In situ observations of stratospheric HCl using three-mirror integrated cavity output spectroscopy

Stratospheric HCl observations are an important diagnostic for the evaluation of catalytic processes that impact the ozone layer. We report here in situ balloon-borne observations of HCl employing an off-axis integrated cavity output spectrometer (OA-ICOS) fitted with a reinjection mirror. Laboratory assessments demonstrated that the spectrometer has a 90% response time of 10 s to changes in HCl and a 30 s precision of 26 pptv. The instrument was deployed alongside an ozone instrument in August 2018 on a balloon-borne descent between 20-80 hPa (29-18 km altitude). The observations agreed with nearby satellite measurements made by the Earth Observing System Microwave Limb Sounder within 10% on average. This is the first time that stratospheric measurements of HCl have been made with ICOS and the first time any cavity-enhanced HCl instrument has been tested in flight.