Sources Of Six Criteria Air Pollutants: An Overview

The Clean Air Act requires the Environmental Protection Agency (EPA) to set National Ambient Air Quality Standards (NAAQS) for six common air pollutants, also known as criteria air pollutants. These pollutants are found all over the United States and can harm human health, the environment, and cause property damage. The six criteria air pollutants include particle pollution, ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. These pollutants are typically released into the atmosphere through the burning of fossil fuels, industrial processes, and natural sources such as wildfires and volcanic eruptions.

Particulate matter sources include charbroiling, construction equipment, and vehicle exhausts

Particulate matter is a major source of outdoor air pollution, especially in highly populated areas. It is classified by size, with diameters of 10.0, 2.5, and 0.1 µm corresponding to PM10, PM2.5, and PM0.1, respectively. The larger particles, such as PM10, are trapped in the airways, while smaller particles, such as PM2.5 and ultra-fine particles (UFP), can pass through barriers and distribute throughout the body, potentially causing inflammation and oxidative stress in tissues. Diesel exhaust (DE) is a primary component of traffic-related PM, especially in the smaller, more harmful PM2.5 and UFP categories.

Charbroiling meat is another source of particulate matter. Smoke from meat charbroiling has a major peak in particle mass distribution at 0.1-0.2 μm particle diameter, with some larger particles also present. This means that charbroiled meat produces a significant amount of the smaller, more harmful particulate matter that can cause health issues.

Construction activities and equipment contribute to air pollution through the emission of particulate matter, volatile organic compounds, and other harmful pollutants. Construction is responsible for a large proportion of particulate matter in the air, with equipment travel, on-site material handling, and equipment exhaust all contributing to emissions.

Vehicle exhaust, particularly from diesel engines, is a significant source of particulate matter. The particulate matter from car exhausts has been shown to alter the function of human iPSC-derived microglia, which are the immune cells of the brain implicated in the progression of neurodegenerative diseases. Large-scale epidemiological studies have associated traffic-related particulate matter with impaired cognitive functions and an increased incidence of Alzheimer's disease.

Ground-level ozone is created by chemical reactions between nitrogen oxides and volatile organic compounds

Ground-level ozone is a harmful pollutant that is formed by chemical reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs). These reactions occur in the presence of sunlight, particularly in the UV spectrum, and result in the production of ground-level ozone, which is a significant health and environmental concern.

Nitrogen oxides (NOx) are produced through high-temperature combustion processes. Significant sources of NOx include power plants, industrial furnaces, boilers, and motor vehicles. The combustion of fossil fuels is a major contributor to the generation of NOx.

On the other hand, volatile organic compounds (VOCs) have both natural and anthropogenic sources. Natural sources of VOCs include emissions from vegetation and biological degradation processes. Anthropogenic sources, on the other hand, include chemical plants, gasoline pumps, oil-based paints, auto body shops, and print shops. Methane, a potent greenhouse gas, is also considered a VOC and contributes to ozone formation on a global scale.

When NOx and VOCs are present in the atmosphere, they undergo photochemical reactions that lead to the formation of ground-level ozone. These reactions are influenced by factors such as sunlight intensity, temperature, and the presence of other pollutants. Ground-level ozone formation typically occurs during the afternoon when sunlight is most intense, but it can also occur in the evening or at night due to the transport of ozone precursors by wind.

Ground-level ozone is a major health concern as it can trigger a variety of health problems, especially for vulnerable individuals such as children, the elderly, and people with lung diseases like asthma. Breathing ozone can lead to chest pain, coughing, throat irritation, airway inflammation, reduced lung function, and harm to lung tissue. It can also worsen respiratory conditions such as bronchitis, emphysema, and asthma.

Carbon monoxide is produced by the incomplete combustion of fossil fuels

Carbon monoxide (CO) is a colourless, odourless, and highly toxic gas that forms when carbon in certain fuels does not burn completely. Fuels that commonly feature carbon include gasoline, heating oil, natural gas, coal, charcoal, wood, and other fossil fuels. When there is a plentiful supply of oxygen, carbon in these fuels combines with oxygen (O2) during combustion to produce carbon dioxide (CO2) and water (H2O). However, when there is an insufficient supply of oxygen, or when combustion is disrupted, carbon does not fully oxidise, instead producing soot or carbon monoxide.

Carbon monoxide is dangerous to human health, even at low concentrations. It is a poisonous gas that can lead to death. It is particularly harmful because it is attracted to haemoglobin in the blood over 200 times more strongly than oxygen. Therefore, when carbon monoxide enters the bloodstream, it prevents red blood cells from carrying sufficient oxygen, which can have fatal consequences.

Carbon monoxide is released into the atmosphere through the incomplete combustion of fossil fuels. This can occur in a variety of ways, often as a result of human activity. For example, motor vehicle exhaust contributes roughly 60% of all carbon monoxide emissions nationwide, and up to 95% in cities. Air concentrations of carbon monoxide can be particularly high in areas with heavy traffic congestion. Carbon monoxide emissions from vehicles tend to be higher during cold winter weather, when vehicles burn fuel less efficiently, and when a strong inversion layer in the atmosphere traps pollution near the ground.

Other sources of carbon monoxide emissions include industrial boilers, waste incinerators, and natural events such as wildfires. Simple actions, such as placing a cold pan on a stove burner or blowing air from a fan across an unvented burner, can also increase carbon monoxide production.

Sulfur dioxide is emitted by power plants and industrial facilities burning sulfur-containing fossil fuels

The Clean Air Act requires the EPA to set National Ambient Air Quality Standards (NAAQS) for six commonly found air pollutants known as criteria air pollutants. These pollutants are particulate matter, ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. Sulfur dioxide (SO2) is one of the six criteria air pollutants and is emitted by power plants and industrial facilities burning sulfur-containing fossil fuels.

SO2 is a major air pollutant and is released into the atmosphere through the burning of fossil fuels, primarily coal and oil. Coal contains impurities of sulfur and coal combustion accounts for almost all of the sulfur emissions from electric utilities. In the Midwest, coal combustion makes up nearly 99% of the sulfur emissions from electric utilities. Sulfur emissions form some of the most harmful and environmentally damaging pollutants in the air. Uncontrolled power plants release twice as much sulfur into the air as cars, factories, and trucks combined.

Power plants are the largest source of SO2 emissions. In addition to power plants, industrial facilities such as factories and boilers also emit SO2 by burning fossil fuels with a high sulfur content. Smaller sources of SO2 emissions include industrial processes such as extracting metal from ore, natural sources such as volcanoes, and vehicles such as locomotives, ships, and other heavy equipment that burn fuel with a high sulfur content.

The release of SO2 into the atmosphere has significant negative consequences for both human health and the environment. SO2 and other sulfur oxides contribute to acid rain, which can harm sensitive ecosystems. These sulfur oxides can also react with other compounds in the atmosphere to form fine particles that reduce visibility, often resulting in hazy parklands and city skylines. Furthermore, the deposition of these particles can stain and damage various materials, including culturally significant objects such as statues and monuments.

Nitrogen dioxide is formed from emissions by cars, trucks, power plants, and industrial equipment

Nitrogen dioxide (NO2) is one of a group of highly reactive gases known as nitrogen oxides (NOx). It is formed when a car engine runs, causing combustion by heating up quickly. This process acts as a catalyst that binds together nitrogen and oxygen to form nitric oxide (NO) or nitrogen dioxide (NO2). Diesel engines, which operate at higher temperatures and pressures than petrol engines, produce stronger and more abundant nitrogen oxide. However, it is important to note that petrol engines also produce nitrogen oxide, albeit in smaller quantities.

Cars, trucks, and other on-road vehicles are significant contributors to nitrogen dioxide emissions. The direct emission of nitrogen dioxide from road vehicle exhaust has been a critical factor in the near-road ambient concentrations of NO2 in many urban areas, particularly in Europe and North America. Diesel vehicles, with their use of emission control technologies, have been associated with higher proportions of NO2/NOx emissions. However, it is important to note that the absolute emissions of NO2 from most vehicles have decreased over the years, and newer models have been designed to reduce nitrogen oxide emissions.

In addition to on-road vehicles, off-road equipment, such as diesel-powered non-road vehicles and industrial processes, also contribute to nitrogen dioxide emissions. This includes industrial boilers, movable engines, and coal-fired power plants. The burning of natural gas, both outdoors and indoors, is another source of nitrogen dioxide. Gas-fired power plants, oil and gas extraction facilities, and appliances such as stoves, dryers, and space heaters that burn natural gas can produce substantial amounts of nitrogen dioxide.

Nitrogen dioxide emissions have been linked to various health risks. Elevated levels of NO2, along with particulate matter and sulfur dioxide, have been associated with heart and lung damage, adverse pregnancy and birth outcomes, and an increased risk of kidney and neurological harm, autoimmune disorders, and cancer. Short-term exposure to high concentrations of NO2 can irritate the airways and aggravate respiratory diseases, especially asthma. Longer-term exposure may contribute to the development of asthma and increased susceptibility to respiratory infections.

Frequently asked questions