The rate at which a pollutant is removed from the atmosphere either by reaction with reactive transient species such as the HO radical, O₃, etc., by photodecomposition initiated by light absorption by the impurity, or by loss at the surface of aerosols, the earth, etc. The decay rate as applied to @R05077@ materials is related to the @R05077@ half-life (\(t_{1/2}\)) of the particular isotopic species A and its concentration \([A]_{t}\), at the given time (\(t\)): \[\text{Rate}=\frac{[A]_{t}\ \ln 2}{t_{1/2}}\]