Detailed calculations of the behavior of the NO3 column content as a function of time after sunset show that it takes nearly 2 hours to increase from daytime values of approximately 1011 cm-2 to nighttime values of 1014 cm-2. These results appear to be in conflict with the measured 3> rise time of 40 min after sunset obtained from the 662-nm absorption data of Noxon et al. (1978). Changes in the rate constants affecting the NO, NO2, NO3, N2O5 system, the atmospheric temperature, and the amount of NO2 are not able to bring the calculated rise time for the NO3 column content into agreement with observations.