How Does Nitrogen Oxide Pollution Affect the Environment?

NOx pollution occurs when nitrogen oxides are released as a gas into the atmosphere during the high-temperature combustion of fossil fuels. These nitrogen oxides consist mainly of two molecules, nitric oxide (NO) and nitrogen dioxide (NO₂); there are other nitrogen-based molecules considered to be NOx, but they occur in much lower concentrations. A closely related molecule, nitrous oxide (N₂O), is a significant greenhouse gas that plays a role in global climate change.

Where Does NOx Pollution Come From?

Nitrogen oxides form when oxygen and nitrogen from the air interact during a high-temperature combustion event. These conditions occur in car engines and fossil fuel-powered electricity plants.

Diesel engines, in particular, produce large amounts of nitrogen oxides. This is due to the combustion features characteristic of this type of engine, including their high operating pressures and temperatures, especially when compared to gasoline engines. In addition, diesel engines allow excess oxygen to exit the cylinders, diminishing the effectiveness of catalytic converters which prevent the release of most NOx gases in gasoline engines.

What Are the Environmental Concerns Associated With NOx?

NOx gases play an important role in the formation of smog, producing the brown haze often observed over cities, particularly during the summer. During exposure to UV rays in sunlight and heat, NOx molecules interact with volatile organic compounds (VOC) and form ground-level (or tropospheric) ozone (O₃). Ozone at the ground level is a serious pollutant, unlike the protective ozone layer much higher up in the stratosphere.

In the presence of rain, nitrogen oxides form nitric acid, contributing to the problem of acid rain. Additionally, NOx deposition in the oceans provides phytoplankton with nutrients, worsening the issue of red tides and other harmful algal blooms.

What Are the Health Concerns Associated With NOx?

Nitrogen oxides, nitric acid, and ozone can all readily enter the lungs, where they create serious damage to delicate lung tissue. Even short-term exposure can irritate the lungs of healthy people. For those with medical conditions like asthma, just a short time spent breathing these pollutants has been shown to increase the risks of an emergency room visit or hospital stay.

Approximately 16% of houses and apartments in the United States are within 300 feet of a major road, increasing exposure to hazardous NOx and their derivatives. For these residents—especially the very young and elderly—this air pollution can lead to respiratory diseases such as emphysema and bronchitis. NOx pollution can also worsen asthma and heart disease and is tied to elevated risks of premature death.

What Role Does NOx Pollution Play in the Volkswagen Diesel Scandal?

In order to transform public opinion about diesel cars, Volkswagen marketed the diesel engines in their fleet of cars as new and clean-burning. They said it was a "new era of diesel" and offered their vehicles as alternatives to hybrid cars, which were gobbling up bigger chunks of the market share. There were concerns over the cars' nitrogen oxide emissions, but those were appeased as the little Volkswagen diesel engines met the stringent requirements policed by the U.S. Environmental Protection Agency and the California Air Resources Board.

Somehow, few other car companies seemed to be able to design and produce their own powerful but thrifty and clean diesel engines. It became clear why in September 2015, when the EPA revealed that VW had been cheating the emissions tests. The automaker had programmed its engines to recognize testing conditions and react by automatically operating under parameters that produce very low amounts of nitrogen oxides. When normally driven, however, these cars produce 10 to 40 times the maximum allowable limit.

This article was written with assistance from Geoffrey Bowers, Assistant Professor of Chemistry at St. Mary's College of Maryland, and author of the book Understanding Chemistry Through Cars (CRC Press).