What Are the Sources of Nitric Oxide?

Ostwald Process

• For industrial purposes, nitric oxide is typically an unwanted byproduct of combustion reactions rather than a desirable end product in itself. One of its few major industrial uses is as part of the Ostwald process, in which ammonia burns or combines with oxygen to yield nitric oxide and water. The nitric oxide is in turn used to make nitrogen dioxide, which is combined with water to form nitric acid. Nitric acid has a variety of uses in the chemical industry, especially in the manufacture of fertilizers like ammonium nitrate, which is produced by combining nitric acid and ammonia.

Lightning

• At room temperature, nitrogen and oxygen do not react. It takes high temperatures to make this reaction take place -- and high temperatures are exactly what you find in the immediate vicinity of a lightning strike. Just as in the Ostwald process, nitric oxide formed by lightning quickly reacts with oxygen to form nitrogen dioxide. Given their close relationship, nitric oxide and nitrogen dioxide are sometimes collectively called NOx or nitrogen oxides.

Automobile Engines

• Another place where a similar sequence of reactions takes place is in the cylinders of an auto engine, where high temperatures cause nitrogen and oxygen to react and form nitric oxide. The nitrogen dioxide produced by car engines and power plants is the primary component of urban smog. Nitrogen dioxide can go on to react with water, forming both nitrous and nitric acid; these acids can decrease the pH of rainwater and contribute to acid rain.

Your Body

• Nitric oxide is a reactive compound that combines with oxygen to form an ugly brown pollutant. It seems counterintuitive at first sight that this unpleasant little molecule would play any role in your biology. As a matter of fact, however, some cells in your body synthesize nitric oxide from arginine (an amino acid) using an enzyme called NO synthase. The nitric oxide acts primarily as a signaling molecule that triggers responses in nearby cells. Among the many pathways that involve nitric oxide signaling are vasodilation (relaxation of the smooth muscle around blood vessels) and penile erection. Some white blood cells also produce nitric oxide to destroy bacteria or viruses after first engulfing them.