Nighttime Ozone Chemical Equilibrium in the Mesopause Region

We examine the applicability of the assumption that nighttime ozone is in photochemical equilibrium. The analysis is based on calculations with a 3‐D chemical transport model. These data are used to determine the ratio of correct (calculated) O3 density to its equilibrium value for the conditions of the nighttime mesosphere depending on the altitude, latitude, and month in the annual cycle. The results obtained demonstrate that the retrieval of O and H distributions using the assumption of photochemical ozone equilibrium may lead to a significant error below 81–87 km depending on season. Possible modifications of the currently used approach that allow improving the quality of retrieval of O and H mesospheric distributions from satellite‐based observations are discussed. Key Points We examine the applicability of the assumption of chemical equilibrium for ozone in the nighttime mesopause by 3‐D chemical transport modeling We demonstrate that the retrieval of O and H distributions using the assumption of chemical ozone equilibrium may lead to a significant error We propose a criterion for determining the borders of the regions where nighttime ozone can be expressed by its equilibrium steady state