Air Pollutants: Primary Examples And Their Sources

Primary air pollutants are those that are emitted directly from a source, as opposed to secondary pollutants, which are formed by chemical reactions between primary pollutants in the atmosphere. Three examples of primary air pollutants are nitrogen oxide, sulfur oxide, and carbon monoxide. These pollutants can be emitted from sources such as vehicle exhaust, industrial boilers, and the incomplete combustion of fossil fuels. They can have detrimental effects on both human health and the environment, contributing to issues such as asthma, lung cancer, and acid rain.

Nitrogen oxide and nitrogen dioxide are linked to asthma and lung cancer

Nitrogen oxides (NOx) and nitrogen dioxide (NO2) are primary air pollutants that are linked to asthma and lung cancer. They are formed and emitted directly from particular sources, such as the burning of natural gas (methane), both outdoors and indoors. Outdoors, sources of nitrogen oxides include gas-fired power plants and facilities that extract, process, or transport oil and gas. Indoors, appliances such as stoves, dryers, and space heaters that burn natural gas or liquified petroleum gas (LPG) can produce substantial amounts of nitrogen dioxide. If these appliances are not adequately ventilated, NO2 levels can rise to unhealthy levels.

Nitrogen dioxide is one of six widespread air pollutants for which there are national air quality standards in place to limit their levels in outdoor air. Exposure to NO2 has been linked to a range of harmful health effects, including an increased likelihood of hospital admissions. Scientific evidence suggests a correlation between NO2 exposure and asthma in children. Additionally, elevated levels of NO2, along with particulate matter and sulfur dioxide, have been associated with heart and lung issues, adverse pregnancy and birth outcomes, and a potentially higher risk of kidney and neurological harm, autoimmune disorders, and cancer.

Several studies have specifically investigated the relationship between nitrogen oxides, nitrogen dioxide, and lung cancer. A systematic review and meta-analysis of these studies found a positive association between exposure to nitrogen dioxide and lung cancer. The meta-estimate for the change in lung cancer risk associated with a 10-μg/m3 increase in NO2 exposure was 4% (with a 95% confidence interval of 1% to 8%). This review also considered the relationship between nitrogen oxides (NOx) and lung cancer, finding a slightly lower meta-estimate of 3% (95% CI: 1%, 5%).

While nitrogen oxides themselves may not be direct carcinogens, they serve as markers for other pollutants formed during the high-temperature combustion of fossil fuels. In urban settings, traffic is often the primary source of NOx in the atmosphere, and the variability in NOx levels can be attributed to traffic pollution. Therefore, the presence of nitrogen oxides and nitrogen dioxide in the air is indicative of other harmful pollutants that can contribute to the development of lung cancer and other health issues.

Carbon monoxide is produced by the incomplete combustion of carbon-containing fuels

Carbon monoxide is a colourless, odourless, and poisonous gas that is produced by the incomplete combustion of carbon-containing fuels. Incomplete combustion occurs when there is a limited supply of oxygen or air during the burning process. This can happen when there is an insufficient amount of oxygen, an inappropriate temperature, or a rapid burn rate, resulting in leftover carbon monoxide in the exhaust.

Carbon-containing fuels, such as hydrocarbons like hexane, propane, or methane, are commonly used fuel sources. When these fuels undergo combustion with an insufficient amount of oxygen, carbon monoxide (CO) is produced instead of carbon dioxide (CO2). This is because, during incomplete combustion, only one oxygen atom reacts with the fuel, resulting in the formation of CO and the release of less energy. In contrast, complete combustion involves two oxygen atoms reacting with the fuel, producing CO2 and a higher amount of energy.

The presence of carbon monoxide can be challenging to detect due to its colourless and odourless nature. However, electronic detectors are often used near boilers to monitor its levels. Incomplete combustion can also lead to the formation of particulate carbon, commonly known as soot. Soot is composed of fine black particles, primarily made of carbon, and is produced by the incomplete combustion of coal, oil, wood, or other fuels.

Carbon monoxide is considered more dangerous than carbon dioxide and can pose significant health risks. It binds to haemoglobin, the red protein in red blood cells responsible for transporting oxygen throughout the body. Additionally, particulate carbon can irritate the lining of the lungs, exacerbate respiratory conditions such as asthma, and potentially lead to cancer.

To mitigate the harmful effects of carbon monoxide, it is essential to ensure complete combustion by providing an adequate supply of oxygen during the burning of carbon-containing fuels. This helps prevent the formation of carbon monoxide and reduces the associated health and environmental risks.

Sulfur oxide and sulfur dioxide cause acid rain, which damages foliage and statues

Primary air pollutants are those that are formed and emitted directly from particular sources. Examples include particulates, carbon monoxide, nitrogen oxide, and sulfur oxide. When sulfur oxide and sulfur dioxide are emitted into the atmosphere, they can cause acid rain. This occurs when these gases react with water molecules in the atmosphere to produce acids.

Acid rain has a pH between 4.2 and 4.4, which is significantly more acidic than normal rain, which has a pH of about 5.6. The acidity of acid rain is due to the formation of sulfuric and nitric acids. These acids then mix with water and other materials before falling to the ground. The major sources of these pollutants are the burning of fossil fuels, vehicles and heavy equipment, manufacturing, and oil refineries.

The effects of acid rain are widespread and detrimental. In ecosystems, acid rain reduces the durability of tree bark, leaving trees more vulnerable to environmental stressors such as drought, extreme temperatures, and pests. It also impacts soil composition by stripping it of essential nutrients such as calcium and magnesium, which are crucial for plant growth and healthy soil maintenance.

Additionally, acid rain causes harm to human infrastructure and cultural heritage. It contributes to the corrosion of steel structures, the weathering of stone buildings and statues, and the deterioration of historical monuments. The corrosive effects of acid rain on statues and monuments are particularly notable on those made of rocks containing large amounts of calcium carbonate, such as limestone and marble. The reactions between the acids in the rain and the calcium compounds in these stones can lead to the formation of gypsum, which then flakes off, causing damage to the structure.

Volatile organic compounds (VOCs) and nitrous oxides (NOx) react to form ground-level ozone

Ground-level ozone, a major secondary air pollutant, is formed when volatile organic compounds (VOCs) and nitrous oxides (NOx) react with each other in the presence of sunlight and heat. VOCs are emitted from sources such as motor vehicles, industrial facilities, power plants, gasoline vapors, and household products. These household products include organic chemicals, which are a type of VOC. Nitrogen oxides are released primarily from vehicle emissions, power plants, and industrial processes.

The presence of sunlight is crucial for the chemical reactions that lead to the formation of ozone. The energy from sunlight triggers the photochemical reactions between VOCs and NOx. These reactions are complex and are not directly proportional to the maximum ozone concentration formed. The maximum ozone concentration formed from a given initial mixture of VOCs and NOx can be plotted on a graph with the initial VOC and NOx concentrations as the axes.

Ozone isopleth diagrams show that the extent of the reduction of peak ozone concentrations resulting from reductions of precursor emissions depends on the initial VOC/NOx ratio. At higher VOC/NOx ratios (greater than ~8-10), ozone concentrations are relatively insensitive to VOC concentrations, and NOx control is more effective in lowering ozone levels.

Ground-level ozone is connected to significant damage to both human and environmental health. It is not emitted directly but is instead created through chemical reactions between NOx and VOCs when exposed to sunlight.

Particulates are caused by industrial processes, farming, and brake and tire abrasion

Particulate matter, one of six criteria air pollutants, is the deadliest form of air pollution. These particles can be extremely small, measuring less than 1 micrometer in diameter, and can remain in the atmosphere for weeks. Due to their size, they can penetrate deep into the lungs and bloodstream unfiltered, causing respiratory diseases, heart attacks, and premature death. There is no safe level of exposure to particulates.

One source of particulate matter is industrial processes. Fossil fuel use, for example, releases particulates that can lodge deep in the lungs and cause birth defects in children. Diesel particulate matter is of particular concern, as it is highest near the source of emission.

Farming practices, such as prescribed burning and the use of stationary engines, also contribute to particulate matter in the air. Backyard burning, for example, releases pollutants at ground level, increasing the risk of inhalation and incorporation into the food chain. It also produces toxic dioxins. The degree to which farming practices impact air quality depends on factors such as location and the type of ignition system used.

Brake and tire wear from vehicles are another source of particulate matter. While vehicle exhaust emissions have decreased due to stricter regulations, non-exhaust emissions from brakes and tires have not been as extensively regulated and are becoming a more significant contributor to particulate matter in urban areas. Studies have shown that braking intensity and vehicle mass impact the amount of particulate matter produced, with heavier vehicles generally producing higher concentrations of particulate matter during braking.

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