Nitrogen Oxides (NOx)
Learn all about nitrogen dioxide and nitric oxide (collectively known as nitrogen oxides or NOx) - from their chemical formulas and sources to their effects on human health and the environment.
Also known as NOx gases, nitrogen oxides (NOx) are a group of highly reactive gases.
NOx is primarily formed when vehicles burn gasoline and diesel. NOx pollution from vehicles is dangerous to both human health and the climate.
The most common members of this group are nitrogen dioxide (NO2) and nitric oxide (NO). Together they are frequently referred to as NOx.
What is Nitrogen Dioxide (NO2)?
Nitrogen dioxide is a reddish-brown pungent gas at ambient temperatures that contributes to smoggy air. It is a nitrogen oxide (NOx) and an atmospheric pollutant. NO2 is poisonous and can be lethal if inhaled in large amounts.
NO2 is used as the indicator for the larger group of nitrogen oxides.
It’s important to distinguish NO2 (nitrogen dioxide) and NO (nitric oxide) from nitrous oxide (N2O). Unlike NO2 and NO, nitrous oxide (N2O) is not considered a nitrogen oxide; instead, it is a greenhouse gas commonly generated through agricultural soil management practices.
What is the Formula for Nitrogen Dioxide?
The chemical formula of NO2 is one atom of nitrogen and two atoms of oxygen.
What is Nitric Oxide (NO)?
Nitric oxide is a colorless, odorless gas at ambient temperatures. It is a member of a family of chemicals comprised of nitrogen and oxygen that are collectively known as nitrogen oxides (NOx). It is oxidized in the atmosphere to form nitrogen dioxide.
What is the Formula for Nitric Oxide?
The chemical formula of nitric oxide is one atom of nitrogen and one atom of oxygen. It has an unpaired electron, often represented by a dot in its chemical formula (•N=O or •NO).
Nitrogen Oxide Sources
Nitrogen oxides (NOx) are primarily produced through the combustion of fossil fuels such as coal, oil, natural gas (methane), or diesel at elevated temperatures.
Where does nitrogen dioxide (NO2) come from?
By far the biggest sources of nitrogen dioxide emissions are trucks, buses, and cars, followed by diesel-powered equipment used off-road.
Due to the high temperatures of internal combustion engines, nitrogen in the air burns. This results in nitrogen oxide (NO) or nitrogen dioxide (NO2), collectively called NOx.
Diesel vehicles account for 60% of the NOx in U.S. transportation. As a result of the compression-ignition design of diesel engines, they use significantly more air for combustion. Consequently, diesel exhaust contains a higher concentration of oxygen compared to gasoline engine exhaust. This high oxygen content creates an unfavorable environment for the chemical reaction needed to reduce NOx emissions.
Industrial processes such as oil and gas production, industrial boilers and other movable engines, and coal-fired power plants also contribute to nitrogen dioxide pollution.
Nitrogen Oxide Effects on Humans
Nitrogen oxide contributes to the formation of ground-level ozone (which causes smog and makes the air hazy) and tiny particulate matter less than 2.5 microns in diameter, called PM2.5 – both of which have their own environmental and human health hazards.
Ozone can cause or worsen chronic lung conditions such as asthma, chronic obstructive pulmonary disease, and emphysema. This is particularly dangerous for vulnerable groups like children and the elderly, for whom it can be fatal. Globally, it is estimated that O3 exposure causes approximately 1 million premature deaths, both from long-term and short-term exposure.
Exposure to PM2.5 is linked to an increased risk of stroke, ischemic heart disease, chronic obstructive pulmonary disease, lung cancer, and lower respiratory infections. PM2.5 is estimated to be responsible for 4.3 and 4.4 million million premature deaths globally.
How Does Nitrogen Dioxide Affect Human Health?
Nitrogen dioxide is poisonous. High levels of exposure can be fatal.
Exposure can make asthma worse. It can also cause other serious health issues including premature death, heart problems, problems with lung development (especially in children), difficulty breathing, and intensified allergic responses.
Oxides of Nitrogen and Their Effects on the Environment
The harmful effects of nitrogen dioxide on the environment include acid rain and deteriorated water quality. It also contributes to acidification of soils and surface waters.
Additionally, the nitrate particles formed from NOx emissions cause haze, making the air difficult to see through. This impacts the scenic views in many of our national parks.
NO2 also contributes to the formation of several other air pollutants, including ozone (O3), nitric acid (HNO3), and nitrate (NO3-).
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Nitrogen Oxide (NOx) Emissions
Nitrogen oxide (NOx) emissions, which include nitric oxide (NO) and nitrogen dioxide (NO2), are major contributors to air pollution. Originating primarily from vehicle exhaust, the nitrogen and oxygen in the air combine to form harmful compounds.
Exposure to these pollutants can lead to respiratory problems, cardiovascular diseases, and even premature death. The environmental impact of NOx emissions extends beyond human health and includes the formation of acid rain. Acid rain can severely damage ecosystems, including forests, lakes, and streams.
Reducing Nitrogen Oxide Pollution
To reduce nitrogen oxide pollution, we need to reduce how much gasoline and diesel we use. For people who use a lot of gasoline, switching to an electric vehicle can be very effective in cutting gasoline use.
In the United States, vehicle emissions constitute the primary contributor to nitrogen oxides (45%). California's transportation sector alone is responsible for nearly 80% of nitrogen oxide pollution and 80% of smog-forming pollutants.
Nitrogen Oxide Policy
Mitigating NOx emissions is a critical environmental priority. Various strategies have been implemented to reduce these emissions, including stricter vehicle emission standards.
In May 2023, the Biden administration proposed standards for cars, SUVs, and light trucks for model years 2027 through 2032 These standards cover greenhouse gases and pollutants such as NOx. The standards would allow less than 1% of the pollution levels that Congress mandated in 1970 – but only in terms of pollution per mile.
Miles driven in the U.S. have tripled since 1970 (to 3.2 TRILLION miles a year), such that per-mile pollution reductions have a smaller effect on overall pollution.
We Can Cut Vehicle Emissions - Including NOx - Faster By Helping The Biggest Gasoline Users Switch To EVs
Prioritizing "Gasoline Superusers," the top 10% of gasoline consumers, to switch to EVs offers a more efficient path to reducing vehicle emissions. This strategy can also provide significant cost savings for lower-income households, who are disproportionately affected by high gasoline expenses.
Learn from our free webinar about how the gasoline superuser approach can help reduce car pollution faster and more equitably.
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