Stability of ammonia in the primitive terrestrial atmosphere

A one‐dimensional photochemical model is used to calculate the rate at which ammonia would have been destroyed in the earth's atmosphere for assumed NH 3 mixing ratios of 10 −8 to 10 −4 . The calculated NH 3 destruction rates are then compared with possible abiotic and biotic sources for ammonia. Abiotic sources were probably sufficient to maintain the NH 3 mixing ratio of 10 −8 needed for life to have evolved but were too small to sustain the mixing ratios of 10 −5 or more necessary to produce significant greenhouse warming. The atmospheric ammonia concentration may have increased during the Archean as a result of biological activities but probably remained below the level required to generate measurable thermal effects.