Meghan Hodges
The colour of the sky can be a good indicator of the level of pollution in the air. Smog, for example, is a type of pollution that gives the sky a reddish-brown haze and reduces visibility. Ground-level ozone, another pollutant, has a pale blue colour and a pungent smell. The Air Quality Index (AQI) is a tool used to communicate about outdoor air quality and health concerns. It features six colour-coded categories, with green representing good air quality and maroon indicating hazardous air quality.
| Characteristics | Values |
|---|---|
| Ground-level ozone | Pale blue |
| Smog | Reddish-brown |
| Air Quality Index (AQI) | Green, yellow, orange, red, purple, maroon |
| AQI value below or at 50 | Good air quality |
| AQI value above 100 | Unhealthy air quality |
| AQI value above 300 | Hazardous air quality |
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What You'll Learn
- Smog: reddish-brown haze over cities, indicating poor air quality
- Ground-level ozone: pale blue, a major component of smog, with a pungent smell
- Air Quality Index: uses green, yellow, orange, red, purple and maroon to indicate air quality
- Sunlight: when polluted, the sun appears orange or yellow
- Clouds: pollution makes clouds look fuzzier and less distinct
Smog: reddish-brown haze over cities, indicating poor air quality
The reddish-brown haze that hangs over many cities is a tell-tale sign of poor air quality. This phenomenon, known as smog, is a type of air pollution that reduces visibility. The term "smog" was coined in the 1900s, combining the words "smoke" and "fog".
Smog has two primary types: photochemical and sulphurous. Photochemical smog is prevalent in urban areas due to its formation from car exhausts, coal power plants, factory emissions, and other sources. It arises when nitrogen oxides (NOx) and volatile organic compounds (VOCs) react with sunlight. Nitrogen oxides, which comprise nitrogen dioxide (NO2) and nitric oxide (NO), are brownish-red in colour and contribute to the reddish-brown hue of smog. NO2, with its pungent smell, is a significant health hazard, causing adverse effects on allergic asthmatics, premature death, cardiopulmonary issues, and reduced lung function growth in children.
Sulphurous smog, on the other hand, results from high concentrations of sulphur oxides (SOx) in the atmosphere, stemming from fossil fuels like coal that have a high sulphur content. This type of smog is more prevalent in damp conditions and during winter due to the absence of wind.
While smog typically conjures images of a reddish-brown haze, it can also manifest as black smog, which originates from soot and smoke, or even blue smog, which is primarily composed of ground-level ozone (O3). O3, a major component of smog, is a pale blue gas with a pungent odour, formed through photochemical reactions involving strong sunlight, UV radiation, and existing pollutants like carbon monoxide (CO), NOx, and VOCs.
The presence of smog and its varying colours underscores the importance of monitoring air quality to safeguard public health. The U.S. Air Quality Index (AQI) is a tool used to communicate outdoor air quality and associated health risks. Each category in the AQI is colour-coded, enabling people to quickly assess the air quality in their communities and take necessary precautions.
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Ground-level ozone: pale blue, a major component of smog, with a pungent smell
While there is no single colour that identifies with pollution, ground-level ozone, a major component of smog, is pale blue in colour and has a pungent smell. Ground-level ozone is a harmful air pollutant that has adverse effects on both human health and the environment.
Tropospheric, or ground-level, ozone is formed by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOCs). These pollutants are emitted by cars, power plants, industrial boilers, refineries, and other sources. Ground-level ozone is a secondary pollutant, as it is not emitted directly into the air but is created when primary pollutants react in sunlight and stagnant air.
Ground-level ozone is particularly harmful to human health, especially for children, the elderly, and people with lung diseases such as asthma. It can trigger a variety of health problems, including breathing difficulties, reduced lung function, and lung diseases. Ozone exposure has also been linked to pre-mature mortality and increased hospital admissions.
In addition to its impacts on human health, ground-level ozone can also affect vegetation and ecosystems. It can harm sensitive vegetation during the growing season, decrease crop productivity, and contribute to forest decline. Furthermore, ozone can damage synthetic materials, cause cracks in rubber, and accelerate the deterioration of paints and coatings.
Ozone levels are typically higher during sunny weather, especially in urban environments. However, wind can transport ozone over long distances, allowing even rural areas to experience high ozone levels. To protect human health and the environment, it is crucial to monitor and regulate ground-level ozone concentrations.
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Air Quality Index: uses green, yellow, orange, red, purple and maroon to indicate air quality
The Air Quality Index (AQI) is a tool used to communicate about outdoor air quality and health. It uses colours, numbers and words to indicate the quality of the air. The AQI uses six colours: green, yellow, orange, red, purple and maroon, each indicating a different level of health concern. The colours make it easy for people to quickly determine whether the air quality is reaching unhealthy levels in their communities.
Green is the best air quality and indicates that the air is clean. Yellow, orange and red are often seen in the summer when air quality isn't at its best. Maroon and purple are the worst air quality levels. Maroon has not been reached in many years due to efforts to clean up the air.
The AQI scale runs from 0 to 500. An AQI value of 50 or below represents good air quality, while a value over 300 indicates hazardous air quality. AQI values at or below 100 are generally thought of as satisfactory. Values above 100 indicate that the air quality is unhealthy for certain sensitive groups of people, and as the value increases, it becomes unhealthy for everyone.
Each of the six categories of the AQI corresponds to a different level of health concern and a specific colour. This colour-coding system makes it easy for people to quickly assess the air quality in their communities and take appropriate actions to protect their health. The AQI is an essential tool for raising awareness about air pollution and its potential health impacts.
The AQI is determined by measuring pollution in the air and then using an index to communicate this information to the public. This index includes colour-coded categories, each corresponding to a range of index values. The specific colours used in the AQI are chosen to indicate the severity of the air quality, with green representing the best air quality and maroon representing the worst.
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Sunlight: when polluted, the sun appears orange or yellow
The colour orange is often associated with pollution. This is because, during sunrise or sunset, when the sun is close to the horizon, sunlight has to travel through more of the atmosphere than when the sun is overhead. This results in more scattering of light, including longer wavelengths such as yellow, orange, and red, which creates colourful skies.
The phenomenon is called Rayleigh scattering, named after British physicist John William Strutt, also known as Lord Rayleigh. When the sun is high in the sky, the light has to travel a shorter distance through the atmosphere. This means that most of the yellow, orange, and red light passes through, while a small amount of blue and purple light is scattered and removed from the mix. The sun, therefore, looks yellow to us here on Earth.
During sunsets and sunrises, when the sun is closer to the horizon, sunlight has to travel through a longer distance and more of the dense environment to reach an observer's eye. Due to Rayleigh scattering, most of the light of shorter wavelengths – blue, violet, and green – are scattered away multiple times, leaving only lights of longer wavelengths – red, orange, and yellow.
The most heavily polluted cities in the world tend to have more orange and red sunsets, resulting from an abundance of human-made aerosols. Intense red sunsets are often visible when forest fires are burning nearby or when volcanic eruptions happen. Although pollution can create more colourful sunsets, too much pollution will dampen the overall sunset experience. Dust and pollution can reduce the vibrancy of sunsets, and the sky takes on dull hues of red and yellow when the air is full of dust and pollutants.
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Clouds: pollution makes clouds look fuzzier and less distinct
While there is no one colour that identifies with pollution, the US Air Quality Index (AQI) is a tool that uses colours to communicate the level of outdoor air quality and the associated health concerns. An AQI value of 50 or below represents good air quality, while an AQI value over 300 represents hazardous air quality. Each category has a specific colour to help people quickly determine the air quality in their communities.
Regarding the appearance of clouds, pollution can indeed make clouds look fuzzier and less distinct. When pollution is high, particles in the air scatter sunlight, causing objects to appear fuzzier and less defined. This effect is visible in clouds, where the edges become less sharp and distinct against the sky. This phenomenon can be observed in heavily polluted cities such as Beijing, where clouds can appear noticeably fuzzier on highly polluted days compared to clearer days with better air quality.
In a study by climate scientist Dr. Qing Yang and the PNNL team, they investigated the impact of different types of particles in the atmosphere on clouds. They found that additional pollution in clean regions of the ocean had a more significant effect on clouds than in already polluted regions. Specifically, they observed that pollution particles attracted water, creating bright and reflective droplets in the clouds, making them appear brighter and hold more water.
The impact of pollution on cloud characteristics is also evident in the formation of "ship tracks". These are cloud trails caused by pollution particles from ship exhaust, particularly off the western coast of Peru. The numerous particles attract water, forming bright and reflective streaks of clouds that stand out against neighbouring clouds. Thus, pollution can significantly alter the appearance of clouds, making them look fuzzier, brighter, and less distinct from the surrounding sky.
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Frequently asked questions
The EPA's Air Quality Index (AQI) uses a colour-coded scale to inform the public about air pollution levels and potential health risks. The AQI features six colour-coded categories: green, yellow, orange, red, purple, and maroon. Each category has corresponding values and levels of concern.
Smog, a type of pollution, gives a reddish-brown haze of colour over cities. Ground-level ozone (O3), an air pollutant, has a pale blue colour.
Maroon indicates hazardous air quality (301+ AQI).
