Nitrogen tetroxide is a chemical compound with the formula N₂O₄. It is a colorless gas at room temperature but can easily turn into a pale yellow liquid when cooled. This compound is often used as an oxidizer in rocket propellants, making it important in the field of aerospace engineering. When nitrogen tetroxide is exposed to light or heat, it can break down into nitrogen dioxide (NO₂), which is a reddish-brown gas. This reaction is significant in environmental science, as nitrogen dioxide is a pollutant that can contribute to smog and respiratory issues in humans.