The term "nitrogen oxide" refers to a combination of gases made up of oxygen and nitrogen. The two most common nitrogen oxide (NOₓ) gases are known as nitrogen monoxide (NO) and nitrogen dioxide (NO2). Nitrogen monoxide is also commonly known as nitric oxide.
These two gases are both harmful to human health, and the environment. Nitrogen oxides also play an important role in the production of other harmful pollutants when they react with other gases in the atmosphere such as ground-level ozone, smog, and acidic rain.
The main environmental concern regarding nitrogen oxides is when they combine with sulphur dioxide (SO₂) to form acid rain. Acid rain forms when nitric and sulphuric acid reacts with water droplets to form a rain cloud. The precipitation that comes out of this rain cloud will be high in acidity. Acid rain also affects infrastructures by corroding cars, buildings, and historical monuments. This costs cities millions in damages every year!