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Lifetime or residence time

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The average time a molecule spends in a reservoir (such as the atmosphere) is called the lifetime or residence time and usually denoted by the symbol t. By definition

t = [X]_ss/R_out   where

[X]_ss  is the steady state concentration of X and

R_out  is the rate of removal.

It follows from this definition that

• if two species have similar atmospheric concentrations and similar rates of input (and removal), they will have similar lifetimes
• if two species have the same atmospheric concentration, but different rates of removal, the species with the higher rate of removal (and input) will have the shorter residence time
• if two species have the same lifetime, but different atmospheric concentrations, the species with the lower concentration will have the shorter lifetime