

The reduction of black carbon and organic carbon emissions is crucial for mitigating climate change, improving air quality, and protecting human health. Strategies to decrease emissions can provide relatively quick climate and health benefits, especially in vulnerable communities that bear the brunt of the consequences of air pollution.

Nitrogen dioxide

NO2 is a significant component of smog and is responsible for the reddish-brown haze that reduces visibility in cities. This reddish-brown colour is caused by the absorption and scattering of solar radiation by NO2, along with other particles like black carbon, fly ash, and soil dust. The presence of NO2 and other pollutants in the atmosphere can alter the intensity and brilliance of sunsets and sunrises, making the sky appear yellow or orange during the day and red at night.

Outdoor sources of NO2 include emissions from vehicles such as cars, trucks, and buses, as well as power plants and industrial sites. Road traffic is the principal outdoor source of NO2, with levels typically higher on or near heavily travelled roadways and in dense urban areas. Indoor sources of NO2 include tobacco smoke and the burning of fuels like gas, wood, oil, kerosene, and coal using appliances such as stoves, ovens, and heaters, especially if they are not properly vented.

The health effects of NO2 pollution are well-documented. Breathing air with high concentrations of NO2 can irritate the airways and aggravate respiratory diseases, especially asthma. Prolonged exposure to elevated levels of NO2 may even contribute to the development of asthma and increase susceptibility to respiratory infections. Vulnerable subpopulations at higher risk from NO2 exposure include people with pre-existing medical conditions and people of colour.

While emissions of NO2 have decreased in recent years due to cleaner power plants, industrial sites, and vehicles, it remains a significant air pollutant. Nitrogen dioxide emissions can be further reduced through continued cleanup efforts and advocacy for clean air policies.

Soot and dust

Soot, also known as black carbon, is a major contributor to the brown colour observed in polluted air. It is a product of incomplete combustion processes, commonly associated with the burning of fossil fuels, such as coal, and biomass. Black carbon has a strong light-absorbing capacity, which means it can absorb and scatter solar radiation, including visible light. This absorption and scattering of sunlight by soot particles contribute to the reddish-brown haze often observed in smoggy conditions.

Dust particles, particularly those composed of soil, also contribute to the yellow or brown colour of polluted air. These fine soil particles can be suspended in the air due to various natural and human-induced factors. For example, dust storms can lift large amounts of dust into the atmosphere, affecting local and regional air quality. Human activities, such as construction and industrial processes, can also generate significant amounts of dust, contributing to airborne particulate matter. Similar to soot, these dust particles interact with sunlight, scattering and absorbing light in a way that influences its colour and intensity as it reaches our eyes.

The impact of soot and dust on the colour of polluted air is particularly evident during sunrise and sunset. At these times, the angle of the sun in the sky causes sunlight to travel through a greater distance in the Earth's atmosphere before reaching our eyes. As a result, the light interacts more strongly with the pollutants, enhancing their scattering and absorption effects. This can lead to the sun appearing more yellow or orange during the day and contributing to reddish-brown hues during sunsets and sunrises.

The presence of soot and dust particles in the air not only affects the colour of the sky but also has significant implications for climate and human health. These particles can remain suspended in the atmosphere for extended periods, forming atmospheric brown clouds. These clouds can extend to altitudes of around 3 kilometres (1.8 miles) and have been observed over various regions, including South Asia, the Indian Ocean, Africa, North America, South America, and Europe. Atmospheric brown clouds influence regional and global climatic patterns, affecting precipitation and agricultural production. Additionally, the fine particulate matter composed of soot and dust can pose risks to human health, contributing to respiratory issues and other adverse effects.

Sulfur dioxide

SO2 emissions that lead to high concentrations of SO2 in the air can also lead to the formation of other sulfur oxides (SOx). These gaseous SOx can react with other compounds in the atmosphere to form small particles, contributing to particulate matter (PM) pollution. These fine particles can reduce visibility, creating a reddish-brown haze that softens sky colors and reduces the total amount of light that reaches the ground. This changes the intensity and brilliance of sunsets and sunrises.

SO2 and other sulfur oxides can contribute to acid rain, which can harm sensitive ecosystems. The deposition of these particles can also stain and damage stone and other materials, including culturally important objects such as statues and monuments. Additionally, at high concentrations, SOx can harm trees and plants by damaging foliage and decreasing growth.

While sulfur dioxide itself does not have a yellow or brown color, the presence of high concentrations of SO2 and other sulfur oxides in the atmosphere can contribute to the formation of particulate matter that scatters sunlight, causing a reddish-brown haze and reducing visibility. This phenomenon is known as smog, which is a type of air pollution that is commonly observed in urban areas with high emissions from vehicles, power plants, and industrial facilities.

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