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Nitrogen oxides (NOx) are a group of highly reactive gases, including nitric oxide (NO) and nitrogen dioxide (NO2). They are considered primary pollutants, emitted directly from combustion sources such as vehicles, power plants, and industrial processes. However, nitrogen oxides can also contribute to the formation of secondary pollutants, reacting with other chemicals in the atmosphere to produce harmful substances such as ozone, particulate matter, and acid rain. This dual nature of nitrogen oxides, as both primary and secondary pollutants, makes them a significant concern in air pollution and its impact on human health and the environment.
| Characteristics | Values |
|---|---|
| Type of Pollutant | Primary and Secondary |
| Primary Pollutant Type | Nitrogen oxide is a type of primary pollutant as it is emitted directly into the environment. |
| Secondary Pollutant Type | Nitrogen oxide is also a secondary pollutant as it can form in the atmosphere from other chemicals. |
| Examples of Primary Nitrogen Oxides | Nitric oxide (NO) and nitrogen dioxide (NO2) |
| Nitrogen Dioxide Appearance | Reddish-brown haze |
| Nitrogen Dioxide Odor | Foul-smelling |
| Nitrogen Dioxide Formula | NO2, composed of one nitrogen atom and two oxygen atoms |
| Nitrogen Oxide Sources | Motor vehicles, power plants, other sources of combustion, industrial sector, biomass burning, natural forest fires, volcanoes |
| Health Effects | Nitrogen dioxide exposure is associated with respiratory health issues, otitis media, eczema, ear/nose/throat infections, and sensitization to food allergens in children. |
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What You'll Learn
Nitrogen oxide is a primary pollutant
Nitrogen dioxide is a highly reactive gas that is a product of vehicle, power plant, and off-road equipment emissions caused by fuels burning at high heat. It is a foul-smelling gas that comes primarily from burning fossil fuels like coal, oil, and gas. Nitrogen dioxide is composed of one nitrogen atom and two oxygen atoms (NO2) in each molecule. It is emitted from combustion devices, such as cars, trucks, and machinery that burn fuel.
Nitric oxide (NO) is primarily produced in fuel combustion processes from nitrogen and oxygen in the atmosphere. It is then converted to NO2 either within the exhaust system of the combustion device or in the atmosphere. The higher the combustion temperature, the more nitric oxide is generated. Indeed, 90–95% of nitrogen oxides are typically emitted as nitric oxide, and only 5–10% as nitrogen dioxide. However, in ambient conditions, nitric oxide is rapidly oxidized in the air to form nitrogen dioxide.
Nitrogen oxides can react with other chemicals in the air to form secondary pollutants, including ozone, particulate matter, and acid rain. These secondary pollutants are challenging to control because their formation is not well understood. They form naturally in the environment and contribute to issues like photochemical smog. For example, nitrogen dioxide combines with volatile organic compounds (VOCs) in the presence of sunlight and heat to create ozone, a harmful secondary pollutant.
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It is formed by burning fossil fuels
Nitrogen oxide is a primary pollutant that is formed and emitted directly from burning fossil fuels. Fossil fuels such as coal, oil, and natural gas contain nitrogen, which cannot be removed prior to combustion. When these fuels are burned to generate electricity, they produce nitrogen oxide, also known as NOx. The combustion of coal and oil produces both thermal and fuel nitrogen oxides, while natural gas combustion primarily produces fuel NOx.
The formation of NOx during fossil fuel combustion is highly temperature-dependent. High-temperature combustion, such as in car engines, produces nitrogen oxides through the reaction between nitrogen and oxygen. At lower temperatures, the oxidation of nitrogen species during the initial stages of combustion can lead to the formation of NOx.
Vehicles, power plants, and industrial sources such as boilers, cement kilns, and turbines are significant emitters of NOx. The burning of fuel at high temperatures in these sources results in the release of NOx into the atmosphere. In areas with high motor vehicle traffic, such as large cities, nitrogen oxide emissions can be a significant source of air pollution.
NOx emissions from fossil fuel combustion are estimated at 28.5 million tonnes. To reduce these emissions, various technologies have been developed, including the use of low NOx burners, selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), and exhaust gas recirculation. These technologies aim to reduce nitrogen oxide emissions during or after combustion.
Nitrogen oxides have detrimental effects on the environment and human health. They contribute to the formation of smog, acid rain, and secondary pollutants such as ozone. Short-term exposure to nitrogen oxides can increase respiratory illnesses, while long-term exposure can lead to lung damage.
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It is emitted from vehicles and power plants
Nitrogen oxide is a primary pollutant. Primary pollutants are those that are formed and emitted directly from particular sources, such as combustion activities. Nitrogen oxide is emitted from vehicles and power plants, as well as other sources of combustion.
Nitrogen dioxide (NO2) is a highly reactive gas and is one of a group of gases known as nitrogen oxides (NOx). It is formed when fossil fuels such as coal, oil, methane gas (natural gas), or diesel are burned at high temperatures. NO2 is used as an indicator for the larger group of nitrogen oxides.
NO2 is emitted from the exhaust systems of vehicles, including cars, trucks, and buses, as well as from power plants and other equipment involving fuel combustion. It is a major air pollutant and has harmful effects on human health, particularly the respiratory system. Exposure to NO2 can irritate the airways and aggravate respiratory diseases, especially asthma.
In addition to vehicles and power plants, NO2 emissions also come from non-road equipment, industrial processes such as oil and gas production, and industrial boilers. The highest concentrations of outdoor NO2 are typically found in large urban areas and near heavily travelled roadways.
To reduce exposure to NO2 and improve air quality, steps have been taken to clean up power plants, industrial sites, and on-road vehicles. These efforts have led to a decline in nitrogen dioxide emissions and an improvement in air quality nationwide. However, there is still a long way to go to ensure healthy air for everyone.
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It can cause respiratory issues
Nitrogen oxide is a primary pollutant. It is formed and emitted directly from particular sources, such as vehicles, power plants, and other sources of combustion. Nitrogen dioxide (NO2) is a highly reactive gas and is one of a group of gases known collectively as nitrogen oxides (NOx).
Nitrogen oxides can cause respiratory issues. NO2, along with other nitrogen oxides, reacts with other chemicals in the air to form particulate matter and ozone, which are harmful when inhaled due to their effects on the respiratory system. Inhaling air with high concentrations of NO2 can irritate the airways in the human respiratory system. Exposure over short periods can aggravate respiratory diseases, especially asthma, leading to symptoms such as coughing, wheezing, or difficulty breathing. Longer exposure to elevated concentrations of NO2 may contribute to the development of asthma and potentially increase susceptibility to respiratory infections.
Ozone, a secondary pollutant, is formed by the reaction of primary pollutants, such as nitrogen oxides, to the presence of sunlight. Exposure to ozone can make people more susceptible to respiratory infections, result in lung inflammation, and aggravate pre-existing respiratory diseases such as asthma. Other health effects of exposure to ozone include a decrease in lung function, chest pain, and coughing.
Nitrogen oxides are primarily produced through the combustion of fossil fuels, such as coal, oil, and gas. The burning of these fuels releases NOx into the atmosphere, where they can react with other chemicals to form secondary pollutants. The highest concentrations of outdoor NO2 are typically found in large urban regions and areas near heavily traveled roadways, where vehicle emissions are a significant source of pollution.
To protect themselves on days with high levels of air pollution, individuals can take proactive steps such as limiting their time outdoors and using air purifiers indoors. Additionally, people can advocate for cleaner air by urging policymakers to implement and enforce regulations that reduce air pollution from various sources, including vehicles, power plants, and industrial sites.
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It can form secondary pollutants
Nitrogen oxide is a primary pollutant, formed and emitted directly from particular sources, such as vehicles, power plants, and other sources of combustion. However, nitrogen oxide can also contribute to the formation of secondary pollutants. Secondary pollutants are those that form in the lower atmosphere through chemical reactions with other primary pollutants.
Nitrogen dioxide (NO2) is a highly reactive gas and is one of the principal nitrogen oxides associated with combustion sources. It is a product of vehicle emissions, power plants, and off-road equipment emissions caused by fuels burning at high heat. NO2 and other nitrogen oxides can react with other chemicals in the air to form secondary pollutants, including ozone, particulate matter, and other toxic chemicals.
Ozone is a well-known secondary pollutant that is formed by the reaction of primary pollutants, including nitrogen oxides and volatile organic compounds (VOCs), in the presence of sunlight. VOCs are emitted from various industrial sources, such as motor vehicles, chemical plants, refineries, and factories. The formation of ozone as a secondary pollutant is of concern as it can have harmful effects on human health, particularly the respiratory system. Exposure to ozone can increase susceptibility to respiratory infections, cause lung inflammation, and aggravate pre-existing respiratory conditions such as asthma.
In addition to ozone, nitrogen oxides can contribute to the formation of other secondary pollutants. For example, nitrogen dioxide can react with water to produce nitrous acid (HONO), a common indoor and ambient air pollutant. Furthermore, nitrogen oxides can interact with other indoor pollutants, creating secondary pollutants that are believed to be responsible for adverse health effects. While the specific mechanisms are still being studied, research suggests that nitrogen dioxide exposure is associated with respiratory health issues, as well as an increased risk of otitis media, eczema, and food allergen sensitization.
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Frequently asked questions
A primary pollutant is any type of pollutant emitted directly into the environment. They are formed and emitted directly from particular sources.
Nitrogen oxide is a primary pollutant. It is emitted directly from vehicles, power plants, and other sources of combustion.
Secondary pollutants are formed in the atmosphere by chemical reactions. Examples include ozone, particulate matter, acid rain, and other toxic chemicals.
