Ozone depletion in the upper stratosphere at the dawn terminator

In situ measurements of atmospheric ozone at an altitude of 40 km reveal a significant depletion event (similar to 50%) coincident with the transit of the dawn terminator. The event developed in about 20 min and then recovered in about half that time. Simultaneous NO data from a detector on the same balloon gondola allow three succeeding photochemical episodes associated with the ozone depletion event to be identified. Qualitative modeling indicates that the phenomenon should be expected to occur daily at all locations on Earth where a well-defined sunrise occurs, and should be easily observed by an in situ ozone detector that is maintained at a constant altitude in the upper stratosphere throughout terminator passage. Several implications of this result are discussed: (I) from an overall perspective, it indicates that ozone depletion in the upper stratosphere at the dawn terminator is in the form of a westward-moving traveling wave; (2) during the initial phase of the phenomenon, ozone depletion can be observed in the absence of ozone formation, thus allowing model calculations of stratospheric photochemistry to be investigated under more nearly ideal experimental conditions; (3) several satellite programs to determine the characteristics of the upper atmosphere are based on measurements of solar radiation on paths through the upper atmosphere at the dawn and dusk terminators; analysis procedures for those data should be re-evaluated if the same ozone variation with altitude has been assumed for the day side and the night side of the terminator. Although the conclusions reached in this paper are based on experimental measurements from a single balloon flight, the high quality of the ozone data is solidly established by its correlation with altitude during conditions of night-time ozone equilibrium; also, these results are in good agreement with previous (but less definitive) reports of ozone depletion at sunrise. Copyright (C) 1996 Elsevier Science Ltd.