Nitrogen Oxides Background Nitrogen oxides are gases composed of nitrogen and oxygen. Nitrogen dioxide is a suffocating, brownish-colored gas that is a strong oxidizing agent. It reacts easily with water vapor to form acid rain.
Nitrogen oxide is more common, but relatively less harmful.
However, nitrogen oxide emitted into the air is converted to nitrogen dioxide by photochemical reactions. When combined with other pollutants, it eventually causes ozone and other kinds of smog. The main sources of nitrogen oxides are motor vehicles, power plants, industry, burning of solid waste, and outdoor burning. Natural sources include volcanoes and forest fires. Washington does not exceed federal health-based standards for nitrogen oxides.