Helena Joyce
Nitrogen dioxide (NO2) is a pungent reddish-brown gas and a member of the nitrogen oxide (NOx) family of chemicals. NO2 is a harmful air pollutant, primarily released into the atmosphere through the burning of fossil fuels, with sources including cars, trucks, power plants, and industrial processes. NO2 is a key contributor to smog and has been linked to adverse health effects, particularly for those living near emission sources, such as people in urban areas, and those with pre-existing medical conditions.
| Characteristics | Values |
|---|---|
| How NO2 is formed | Nitrogen Dioxide (NO2) is formed by the combustion of fossil fuels, along with nitric oxide (NO). NO2 is also formed when NO reacts with certain gases in the atmosphere. |
| Sources of NO2 emissions | Cars, trucks, buses, power plants, industrial boilers, non-road equipment, and other movable engines. |
| Health effects | Exposure to NO2 can irritate airways, increase susceptibility to respiratory infections, and aggravate respiratory diseases, especially asthma. Long-term exposure during childhood can lead to smaller lungs at maturity. |
| Vulnerable subpopulations | People who live near emission sources, people with pre-existing medical conditions such as asthma, COPD, cardiovascular disease, diabetes, and lung cancer, and people of color. |
| Indoor sources | Tobacco smoke, gas-, wood-, oil-, kerosene-, and coal-burning appliances such as stoves, ovens, and heaters. |
| Ambient concentrations | Can exceed a total concentration (NO + NO2) of 500 μg/m3 in dense urban areas. |
| Rural background NO2 pollution | Has decreased over time, with a 74% reduction from 1997 to 4.7 µg/m3 in 2024. |
| Urban background NO2 pollution | Has decreased over time, with an annual mean concentration of 13.1 µg/m3 in 2024, an 8% decrease since 2023. |
| National standards | The one-hour standard for NO2 was lowered to 0.18 ppm in 2007, and the annual average standard is 0.030 ppm. |
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![Natural and anthropogenic sources of oxides of nitrogen (NOx) for the troposphere Ernest Bauer. 1982 [Leather Bound]](https://m.media-amazon.com/images/I/61IX47b4r9L._AC_UY218_.jpg)
What You'll Learn
Fossil fuel combustion
Nitrogen dioxide (NO2) is one of a group of highly reactive gases known as oxides of nitrogen or nitrogen oxides (NOx). It is formed when fossil fuels such as coal, oil, methane gas (natural gas), or diesel are burned at high temperatures. The higher the combustion temperature, the more nitric oxide is generated. Nitrogen dioxide is a significant component of smog and acid rain and has detrimental effects on human health and the environment.
The burning of fossil fuels is a major contributor to NO2 pollution. When these fuels are burned, they release nitrogen oxides into the atmosphere. In ambient air, nitrogen oxides are formed by various combinations of oxygen and nitrogen during the combustion process. While nitric oxide (NO) is the dominant nitrogen oxide emitted (90-95%), a significant proportion (5-10%) is still emitted as nitrogen dioxide.
Major sources of nitrogen oxide emissions from fossil fuel combustion include vehicles and industrial processes. Cars, trucks, and buses are the largest sources of NO2 emissions, with diesel-powered vehicles being a notable contributor. Industrial processes such as oil and gas production, industrial boilers, and coal-fired power plants also emit substantial amounts of NO2. Additionally, indoor sources of NO2 include appliances such as stoves, dryers, and space heaters that burn natural gas, liquefied petroleum gas, or kerosene.
The impact of nitrogen dioxide pollution on human health is significant. Breathing air with high concentrations of NO2 can irritate the airways and aggravate respiratory diseases, especially asthma. Prolonged exposure to elevated levels of NO2 may even contribute to the development of asthma and increase susceptibility to respiratory infections. Vulnerable subpopulations at higher risk from nitrogen dioxide exposure include individuals with pre-existing medical conditions and people of color.
To address NO2 pollution from fossil fuel combustion, regulatory actions and emission reduction strategies are crucial. Leading businesses are taking steps to understand and manage their greenhouse gas emissions, and improvements in energy efficiency can also help reduce emissions. Additionally, individuals can play a role by reducing their driving and opting for more environmentally friendly transportation options, such as carpooling, public transportation, walking, or biking.
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Industrial processes
Nitrogen dioxide is a harmful gas that is formed by the combustion of fossil fuels, such as coal, oil, and gas. It is a member of the NOx family, which includes nitric oxide (NO) and nitrogen dioxide (NO2). These gases are produced during fuel burning processes, such as in power plants, industrial sites, and vehicles. While power plants and vehicles are the dominant sources of NO2 emissions, industrial processes also contribute significantly to its presence in the atmosphere.
Another industrial process that can lead to nitrogen dioxide pollution is the use of tobacco smoke and gas-, wood-, oil-, kerosene- and coal-burning appliances. These appliances, such as stoves, ovens, and heaters, can emit nitrogen oxides, particularly in poorly ventilated indoor spaces. The oxidation of nitric oxide (NO) to nitrogen dioxide (NO2) can occur within the exhaust systems of these combustion devices, further contributing to the presence of NO2 in the air.
Furthermore, certain industrial activities can indirectly increase nitrogen dioxide levels by influencing other factors. For instance, urban centres with high vehicle traffic, which is a significant source of NO2, are often associated with industrial development. Additionally, the seasonal patterns observed in NO2 concentrations are influenced by greater heating during winter, which is driven by industrial energy consumption.
It is important to note that the impact of industrial processes on nitrogen dioxide pollution extends beyond the immediate vicinity of the emission sources. The wind can carry NO2 over long distances, affecting the air quality in downwind regions. Therefore, even those who live far from industrial areas may still be at risk of exposure to harmful levels of nitrogen dioxide.
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Vehicle emissions
One factor is the type of fuel used. Diesel vehicles have been associated with higher proportions of NO2/NOx emissions compared to gasoline engines. However, it's important to note that gasoline engines are not exempt from NOx emissions. Even with proper tuning and normal operating conditions, gasoline-engine vehicles can produce negligible amounts of NOx, which can accumulate and contribute to air pollution, particularly in traffic congestion.
Vehicle mileage also plays a role in NO2 emissions. Studies have shown that the amount of NO2 decreases as vehicle mileage increases, leading to reduced NO2 concentrations near roadsides. This trend has been observed in light-duty diesel vehicles, indicating a decrease in absolute NOx and NO2 emissions over time. However, it is important for emission factor models to account for these behavioural changes to accurately assess the impact of vehicle emissions on NO2 pollution.
Additionally, it is worth noting that while electric vehicles (EVs) do not have tailpipe emissions, they are not entirely emission-free. The production and distribution of electricity used to power EVs can generate emissions, and the impact of these emissions may vary depending on the region's energy sources.
To address vehicle NOx emissions, several strategies are employed. One approach is to minimise the creation of NOx during the combustion process by lowering the combustion temperature. Aftertreatment devices are also used to chemically reduce NOx in the exhaust, converting it into nitrogen, water, and/or CO2. However, the presence of too much or too little oxygen in the vehicle exhaust can hinder the effectiveness of these chemical reactions, making it challenging to balance the removal of NOx and other pollutants.
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Indoor sources
Nitrogen dioxide (NO2) is a highly reactive oxidant and corrosive gas that is harmful to human health. It is one of the six common air pollutants called "criteria pollutants", which are subject to primary and secondary national ambient air quality standards.
The primary indoor sources of NO2 are combustion processes, such as tobacco smoke and gas, wood, oil, kerosene, and coal-burning appliances. Gas stoves, space heaters, and water heaters are the most common indoor sources of NO2 emissions. The presence of these appliances is the primary determinant of indoor NO2 levels. The average nitrogen dioxide concentration over a period of several days may exceed 150 μg/m3 when unvented gas stoves are used. In addition, indoor levels of NO2 are influenced by the indoor-outdoor air exchange rate and the effects of the season.
Tobacco smoke is a major source of indoor NO2, containing several thousand substances distributed as gases, vapors, or particulates. Smoke from wood stoves and fireplaces is also a potential source of indoor NO2, particularly in lesser-developed nations where biomass fuels are commonly used for cooking and heating.
In addition to the direct release of NO2, indoor combustion sources emit various co-pollutants, including ultrafine particles produced during cooking. These particles, along with NO2, contribute to the formation of several other indoor air pollutants, such as ozone (O3), nitric acid (HNO3), and nitrate (NO3)-containing particles.
It is important to note that indoor concentrations of ozone are usually lower than outdoor concentrations due to the penetration of outdoor ozone indoors. However, even small amounts of indoor ozone are significant in locations with potential NO2 sources, as the reaction between NO2 and ozone forms other harmful pollutants.
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Urban areas
Nitrogen dioxide (NO2) is one of a group of highly reactive gases known as oxides of nitrogen or nitrogen oxides (NOx). NO2 is a major air pollutant in urban areas, with emissions from vehicles, power plants, and industrial processes being the primary sources.
In urban areas, road traffic is the principal source of outdoor NO2 pollution. The combustion of fossil fuels, such as gasoline and diesel, in vehicles like trucks, buses, and cars releases NO2 into the atmosphere. The concentration of NO2 is particularly high on or near heavily travelled roadways, and in areas with tall buildings that prevent the dispersion of pollutants. This is why large urban regions like the Northeast corridor, Chicago, and Los Angeles have higher levels of NO2 pollution.
In addition to road traffic, power plants and industrial sites contribute to NO2 emissions in urban areas. The burning of fossil fuels, such as natural gas and coal, in power plants can release NO2 into the atmosphere. Similarly, industrial processes such as oil and gas production, and the use of diesel-powered equipment, can also emit NO2.
Indoor sources of NO2 in urban areas include the burning of fuels like wood or natural gas for cooking, heating, or other appliances. Tobacco smoke is another indoor source of NO2 pollution.
The health impacts of NO2 pollution are significant, and people living near emission sources are at higher risk. Short-term exposure to NO2 can irritate airways and exacerbate respiratory conditions, especially asthma. Long-term exposure has been linked to adverse effects on lung and heart health, increased risk of allergies, and potential harm to pregnancy and birth outcomes.
Efforts to reduce NO2 emissions in urban areas have shown progress, with a decrease in annual mean concentrations observed in recent years. However, more work is needed to ensure that urban air quality meets the standards required to protect public health.
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Frequently asked questions
Nitrogen dioxide (NO2) is a gas that is mainly produced during the combustion of fossil fuels. It is one of a group of highly reactive gases known as oxides of nitrogen or nitrogen oxides (NOx).
Trucks, buses, and cars are the largest sources of NO2 emissions. Other sources include industrial processes, such as oil and gas production, and coal-fired power plants.
NO2 is a primary pollutant and a key agent in the formation of several airborne toxic substances, including nitric acid (HNO3) and fine particles. It contributes to the reddish-brown haze characteristic of smoggy air.
Exposure to NO2 can cause adverse health effects, particularly for people with pre-existing medical conditions such as asthma, cardiovascular disease, and diabetes. Short-term exposure can irritate airways and increase susceptibility to respiratory infections. Long-term exposure has been linked to heart and lung harm, affected pregnancy and birth outcomes, and potentially increased risk of kidney and neurological harm.
Emissions of nitrogen dioxide can be reduced through cleanup efforts and regulatory standards. For example, the federal Clean Air Act has helped drive down nitrogen dioxide emissions, leading to improvements in air quality nationwide.
