Air Pollution: Understanding The Hazards And Impacts

APES Chapter 18 focuses on air pollution, its causes, and its effects on human health and the environment. Air pollution is caused by the presence of harmful chemicals in the atmosphere, which can be released by natural sources such as volcanic eruptions and wildfires, or human activity, including the burning of fossil fuels and industrial processes. The chapter covers the different layers of the Earth's atmosphere, including the troposphere, where most of the air we breathe is found, and the stratosphere, which contains the ozone layer that protects life on Earth by absorbing UV radiation. It also discusses the health impacts of air pollution, such as respiratory problems and increased risk of cancer, as well as strategies to reduce pollution, like stricter emission standards and the distribution of efficient cook stoves in developing countries.

Natural and anthropogenic sources of air pollution

Natural sources of air pollution include wind-blown dust, pollutants from wildfires and volcanic eruptions, and volatile organic chemicals released by some plants. Volcanic eruptions, for instance, release carbon and nitrogen emissions into the atmosphere.

Anthropogenic sources of air pollution, on the other hand, are largely a result of human activities such as industrialization and urbanization. The burning of fossil fuels in power plants, industrial facilities, and motor vehicles contributes significantly to air pollution. Specifically, the combustion of coal and oil leads to the release of carbon dioxide, carbon monoxide, and sulfur dioxide, among other pollutants.

In addition, the use of fossil fuels and natural gases, forest fires, fracking, flame retardants, and aerosols further exacerbate air pollution. For example, the presence of chlorofluorocarbons (CFCs) in aerosol cans increases the levels of harmful ozone in the troposphere, particularly in urban areas. The troposphere, being the lowest level of the Earth's atmosphere, is where tropospheric ozone is formed as an indirect result of air pollution from automobile exhaust and industrial emissions.

Furthermore, human activities such as burning coal to generate electricity, industrial processes, and vehicle emissions contribute to the formation of photochemical smog. This type of smog is a mixture of primary and secondary pollutants influenced by UV radiation, resulting in the reddish-brown haze often observed in cities with warm, sunny, and dry climates.

To address air pollution, various methods and regulations have been proposed and implemented. These include emission control devices, regular car exhaust inspections, and stricter emission standards. Additionally, economic incentives, such as taxing the production of air pollutants, can encourage the reduction of emissions. The distribution of efficient cook stoves and the transfer of renewable energy and pollution control technologies to developing countries are also suggested strategies to combat air pollution.

Carbon oxides: carbon monoxide and carbon dioxide

Carbon monoxide (CO) is a highly toxic, colourless, and odourless gas that is formed during the incomplete combustion of carbon-containing materials. It is emitted from sources such as motor vehicle exhaust, the burning of forests and grasslands, fossil fuel-burning power plants, tobacco smoke, and open fires or inefficient cooking stoves. Long-term exposure to carbon monoxide can trigger heart attacks and aggravate lung diseases such as asthma and emphysema.

The chemical reaction for carbon monoxide is:

C + O2 → CO

Carbon dioxide (CO2) is also a colourless and odourless gas, and about 93% of atmospheric CO2 is the result of the natural carbon cycle. However, human activities, particularly the burning of fossil fuels and deforestation, have significantly increased the concentration of CO2 in the atmosphere, leading to global warming and climate change.

The sources of carbon dioxide include both natural and anthropogenic processes. Natural sources include volcanic eruptions, while anthropogenic sources are primarily associated with the burning of fossil fuels, industrial facilities, power plants, and motor vehicles. The chemical reaction for carbon dioxide formation is:

2C + O2 → 2CO2

To mitigate the emissions of carbon monoxide and carbon dioxide, various measures have been implemented, including the Clean Air Acts of 1970, 1977, and 1990, which set standards for acceptable levels of air pollutants from vehicles, transportation, and industries. Additionally, the EPA has established national emission standards and regulations to reduce the release of hazardous air pollutants.

Nitrogen oxides and nitric acid

Nitrogen is the most abundant gas in the atmosphere, constituting 79% of air volume. While most nitrogen is present as inert nitrogen gas (N2), various processes, including oxidation by bacteria, lightning, organic protein decay, and high-temperature combustion, can lead to the formation of different nitrogen compounds. Among these compounds, nitric oxide (NO), nitrogen dioxide (NO2), ammonia (NH3), and, to a lesser extent, nitrous oxide (N2O) are the most significant due to their health effects and reactivity.

Nitrogen oxides (NOx) are a group of highly reactive gases that include nitrogen dioxide (NO2), nitrous acid, and nitric acid. NOx compounds are converted into reddish-brown smog, commonly referred to as "brown-air smog." This phenomenon is particularly prevalent in modern cities with warm, sunny, and dry climates and a high number of motor vehicles, such as Los Angeles.

The primary sources of NO and NO2 pollutants are high-temperature combustion processes, where nitrogen in fuel sources like coal and oil is oxidized to form nitrogen oxides. During combustion, approximately 5-40% of the nitrogen in coal and 20-100% of the nitrogen in oil is converted into nitrogen oxides. NO is further oxidized to NO2 in stack gas or diluted plume conditions. The reaction of NO with tropospheric ozone (O3) also rapidly forms NO2.

Gasoline-powered vehicles are the predominant source of transportation-related NOx emissions, contributing 32% of total NOx and 82% of transportation-related NOx. Industrial processes, particularly the manufacturing and use of nitric acid, petroleum refining, electroplating, engraving, welding, and metal cleaning, are also significant contributors to NOx emissions. In highly populated areas, NOx emissions can be substantial, with about 77% of total NOx emissions occurring in these regions.

Nitrogen dioxide (NO2) is a harmful pollutant that primarily enters the air through the burning of fuel. It is produced by emissions from cars, trucks, buses, power plants, and off-road equipment. Exposure to high concentrations of NO2 can irritate the human respiratory system, exacerbating respiratory diseases like asthma and leading to symptoms such as coughing, wheezing, and difficulty breathing. Prolonged exposure to elevated NO2 levels may even contribute to the development of asthma and increased susceptibility to respiratory infections.

Sulfur dioxide and sulfuric acid

Sulfur dioxide (SO2) is one of the six common air pollutants known as criteria pollutants. It is produced when fossil fuels like coal and oil are burned, and it is a major component of emissions from power plants, metals processing and smelting facilities, and vehicles. Diesel vehicles, in particular, were once a significant source of SO2, but federal regulations to reduce the sulfur content in diesel fuels have significantly lowered emissions.

SO2 emissions contribute to the formation of other sulfur oxides (SOx), which can have harmful effects on human health and the environment. High concentrations of SO2 and SOx in the air can lead to the formation of small particles, contributing to particulate matter (PM) pollution. These particles can penetrate deeply into the lungs and cause respiratory issues, especially for children, the elderly, and those with pre-existing conditions. Longer exposures can aggravate existing heart and lung conditions.

Additionally, SO2 and SOx contribute to the formation of acid rain, which can damage trees, plants, and sensitive ecosystems. Acid rain occurs when SO2 and SOx react with other compounds in the atmosphere and then mix with water droplets in the air, forming sulfuric acid. This acid rain falls to the ground and can harm vegetation, waterways, and even cultural monuments. The deposition of sulfur oxides on soil may also be converted back into hydrogen sulfide by anaerobic bacteria and released into the atmosphere, creating a continuous cycle.

To address the harmful effects of SO2 and sulfuric acid, the EPA has established national ambient air quality standards (NAAQS) and regional rules to reduce emissions. These regulations aim to protect human health and the environment by reducing exposure to SO2 and other sulfur oxides. As a result, SO2 emissions from coal have been cut in half, and the use of low-sulfur fuels has helped most urban areas achieve the US standard of 0.03 ppm for SO2 concentrations.

Photochemical and industrial smog

Industrial smog is formed when coal and heavy oils are burned. This process releases an unhealthy mix of sulfur dioxide, suspended droplets of sulfuric acid, and a variety of suspended solid particles into the air. When coal and oil are burned, the carbon in them is converted to carbon monoxide and carbon dioxide. The sulfur compounds in the coal and oil react with oxygen to produce sulfur dioxide gas, which can then become tiny suspended droplets of sulfuric acid. These droplets can then react with ammonia in the atmosphere to form solid particles of ammonium sulfate. The suspended particles of salt and soot give the resulting smog a grey colour, which is why it is sometimes called grey-air smog.

Photochemical smog, on the other hand, is a mix of primary and secondary pollutants that are formed under the influence of sunlight. The formation of photochemical smog begins when exhaust from morning commuter traffic releases large amounts of NO and VOCs into the air over a city. The NO is converted to reddish-brown NO2, which is why photochemical smog is sometimes called brown-air smog. When exposed to ultraviolet radiation from the sun, NO2 reacts with VOCs released by certain trees, motor vehicles, and businesses. The resulting smog is a mixture of ozone, nitric acid, aldehydes, peroxyacyl nitrates (PANs), and other secondary pollutants. Collectively, NO2, O3, and PANs in this chemical brew are called photochemical oxidants because they can react with and oxidize certain compounds in the atmosphere or inside our lungs.

High temperatures, urban buildings, and hills or mountains can all promote the chemical reactions that lead to the formation of photochemical smog. Urban buildings slow wind speed and reduce the dilution and removal of pollutants, while hills and mountains reduce the flow of air in valleys, allowing pollutant levels to build up at ground level. Certain trees and plants in heavily wooded urban areas can also play a large role in the formation of photochemical smog, as they emit volatile organic compounds (VOCs).

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