Atmospheric lifetime

Atmospheric lifetime is the average time a substance remains in the atmosphere before chemical or physical removal. It depends on the substance’s removal processes and the timescale being considered.

Chemical reaction: A process in which substances transform into different chemical substances through rearrangement of atoms. Atmospheric oxidation and other reactions remove gases by transforming them.
Residence time: The average time material remains within a defined reservoir before leaving it. Atmospheric lifetime is often treated as the residence time of a substance in air.
Global warming potential: An index comparing the integrated radiative forcing of a greenhouse-gas pulse with that of carbon dioxide over a specified period. A gas’s atmospheric persistence shapes how its warming effect accumulates over the chosen horizon.
Perturbation lifetime: The characteristic time for an atmospheric concentration increase above its baseline to decline after an emission perturbation. It tracks the decay of an added pulse, rather than the average turnover of the entire atmospheric burden.
Photolysis: Chemical decomposition caused by a substance absorbing light. Sunlight breaks down some atmospheric gases, providing a removal pathway.
Turnover time: The time required for a reservoir to lose or replace an amount equal to its contents at a given flux. This closely related measure is commonly calculated from atmospheric burden and removal flux.
Ozone depletion potential: A relative measure of a substance’s capacity to deplete stratospheric ozone compared with a reference substance. Long-lived ozone-depleting substances can reach the stratosphere before breaking down.
Adjustment time: The time for an atmospheric concentration perturbation to approach equilibrium after emissions or conditions change. Feedbacks and coupled reservoirs can make adjustment slower than direct chemical removal.
Dry deposition: Transfer of gases or particles from the atmosphere to surfaces without precipitation. Uptake by land, water, and vegetation removes substances from the air.
E-folding time: The time over which an exponentially declining quantity falls to 1/e of its initial value. A first-order atmospheric loss process has an e-folding time equal to its lifetime.

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