How are nitrogen oxides formed in the atmosphere?

• A. Formed by nitrogen reacting with carbon dioxide.
• B. Formed when nitrogen and oxygen react at high temperatures, such as in vehicle engines.
• C. Formed by water splitting to hydrogen and oxygen.
• D. Formed when nitrogen dissolves in oxygen under pressure.

Answer: (B)

Nitrogen oxides form in the atmosphere mainly from high-temperature combustion where nitrogen from the air reacts with oxygen. In engines and furnaces the temperatures are high enough to break the strong N≡N bond in N2, allowing it to react with O2 to make NO: N2 + O2 → 2 NO. This NO can then further react with O2 to produce NO2: 2 NO + O2 → 2 NO2. These NOx gases are produced by engines and other high-temperature combustion processes and then contribute to air pollutants.

The other scenarios don’t fit this process: nitrogen reacting with carbon dioxide isn’t a typical route to NOx; splitting water gives hydrogen and oxygen, not nitrogen oxides; and nitrogen dissolving in oxygen under pressure doesn’t generate NOx in the atmosphere.