Colourless gas, formula N₂O, released by bacterial activity at the earth's surface. It has an atmospheric lifetime of about 160 years and is currently present at a level of about 330 ppb. Its atmospheric significance is that it is transported into the stratosphere, where its reaction with excited oxygen atoms (O¹D) is the major source of active nitrogen; it is also a major greenhouse gas. In large amounts, it has anaesthetic properties (laughing gas).