The most important anthropogenic sources of nitrogen oxides are combustion processes. Nitrogen oxides can be formed in combustion processes from the nitrogen contained in the fuel or from the nitrogen that is part of the air.

In most of the external fossil fuel combustion systems, around 95% of the nitrogen oxides emitted are in the form of nitrogen monoxide (NO), whereas the remaining 5% is in the form of nitrogen dioxide (NO₂).

The NO emitted oxidizes further in the atmosphere to NO₂. The term NO_X refers to the sum of nitric oxide (NO) and nitrogen dioxide (NO₂), expressed as NO₂.

NO₂ is a highly reactive gas whose color gives the peculiar tone of reddish-brown to photochemical smog. Also, these oxides react with water to produce nitric acid (HNO₃), which, together with sulfuric acid, results in acid rain.

Another harmful effect of nitrogen oxides is the atmospheric deposition of nitrogen as nitrates and nitrites derived from NO_X, which leads to eutrophication of inland waters and coastal seas [46–48].