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TL;DR: Nitrogen oxides (NOₓ) represent a subgroup of pollutants formed from nitrogen and oxygen. NOₓ primarily refers to nitric oxide (NO) and nitrogen dioxide (NO₂), two outdoor air pollutants produced by vehicle emissions and fossil fuel combustion, contributing to smog, acid rain, and ozone formation. Exposure to NOₓ gases can cause respiratory issues, with children being particularly vulnerable. In the U.S., NOₓ emissions have decreased by 70% from 2002 to 2022 thanks to the efforts of the US Environmental Protection Agency (EPA) and regional air quality management districts. Measuring both NO and NO₂ helps track pollution sources, improve air quality models, and develop effective mitigation strategies.
Nitrogen oxides composition:
Nitrogen oxides (NOx) are a subgroup of gaseous pollutants that can have adverse effects on human health and the environment. This group consists of a family of seven compounds, including nitric oxide (NO) and nitrogen dioxide (NO2). NOx is often used as a shorthand to collectively refer to these two gases.
Diatomic molecular nitrogen (N2) makes up about 80% of the atmosphere. Yet, nitrogen (N) by itself can be quite reactive and exist in different ionization states, allowing it to form various oxides with oxygen ions.
Nitrous oxide (N2O), commonly known as “laughing gas,” is sometimes also considered part of this group, along with dinitrogen dioxide (N2O2), dinitrogen trioxide (N2O3), dinitrogen tetroxide (N2O4), and dinitrogen pentoxide (N2O5). None of these are pollutants of major concern when it comes to ambient air pollution, so we will not focus on them for this blog.
Nitrogen dioxide (NO2) is the most prevalent form of (NOx) in the atmosphere produced by human activities. More in-depth information about nitrogen dioxide can be found in our NO2 air quality measurements series page.
Nitrogen oxides sources and distribution:
The most prevalent nitrogen oxides (NOx) are nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O). In the atmosphere, nitrogen oxides are a critical component of smog, with NO2 in particular producing the visible yellowish-brown color over cities. In urban areas, nitrogen oxide levels are especially high due to vehicle emissions. Nitrogen oxides can remain in the atmosphere long enough to travel many hundreds of kilometers before their eventual conversion to nitric acid or nitrates. This means that NOx produced in one country can easily travel to and impact other countries.
Nitrous oxide (N2O) is often produced by naturally occurring non-human sources such as yeast and plants at the Earth’s surface. Indeed, 70% of it comes from biological processes which include the decaying processes in nature. The other 30% of it, however, comes from human activities, in particular, agriculture and fossil fuel burning. N2O is unfortunately being produced at an increasing rate. When nitrous oxide (N2O) is oxidized by ozone (O3), it can form nitric oxide (NO) or sometimes dinitrogen dioxide (N2O2).
Nitric oxide (NO) is produced during combustion. Man-made sources include automotive engines and thermal power-generating plants. The heat produced by combustion allows nitrogen to react with oxygen, producing NO. The higher the combustion temperature, the more nitric oxide (NO) is produced. Nitrogen dioxide (NO2) is also produced by combustion, but far more NO tends to be emitted under these conditions than NO2. Although it can be produced by soils, lightning, and fires, NO is largely generated by human activity. Biogenic sources are only responsible for about 10% of total emissions.
In as little as two hours, nitric oxide (NO), or two NO molecules joined together as dinitrogen dioxide (N2O2), oxidizes to form nitrogen dioxide (NO2). This is why NO2 is often used as a shorthand to measure other NOx gases. However, nitrogen dioxide (NO2) can, in turn, react with a photon of sunlight in the atmosphere to produce both ozone (O3) and nitric oxide (NO), meaning that the balance of these three gases can change rapidly in the atmosphere depending on sunlight and other environmental conditions.
How nitrogen oxides impact human health:
Of the nitrogen oxides (NOx) group, nitric oxide (NO) and nitrogen dioxide (NO2) are the most hazardous to human health. The most common route of exposure to NOx is through inhalation. Even at low concentrations, most NOx gases, including NO2, N2O4, N2O3, and N2O5, irritate the upper respiratory tract and lungs. Breathing relatively low levels of nitrogen oxides (NOx) may cause coughing, shortness of breath, nausea, and tiredness. Repeated exposure may result in asthma.
At high concentrations, the above-listed NOx gases can be severely toxic. Nitrogen dioxide (NO2) is heavier than air and can cause asphyxiation in tight or enclosed spaces. Nitric oxide (NO) can also cause the same failure to absorb oxygen into the bloodstream as carbon monoxide (CO). However, since NO is not as soluble, this threat mostly only applies to very sensitive individuals and infants. Still, lung injury, throat swelling, and choking caused by very high concentrations of most NOx gases can be fatal.
Symptoms can be delayed from the moment of exposure. Severe and life-threatening symptoms from very high levels of NOx gases, such as noncardiogenic pulmonary edema, may occur days and even weeks after exposure. Additionally, when exposed to moisture, including bodily moisture, certain NOx gases can convert into nitrous acid, damaging airways and skin, causing eye and skin irritation, inflammation, and even burning. NOx gases are, moreover, a known carcinogen.
Children are more vulnerable to the effects of most NOx gases because they have a greater lung surface area in relation to their body weight. They also have a greater skin surface area-to-body-weight ratio, exacerbating their chances of experiencing NOx skin irritation. Moreover, since nitrogen dioxide (NO2) can be found near the ground, their short stature puts them at a higher risk.
Nitrous oxide (N2O), or “laughing gas,” does not have as much of an alarming effect on human health and can be used as a sedative in medical procedures. However, chronic misuse can result in adverse side effects.
Environmental impacts of nitrogen oxides:
Nitrogen oxides significantly impact the environment in a variety of ways. For instance, nitrous oxide (N2O) by itself is a powerful greenhouse gas. In fact, it is roughly 100 times more potent per pound than carbon dioxide over a period of 100 years, cementing itself as a strong climate-forcing agent.
Nitric oxide (NO), on the other hand, does not directly impact climate change but has a significant indirect effect on the environment. Together with nitrogen dioxide (NO2), NO goes through a series of chemical reactions that have multiple negative environmental impacts.
When NO2 reacts with sunlight, it produces tropospheric ozone (O3) in addition to nitric oxide (NO). The resulting NO, moreover, reacts with sunlight and volatile organic compounds (VOCs) to form more NO2. This occurs repeatedly until the VOCs cease to be photoreactive (usually after about five cycles). In this way, a single NO or NO2 molecule can produce ozone multiple times.
Tropospheric (aka ground-level) ozone (O3) negatively affects human health and ecosystems. It also traps heat, contributing to climate change. Clarity Movement also provides an FEM Ozone Module to help monitor tropospheric ozone.
Nitric oxide (NO) and nitrogen dioxide (NO2) also combine with water vapor in the atmosphere, forming nitric acid, a critical component of acid rain. Acid rain is toxic to aquatic organisms, trees, foliage, and many other ecosystems. Acidification can release dissolved aluminum in soil which can be toxic to both animals and plants. In wetlands, high acidity can accelerate the production of the neurological toxin methylmercury. Acid rain can even erode historical man-made monuments.
Regulation & prevalence of nitrogen oxides:
The Clean Air Act requires the EPA to set National Ambient Air Quality Standards (NAAQS) for criteria pollutants. Nitrogen dioxide (NO2) is considered a criteria pollutant with a 1-hour average of 100 ppb as a primary standard and an annual mean of 53 ppb as a primary and secondary standard. The United States EPA reviewed and decided to uphold these NAAQS for NO2 in 2024. The World Health Organization (WHO) also sets recommended maximum levels for ambient NO2 air pollution, which are significantly stricter than existing EPA regulations for the pollutant.
Nitrogen dioxide (NO2) regulation is often used as a shorthand way to regulate nitrogen oxides (NOx) as a whole (particularly the combination of nitric oxide (NO) and nitrogen dioxide) since NO quickly oxidizes into NO2.
Fortunately, in certain parts of the world, including the United States, nitrogen oxide (NOx) emissions have significantly declined over the last two decades. From 2002 to 2022, man-made emissions in the U.S. have impressively decreased by 70%. There has been an 84% reduction in highway vehicle emissions, and a 68% reduction in fuel combustion emissions.
How are nitrogen oxides measured:
Nitrogen oxides (NOx) are typically measured through a method known as chemiluminescence. When nitric oxide (NO) reacts with ozone in the air to become nitrogen dioxide (NO2), a very small amount of light is emitted. This allows a photomultiplier tube to measure the intensity of the light and determine the NO concentration. Because of the relationship between nitric oxide (NO) and nitrogen dioxide (NO2), the latter is often measured indirectly via this method as well.
Nitrogen oxides (NOx) can also be measured through other methods, such as laser-induced fluorescence. Indeed, NO2 can even be seen with the naked eye sometimes as it is a reddish-brown gas above 70 degrees Fahrenheit, or roughly 21 degrees Celsius. Very high levels of NO2, N2O4, and N2O5 have an odor that can be detected with one’s nose. This smell can help one recognize and avoid acute exposure.
In contrast to chemiluminescence and laser-based methods, electrochemical cell sensors measure NO and NO2 by relying on chemical reactions that generate an electrical current—an approach which offers a compact, low-power alternative for continuous monitoring, especially in portable or lower-cost devices like the Clarity Multi-Gas Module. Inside these sensors, the target gas diffuses through a membrane and reaches an electrode coated with a catalyst. When NO or NO2 comes into contact with this electrode, it undergoes a redox reaction, producing electrons that flow as current. The strength of this current is directly proportional to the concentration of the gas.
Use cases for measuring nitrogen oxides:
It can be especially useful to measure both nitric oxide (NO) and nitrogen dioxide (NO2) rather than just one in isolation. For instance, the ratio of NO2 to NOx in the air affects overall NOx ozone production and distribution. Knowing this ratio helps to understand the complex dynamics at play between air pollutants in the atmosphere, which can aid in forming the right simulations to help in mitigation strategies.
Measuring nitric oxide (NO) in addition to nitrogen dioxide (NO2) can also better help determine ambient air pollution hotspots. Since NO takes time to convert to NO2 in the atmosphere, more NO findings will occur closer to the source of the pollution, and more NO2 findings will occur slightly farther away. This is especially useful in determining traffic hotspots.
Clarity’s take on nitrogen oxide measurement:
Clarity’s Multi-Gas Module measures NOx ambient air pollution. In particular, it measures nitric oxide (NO) and nitrogen dioxide (NO2). The Multi-Gas Module also conveniently measures carbon monoxide (CO) and ozone (O3). It attaches seamlessly to our flagship Node-S air quality sensor, which measures particulate matter (PM) and nitrogen dioxide (NO2).
If you’re interested in measuring nitrogen oxides (NOx) with the Multi-Gas Module, feel free to reach out to learn more from our team!