Kiera Jefferson
When two primary pollutants combine, they form secondary pollutants. Primary pollutants are emitted directly from particular sources, such as industrial processes or vehicles. Examples include carbon monoxide, nitrogen oxide, and sulfur oxide. Secondary pollutants, on the other hand, are formed in the lower atmosphere by chemical reactions between primary pollutants. One major secondary pollutant is ground-level ozone, which forms when volatile organic compounds (VOCs) and nitrous oxides (NOx) react with sunlight and heat. These reactions contribute to photochemical smog, which can cause eye irritation and respiratory issues, as well as reduced visibility and acid rain.
| Characteristics | Values |
|---|---|
| Name | Ground-level ozone, secondary organic aerosol (haze), photochemical smog |
| Formation | Reactions between volatile organic compounds (VOCs) and nitrous oxides (NOx) with sunlight and heat |
| Transport | Can be transported long distances by wind |
| Health Impact | Significant damage to human and environmental health, including eye irritation, respiratory issues, and damage to crops and materials |
| Geography | More common in big cities with heavy industry and traffic, especially those situated in basins |
| Examples | Los Angeles, Mexico City, London |
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What You'll Learn
- Ground-level ozone is formed from volatile organic compounds (VOCs) and nitrous oxides (NOx)
- Secondary organic aerosol (haze)
- Nitrates are formed from the oxidation of nitrogen oxide gases
- Sulfates are formed from sulfur-containing compounds
- Photochemical smog is formed from VOCs, NOx, nitrogen oxides, ozone, and other air pollutants
Ground-level ozone is formed from volatile organic compounds (VOCs) and nitrous oxides (NOx)
Ground-level ozone is a secondary air pollutant that forms when two primary pollutants, volatile organic compounds (VOCs) and nitrous oxides (NOx), interact with sunlight in the presence of heat. VOCs are emitted from a range of sources, including motor vehicles, chemical plants, refineries, and natural (biogenic) sources. NOx, on the other hand, is formed when fuel is burned at extremely high temperatures (above 1200 °F), typically in automobiles, power plants, industrial boilers, and motor vehicles.
The formation of ground-level ozone occurs through a complex series of reactions between VOCs and NOx. These reactions result in the creation of a powerful irritant that affects both human health and the environment. Ozone irritates the eyes and upper respiratory system, making breathing difficult. It also inflicts damage on crops and man-made structures like monuments and statues.
Ozone and its precursor pollutants can be transported by wind over long distances. This means that even rural areas with relatively low emissions of these primary pollutants can still experience elevated levels of ozone. The presence of ground-level ozone is a significant contributor to the formation of photochemical smog, which is characterized by a brown haze that can be painful to the eyes.
Nitrogen oxides, including NOx, play a crucial role in the formation of ground-level ozone. They are a class of pollutants formed during high-temperature combustion processes. While NOx does not have a specific air quality standard, its impact on the creation of ground-level ozone is significant. Through intricate reactions with VOCs, NOx contributes to the complex mixture that constitutes ground-level ozone.
The adverse effects of ground-level ozone extend beyond its immediate impact on human health and the environment. It also contributes to the formation of acid rain, further exacerbating its environmental impact. The formation of ground-level ozone from VOCs and NOx is a complex process that has substantial consequences for the ecosystems and communities exposed to it.
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Secondary organic aerosol (haze)
Secondary organic aerosols (SOAs) are fine particulates that are found in the Earth's atmosphere and can impact human health and air quality. They are formed through a series of chemical reactions between sunlight, primary organic matter, and volatile organic compounds (VOCs). SOAs are produced from both anthropogenic and biogenic sources.
SOAs are a major component of haze pollution, especially in large cities with a lot of industry and traffic. They are formed through the atmospheric oxidation of VOCs. These VOCs can be emitted from a variety of sources, including motor vehicles, chemical plants, refineries, and natural (biogenic) sources. The oxidation of VOCs can lead to the formation of gas-phase SOA (gasSOA) through homogeneous nucleation or the condensation of gas species. In the presence of environmental aerosols and specific weather conditions, water-soluble organics (WSOCs) can form aqueous-phase SOA (aqSOA) through chemical reactions.
SOAs have complex mixing states, and a single SOA particle formed from a single precursor can consist of hundreds of different compounds. The formation of SOAs can be difficult to describe due to the complexity of these mixing states. SOAs are formed through the oxidation of gas-phase organic compounds or through condensation on pre-existing particles. The oxidation of these compounds leads to increased polarity and reduced volatility of the molecules, resulting in reduced vapor pressure. As the vapor pressure decreases, the gas-phase compound partitions into the solid phase, resulting in the production of secondary organic matter.
SOAs represent a significant sum of aerosols contained in the troposphere and contribute to a large number of fine particulates (PM2.5). These fine particles are small enough to penetrate deep into the lungs, causing various respiratory health effects. SOAs can also have significant impacts on the Earth's energy balance through their role in scattering and absorbing solar radiation, heterogeneous chemistry, and cloud formation. They contribute to reduced visibility, worsened air quality, and haze formation.
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Nitrates are formed from the oxidation of nitrogen oxide gases
Nitrogen oxides (NOx) are formed when fuel is burned at very high temperatures (above 1200 °F) in automobiles and power plants. Nitrogen dioxide (NO2), nitric oxide (NO), and other nitrogen oxides are all part of the NOx class of pollutants. Nitrogen oxides are among the most significant contributors to air pollution.
Nitric oxide (NO) is formed by the combustion of nitrogen-containing compounds, including fossil fuels, and by the thermal fixation of atmospheric nitrogen. Nitric oxide is further oxidized to nitrogen dioxide (NO2) in the ambient air. The oxidation of ammonia to nitrite, and then to nitrate, is another important step in the global nitrogen cycle. This process is known as nitrification and is carried out by microbes called ammonia-oxidizers.
Nitrates are used as oxidizing agents in various applications, such as explosives and the removal of air bubbles from molten glass and ceramics. They are also used in the medical field for the treatment of acute coronary syndrome, myocardial infarction, and acute pulmonary oedema. Additionally, nitrates are a primary form of nitrogen for many plants, which use them to synthesize proteins, nucleic acids, and other vital organic molecules.
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Sulfates are formed from sulfur-containing compounds
Sulfates are a family of chemicals that contain the fully oxidized ionic form of sulfur (SO42-) in combination with metal and/or hydrogen ions. They are formed from sulfur-containing compounds, which are emitted primarily from the combustion of petroleum-derived fuels (e.g. gasoline and diesel fuel) that contain sulfur. Sulfur is also released during the combustion of coal and oil by industrial furnaces or power plants.
Sulfur-containing compounds can also be released from household wastes, including detergents, and industrial effluents from tanneries, steel mills, sulfate-pulp mills, and textile plants. In California, a small amount of sulfate is directly emitted from the combustion of sulfur-containing fuels, but most ambient sulfate is formed in the atmosphere through chemical reactions.
Similarly, secondary sulfates are formed in the atmosphere from other sulfur-containing compounds under mechanisms that involve photochemical processes. Sulfur dioxide (SO2), a gaseous pollutant, is converted to sulfate particulate matter through chemical reactions in the atmosphere. This process occurs rapidly and completely in urban areas of California due to regional meteorological characteristics.
Sulfates are salts of sulfuric acid, and many are prepared from that acid. Sulfuric acid is a strong acid that behaves as a Brønsted-Lowry acid and is deprotonated to form the hydrogensulfate ion. Sulfates are widely used in industry, including in steel and metal production, manufacturing, and fertilizers. They are also used as disinfectants and exist as an end-product in the form of copper sulfate, a fungicide and algicide.
In nature, sulfur is available in both inorganic and organic forms, and plants utilize sulfate in the soil solution as a sulfur source to synthesize organic sulfur compounds for vitalization. Sulfate assimilation in plants occurs primarily in the chloroplasts and is then a part of photosynthesis, but it also takes place in the plastids of the roots.
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Photochemical smog is formed from VOCs, NOx, nitrogen oxides, ozone, and other air pollutants
Photochemical smog is a type of air pollution that occurs in urban areas with high traffic. It is formed from a combination of primary and secondary pollutants, including VOCs, NOx, nitrogen oxides, ozone, and other airborne particles.
VOCs, or volatile organic compounds, are emitted from a variety of sources such as motor vehicles, chemical plants, refineries, and natural (biogenic) sources. NOx, or nitrogen oxides, are formed when fuel is burned at very high temperatures, typically above 1200 °F, in automobiles and power plants. These two primary pollutants can react with sunlight in the presence of heat to form ground-level ozone, a major component of photochemical smog.
The formation of photochemical smog involves a complex series of chemical reactions. When exposed to sunlight or ultraviolet radiation, nitrogen dioxide (NO2) undergoes a series of hydrocarbon reactions to produce ozone, aldehydes, nitric acid, peroxyacyl nitrates (PANs), and other secondary pollutants. These reactions result in the formation of a brownish-gray haze, characteristic of photochemical smog.
The presence of hydrocarbons facilitates the formation of photochemical smog. Hydrocarbons, including unburned hydrocarbons from incomplete combustion and the evaporation of solvents and liquid fuels, react with nitrogen oxides to produce peroxy radicals. These peroxy radicals then convert nitric oxide (NO) into nitrogen dioxide (NO2), contributing to the overall smog formation.
Photochemical smog has significant negative impacts on both the environment and human health. It can cause eye irritation, decreased vision, respiratory issues, and shortness of breath. The ground-level ozone present in photochemical smog is particularly harmful to human health, triggering asthma, causing breathing problems, reducing lung function, and contributing to lung diseases.
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Frequently asked questions
Primary pollutants are pollutants that are formed and emitted directly from particular sources. Examples include particulates, carbon monoxide, nitrogen oxide, and sulfur oxide.
When two primary pollutants, volatile organic compounds (VOCs) and nitrous oxides (NOx), react with sunlight in the presence of heat, ground-level ozone is formed.
Ground-level ozone is a strong irritant to the eyes and upper respiratory system. It hampers breathing and damages crops and man-made materials such as monuments and statues.
