Applicability of the steady state approximation to the interpretation of atmospheric observations of NO₃ and N₂O₅

This paper examines the conditions under which a steady state analysis is valid for modeling NO 3 and N 2 O 5 chemistry in the atmosphere. The conclusions come from a simple box model analysis that considers a limited number of reactions between NO 2 , O 3 , NO 3 , N 2 O 5 and the presumed sinks for the latter two. The applicability of the steady state depends on the strength of the sinks for NO 3 and N 2 O 5 , the concentration of NO 2 , and the ambient temperature. Under clean conditions, weak sinks for NO 3 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 NO 2 concentrations) or cold conditions, the presence of an equilibrium between NO 3 and N 2 O 5 markedly slows the approach to steady state even though the two species are close to equilibrium. The time required to approach equilibrium between NO 2 , NO 3 , and N 2 O 5 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 NO 3 and N 2 O 5 from observed concentration measurements for the steady state case.