Recent measurements by Webster et al. (1990) have confirmed quantitatively the chemistry controlling the night--time decay of NO2. Simple equations describing the night--time behavior of NO2 and N2O5 are presented here. With measurements of the night--time ozone and NO2 concentrations, these equations can be used to predict the amount of N2O5 produced at any time during the night. In this way the N2O5 night--time emission measurement of Roscoe (1982), Kunde et al. (1988) and sunrise measurements of the ATMOS experiment (Rinsland et al., 1989) are all used to test theory. The measurements are found to be both self consistent and confirm our understanding of night--time NO2 conversion to N2O5. The variation of N2O5 by a factor of two between measurements is found to be consistent with theory. ¿ American Geophysical Union 1991