This paper examines the conditions under which a steady state analysis is valid for modeling NO3 and N2O5 chemistry in the atmosphere. The conclusions come from a simple box model analysis that considers a limited number of reactions between NO2, O3, NO3, N2O5 and the presumed sinks for the latter two. The applicability of the steady state depends on the strength of the sinks for NO3 and N2O5, the concentration of NO2, and the ambient temperature. Under clean conditions, weak sinks for NO3 prevent the system from passing through the induction period during the time between sunset and sunrise, thus keeping the system out of steady state. Under polluted (i.e., large NO2 concentrations) or cold conditions, the presence of an equilibrium between NO3 and N2O5 markedly slows the approach to steady state even though the two species are close to equilibrium. The time required to approach equilibrium between NO2, NO3, and N2O5 is not a good measure of the time required to achieve a steady state among these compounds. The paper considers the conditions for which steady state may be valid and outlines a method for identification of individual sinks for NO3 and N2O5 from observed concentration measurements for the steady state case.