Evaluation of processes that affect the photochemical timescale of the sodium layer

We apply the method of the eigenvalue and eigenvector analysis (EEA method) to analyze the photochemical timescale of the sodium layer chemical system to perturbations. The results show that the timescales of three important sodium species, Na, Na⁺ and NaHCO₃, vary strongly with altitude and are significantly different during day and night. For these three species, the EEA method gives a photochemical timescale that differs strongly from the traditional lifetime, defined as the inverse of the photochemical loss rate. For other chemical species in the sodium layer, the two methods of predicting chemical lifetime give virtually identical results. Tests of the sensitivity of the photochemical timescale to changes of the background conditions indicate that the photochemical timescale of Na is sensitive to the changes of the background densities of O₃, H, and O.