AbstractCarbonyl sulfide (OCS or COS) is a ubiquitous trace gas and plays a role in forming stratospheric sulfate aerosol particles, thereby influencing climate. In this study, whole‐air samples containing OCS were collected onboard a passenger aircraft (IAGOS‐CARIBIC) from the upper troposphere/lowermost stratosphere (UT/LMS, 10–12 km) region and analyzed with CryoTrap–GC–AED system in the laboratory. Global OCS mixing ratios are presented and by using the OCS measurements in conjunction with other trace gases, an atmospheric OCS lifetime of 2.1 ± 1.3 years, and lowermost stratospheric OCS lifetime of 47 ± 16 years were determined. A total flux of 137 GgS a⁻¹ of OCS from the troposphere into the stratosphere was estimated, and the stratospheric sink estimate yielded 55 ± 23 GgS a⁻¹ of OCS. The 60% smaller sink can be interpreted as 82 GgS a⁻¹ OCS which is transported back from the stratosphere into the troposphere.