Air Pollution: Can We See The Invisible?

Air pollution is a pressing issue that affects visibility and public health. Haze, a common manifestation of air pollution, is caused by sunlight interacting with tiny pollution particles in the air, resulting in reduced clarity and colour intensity of our surroundings. This haze can be attributed to various sources, including local traffic, industrial activities, and natural occurrences like dust storms and wildfires. While some air pollutants are invisible to the human eye, such as ozone and carbon dioxide, others form visible haze that obscures distant objects, diminishes contrast, and discolours the sky.

The impact of air pollution on visibility is significant, with scenic areas experiencing reduced visual range due to haze. For example, in eastern parks, the average visual range has decreased from 90 miles to 15-25 miles. Additionally, certain pollutants have been linked to respiratory illnesses, decreased lung function, and even premature death.

To address air pollution and its consequences, various techniques are employed, including human observation, light transmission measurements, digital photography, and satellite imaging. By understanding the sources and effects of air pollution, we can implement measures to protect visibility and public health, ensuring that we can continue to appreciate the beauty of our surroundings.

Haze and smog

Haze is an atmospheric phenomenon where dust, smoke, and other dry particles obscure the clarity of the sky. Haze is often the result of human activity, with sources including traffic, industry, coal-burning power plants, and wildfires. Natural sources can include windblown dust and soot from wildfires.

Haze is made up of very small, dry molecules in the air, including aerosols, dust, salt, and photochemical smog. These particles are so small that they cannot be seen individually with the naked eye, but they reduce horizontal visibility and give the atmosphere an opalescent appearance. Haze appears as a bluish or yellowish veil depending on the background.

Smog, on the other hand, is a type of air pollution and is a mixture of smoke, gases, and chemicals that makes the atmosphere appear thick and dark or yellow. It is caused by the burning of large amounts of coal, vehicular emissions, and industrial fumes. Smog is similar to haze but with condensation.

Both haze and smog can be observed from space through satellite images, which help identify large areas of pollution caused by fires, dust or sand storms, volcanic eruptions, industrial sources, or the transport of man-made pollution from other regions.

Natural vs man-made sources of air pollution

Haze in the sky is a common indicator of air pollution. This haze is caused by sunlight encountering tiny pollution particles in the air. Some light is absorbed by these particles, while other light is scattered before it reaches an observer. The more particles there are, the more light is absorbed and scattered, reducing the clarity and colour of what we see.

Air pollution comes from a variety of natural and man-made sources. Natural sources include windblown dust, soot from wildfires, organic compounds from plants, sea salt, suspended soils and dusts (like the dust from the Sahara Desert). Natural sources of air pollution can also include volcanic eruptions, which spew massive amounts of sulphur dioxide into the atmosphere, and forest fires, which release large amounts of harmful gases and smoke that can increase background pollution levels for years, even in areas far away from the original source.

Man-made sources of air pollution include motor vehicles, electric utilities, industrial fuel burning, and manufacturing operations. Mobile sources, such as cars, buses, planes, trucks, and trains, account for more than half of all air pollution in the United States, with automobiles being the primary source. Stationary sources, like power plants, emit large amounts of pollution from a single location. Area sources are made up of smaller pollution sources that can be significant when considered as a group. These include agricultural areas, cities, and wood-burning fireplaces.

How to measure air pollution

While air pollution is often invisible to the naked eye, there are several ways to measure it.

One way is through the use of satellites. Satellites can identify large areas of pollution caused by fires, dust or sand storms, volcanic eruptions, large industrial sources, or the transport of man-made pollution from other regions. They do this by measuring the concentration of particles (aerosols) in the atmosphere, observing how much light reaches the surface of the Earth and how much is reflected off the aerosols. This measurement is called aerosol optical depth or aerosol optical thickness.

Ground-based instruments are also used to collect information about what is in the air. These instruments are often outfitted with sensors designed to detect specific pollutants. Some use lasers to scan particulate matter density in a cubic metre of air. The data collected by these instruments is then processed to produce an Air Quality Index (AQI) reading. The AQI tracks five major air pollutants: particulate matter (PM2.5 and PM10), ground-level ozone, nitrogen dioxide, and sulfur dioxide. The greater the density of these pollutants in the air, the higher the AQI, which runs on a scale from 0 to 500, with 50 or below considered safe and readings above 100 deemed unhealthy.

In addition to satellite and ground-based instruments, low-cost air quality monitors can also be used to improve air quality management, especially in developing nations or areas that lack government-operated stations. These monitors are easier to deploy and have significantly reduced operational costs, making them a viable public alternative.

The impact of air pollution on visibility

Air pollution can significantly impact visibility, and this effect is perhaps the most apparent symptom of air pollution. The pollution creates a haze that affects how far we can see, and how well we are able to distinguish colours, forms, and textures of natural and historic vistas. This haze is caused when sunlight encounters tiny particles in the air, which scatter and absorb light before it reaches the observer. The more particles in the air, the more light is scattered and absorbed, reducing the clarity and colour of what we see.

The reduction in visibility caused by air pollution can have economic consequences, particularly for areas that rely on tourism. For example, haze affecting national parks can obscure the views that visitors expect to experience, potentially leading to a decrease in the number of visitors or shorter visits. Therefore, maintaining good visibility is important not only for the aesthetic value but also for the economic well-being of these areas.

The impact of air pollution on health

Air pollution is a mix of hazardous substances from both human-made and natural sources. It is a major threat to global health and prosperity, causing more than 6.5 million deaths each year, a number that has increased over the past two decades. The primary sources of human-made air pollution are vehicle emissions, fuel oils, natural gas used to heat homes, by-products of manufacturing and power generation, and fumes from chemical production. Natural sources include smoke from wildfires, ash and gases from volcanic eruptions, and gases like methane emitted from decomposing organic matter in soils.

The main pathway of exposure to air pollution is through the respiratory tract. Breathing in these pollutants leads to inflammation, oxidative stress, immunosuppression, and mutagenicity in cells throughout the body, impacting the lungs, heart, and brain, among other organs, and ultimately leading to disease. Almost every organ in the body can be affected by air pollution. Some pollutants are so small that they can penetrate the bloodstream via the lungs and circulate throughout the entire body, leading to systemic inflammation and carcinogenicity.

Specific disease outcomes that have been strongly linked to exposure to air pollution include stroke, ischaemic heart disease, chronic obstructive pulmonary disease, lung cancer, pneumonia, and cataract (household air pollution only). There is also suggestive evidence linking air pollution exposure to an increased risk of adverse pregnancy outcomes, other cancers, diabetes, cognitive impairment, and neurological diseases.

Fine particulate matter (PM2.5) is of particular concern as it can be inhaled deeply into the lungs and contribute to serious health problems. It accounts for most health effects due to air pollution in the US. Exposure to PM2.5 has been associated with an increased risk of death, respiratory infections, aggravated asthma, and reduced lung function. Long-term exposure increases the risk of diseases with a longer onset, such as stroke, heart disease, chronic obstructive pulmonary disease, and cancer.

Children, the elderly, and pregnant women are more susceptible to air pollution-related diseases. Additionally, certain racial and ethnic groups are disproportionately affected by air pollution due to systemic racism and the historical placement of sources of pollution in economically disadvantaged communities of color. Research has shown that people of color are more likely to be exposed to air pollution and to suffer harm to their health as a result.

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