Air Pollution: The Invisible Danger?

Air pollution is a pressing issue that affects both human health and the environment. It is caused by a complex mixture of gases and particles, which can come from natural sources such as forest fires and dust, as well as human activities like industrial processes, transportation, and agriculture. While some pollutants are invisible, air pollution often manifests as a haze that reduces visibility and the clarity and colour of what we see. This haze, also known as smog, is composed of various pollutants that scatter and absorb light, obscuring distant features and softening colours and textures. The impact of air pollution on visibility can be observed through tools like sun photometers and webcams, which help monitor changes in air quality over time.

Haze and smog

Haze is a common phenomenon where dust, smoke, and other dry particles are suspended in the air, reducing visibility and the clarity of the sky. It is often caused by industrial pollution, wildfires, and farming practices, and can have severe health and environmental impacts. For example, haze can cause respiratory issues and reduce solar photovoltaic production and agricultural yield. Haze is not limited to a specific geographic area and can be carried by wind over long distances, affecting air quality in adjacent regions.

In contrast, smog is a combination of the words "smoke" and "fog", and it specifically refers to a state of extreme air pollution caused by human activities such as burning fossil fuels and industrial processes. Smog contains gaseous components in addition to solid and liquid particles, resulting in grey, yellow, or brown shades in the air. It tends to manifest more locally, appearing as a low-hanging layer over cities. Smog has been a significant issue in cities like Los Angeles, Donora, and New York City, as well as Delhi and Ulaanbaatar, leading to strict regulations and efforts to improve air quality.

The distinction between haze and smog lies primarily in their composition and scale. Haze is typically characterised by the presence of dust and smoke particles, while smog includes gaseous components alongside particles. Haze often covers large geographical areas, while smog tends to be more localised. However, both phenomena can significantly reduce visibility and have detrimental health and environmental consequences.

To address the issues of haze and smog, various measures have been implemented. Internationally, the Association of Southeast Asian Nations (ASEAN) established the Regional Haze Action Plan (RHAP) and a coordination and support unit to tackle haze pollution. Similarly, all ASEAN countries signed the Agreement on Transboundary Haze Pollution in 2002. At the national level, agencies like the California Air Resources Board and the United States Environmental Protection Agency have enforced strict regulations on emission levels, leading to significant improvements in air quality.

In conclusion, haze and smog are both forms of air pollution that impair visibility and pose health risks. Haze is characterised by dust and smoke particles and can cover extensive areas, while smog is a mix of gases, solids, and liquids, typically forming over cities. Effective measures to combat haze and smog include international agreements, regional action plans, and stringent emission regulations. These efforts are crucial to mitigate the negative impacts of these phenomena on human health, the environment, and visibility.

Natural vs human sources

Air pollution is a complex mixture of gases and particles, which can be solid, liquid, or gas and often takes the form of an aerosol. It is the contamination of the indoor or outdoor environment by any chemical, physical, or biological agent that modifies the natural characteristics of the atmosphere. It is caused by a variety of natural and human sources.

Natural Sources

Natural sources of air pollution include wildfires, dust storms, and volcanic eruptions. Vegetation, in some regions, emits volatile organic compounds (VOCs) on warmer days. These VOCs react with human-made pollution sources to produce a seasonal haze of secondary pollutants. Black gum, poplar, oak, and willow are some examples of vegetation that can produce abundant VOCs. Volcanic eruptions produce mostly steam, carbon dioxide, sulfur dioxide, and small amounts of other pollutants, such as chlorine and ash particulates. Other natural sources include windblown dust and soot from wildfires.

Human Sources

Human sources of air pollution are often the result of industrial processes, the burning of fossil fuels for electricity and transport, waste management, and agriculture. The burning of fuels to produce electricity causes air pollution, with lignite and coal producing the most air pollution, followed by oil. The burning of fossil gas and biomass causes less air pollution. Methane leaks are common in oil and gas production. A 2014 study found that in China, the manufacturing and construction sectors contributed more than 50% of air pollution.

Other human 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, such as power plants, oil refineries, and industrial facilities, emit large amounts of pollution from a single location. Area sources, such as agricultural areas, cities, and wood-burning fireplaces, are made up of smaller pollution sources that can have a significant impact when considered as a group.

Visibility

The visibility of air pollution can vary depending on the source and the specific pollutants involved. Particulate matter pollution, or particle pollution, is a major cause of reduced visibility (haze) in many parts of the United States, including national parks. This haze is caused by the absorption and scattering of light by particles and gases in the atmosphere before it reaches the observer. As the number of fine particles increases, more light is absorbed and scattered, resulting in reduced clarity, color, and visual range. This can lead to a substantial reduction in visual range, as seen in many scenic areas.

Health and environmental problems

Air pollution is a mix of hazardous substances from both human-made and natural sources. It is the presence of one or more contaminants in the atmosphere, such as dust, fumes, gas, mist, odour, smoke or vapour, in quantities that can be harmful to human health and the environment.

In 2019, air pollution was the largest environmental risk to health globally, causing an estimated 6.5 million deaths each year. This number has increased over the past two decades. According to the World Health Organization (WHO), almost the entire global population (99%) breathes air that exceeds healthy levels. Low- and middle-income countries suffer the highest exposures.

Both short- and long-term exposure to air pollution can lead to a wide range of health issues. Fine particulate matter (PM2.5) is the air pollutant driving the most significant health problems and premature mortality. In 2021, 97% of the urban population was exposed to concentrations of PM2.5 above the health-based guideline level set by the WHO. Exposure to high levels of PM2.5 can lead to reduced lung function, respiratory infections, aggravated asthma, and an increased risk of developing non-communicable diseases, including stroke, heart disease, chronic obstructive pulmonary disease, and cancer.

Other health issues associated with air pollution include an increased risk of type 2 diabetes, obesity, systemic inflammation, Alzheimer's disease, and dementia. Air pollution has also been linked to behavioural problems, learning deficits, and lowered IQ in children, as well as high blood pressure and heart disease in adults.

In addition to the direct health impacts, air pollution also has significant environmental consequences. It contributes to climate change, ozone layer depletion, and environmental damage. For example, nitrogen dioxide and sulfur dioxide, which are byproducts of fossil fuel combustion, cause multiple adverse respiratory effects and are associated with increased hospital admissions for respiratory illnesses.

To address these health and environmental problems, interventions and initiatives that promote sustainable land use, cleaner household energy, improved transport, energy-efficient housing, and better waste management are crucial. The WHO and other organizations have developed strategies to raise awareness and implement solutions to mitigate the risks associated with air pollution.

Light absorption and scattering

Light scattering and absorption by aerosols play a crucial role in determining visibility, which is influenced by both the physical interaction of light with particles and gases, as well as the psychophysical processes of human perception.

Light Scattering by Aerosols

Aerosols are fine particles suspended in the air, which can include pollution particles such as sulfate, nitrate, ammonium, carbonaceous particles, and organic carbon. When light encounters these particles, it can be scattered in different directions, reducing the amount of light that reaches our eyes. This scattering process affects the colours we perceive, with particles scattering sunlight and diluting colours, resulting in a hazy appearance on polluted days. The scattering coefficient is used as a proxy for particulate matter mass concentration, and it is influenced by various city-specific conditions such as latitude, altitude, and local meteorological conditions.

Light Absorption by Aerosols

In addition to scattering, aerosols can also absorb light. Carbonaceous particulates, for instance, predominantly absorb rather than scatter visible light. This absorption process involves converting photons into thermal energy, making it challenging to detect absorption by aerosols. The measurement of aerosol light absorption is complex and can be influenced by factors such as particle concentration, particle deposition, and optical interaction with filters. Techniques like laser-induced incandescence and photoacoustic measurements help quantify aerosol light absorption by monitoring particle heating during absorption.

Impact on Visibility

The combination of light scattering and absorption by aerosols in the atmosphere leads to impaired visibility. Haze, a common consequence of air pollution, is a result of fine particles and gaseous pollutants scattering and absorbing light. This scattering and absorption reduce the amount of light that reaches our eyes, affecting our ability to see distant objects, diminishing contrast, and discolouring the sky. The impact of haze on visibility can vary depending on climatic conditions, with visual ranges being greater in dry climates compared to humid climates.

Mitigation Strategies

To address the issue of air pollution affecting visibility, various interventions can be implemented. Reducing air pollution, specifically aerosols, has been shown to decrease night sky brightness by tens of percent near light sources. Additionally, adopting modern lighting technologies and reducing light pollution can have positive environmental consequences, including the preservation of global biodiversity and nocturnal animal migration.

Air quality measurement

Air pollution is broadly measured in two ways: passively or actively. Passive devices are relatively simple, low-cost, and work by soaking up or otherwise passively collecting a sample of the ambient air, which then has to be analysed in a laboratory. Deposit gauges, for example, are large funnels that collect soot or other particulates and drain them into sampling bottles. Active measurement devices, on the other hand, tend to be more complex and sophisticated than passive devices, though they are not always more sensitive or reliable. They use fans to suck in the air, filter it, and either analyse it automatically on the spot or collect and store it for later analysis in a laboratory. Active sensors use either physical or chemical methods.

Air quality is reported using the Air Quality Index (AQI), which is a way of showing changes in the amount of pollution in the air. The AQI is divided into six colour-coded categories, each corresponding to a range of index values. The higher the AQI value, the greater the level of air pollution and the greater the health concern. For example, an AQI value of 50 or below represents good air quality, while an AQI value over 300 represents hazardous air quality. The AQI is based on the measurement of particulate matter (PM2.5 and PM10), ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2), and carbon monoxide (CO) emissions.

Air quality databanks process readings from governmental, crowd-sourced, and satellite-derived air quality monitors to produce an aggregated AQI reading. Satellites orbiting above Earth monitor the contents of our air, while air quality monitors on the ground are outfitted with sensors designed to detect specific pollutants. Some use lasers to scan particulate matter density in a cubic metre of air, while others rely on satellite imaging to measure energy reflected or emitted by the Earth.

The World Health Organization (WHO) has been compiling and publishing ground measurements of air quality and the annual mean concentrations of particulate matter since 2011. The WHO Ambient Air Quality Database compiles data on ground measurements of annual mean concentrations of nitrogen dioxide (NO2), particulate matter of a diameter equal to or smaller than 10 μm (PM10) or equal to or smaller than 2.5 μm (PM2.5). The database is updated regularly every 2-3 years and is used to derive the Sustainable Development Goal Indicator 11.6.2, Air quality in cities.

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