Used in an ecological context to describe a large class of nitrogen oxide gases produced in combustion which occur as atmospheric pollutants. Colorless, poisonous gas formed when nitric acid reacts with copper or when ammonia reacts with platinum. It is produced in combustion engine processes such as automobile emissions, resulting in acid rain and photochemical smog. It is also an important messenger molecule in mammals and other animals, produced through cellular metabolism, and involved in oxygen transmission, blood pressure regulation, and other physiological activities.
