The sunrise and sunset variation of ClO in the lower stratosphere

The abundances of ClO have been measured, in situ, in the lower stratosphere during sunrise and sunset. Measurements were made with an instrument mounted on the NASA ER‐2 aircraft, which was flown at an altitude of 20 km and latitudes between 35°N and 47°N during a morning and an evening flight. The abundances of ClO were observed over a dynamic range of 20 from a detection threshold of ∼ 1 part per trillion volume (pptv). These data confirm the sunrise variation of the photolysis of chlorine nitrate that is predicted by a zero‐dimensional photochemical model. They also suggest that the absolute photolysis and termolecular formation of chlorine nitrate occur at rates consistent with nominal ClONO 2 and NO 2 concentrations.