lifetime in atmospheric chemistry

https://dx.doi.org/10.1351/goldbook.L03516
The time required for the luminescence intensity to decay from some initial value to 1 e of that value (e = 2.718 ...). Lifetimes can be measured by phase fluorimetry (phosphorimetry) where the phase shift between the sinusoidally modulated exciting light and the emitted light is measured.
Source:
PAC, 1984, 56, 231 (Nomenclature, symbols, units and their usage in spectrochemical analysis-Part VI: molecular luminescence spectroscopy) on page 242