From AMS Glossary
The time required for the concentration of a chemical species to decrease to 1/e of its original value.
The definition arises from assuming a pseudo-first-order decay of the species concentration,
. When the time t is equal to the inverse of the first-order rate coefficient k, the concentration [A] will have decreased to 1/e of the initial concentration [A]0. The atmospheric lifetime of a species in steady state can also be defined as the total atmospheric burden divided by either the total loss rate or the total production rate.