Recent Decline in Global Atmospheric COS Driven by Reduced Anthropogenic CS 2 Emissions

Abstract Carbonyl sulfide (COS) is an atmospheric tracer for carbon cycle processes. Global observations reveal a rapid decline in COS concentrations beginning in 2016, but the cause of this decline is unknown. Here we show that this decrease is unexpected. Previous COS trends followed rayon production which emits CS 2 , a COS precursor. However, the recent COS decline coincides with increasing rayon output. Multiple independent observing systems, including NOAA's global flask network and NDACC FTIR retrievals, show a consistent tropospheric COS trend of −1.2 ± 0.1% yr −1 from 2016 to 2022, even as rayon manufacturing increased by 4.1 ± 1.3% yr −1 . Using atmospheric models, we demonstrate that the COS decline is consistent with a 2.5% yr −1 reduction in emissions, driven by decreasing CS 2 emission factors. Variations in other sources and sinks are unable to account for this trend. These results highlight the need for improved anthropogenic inventories and point toward robust COS applications to the carbon‐cycle through regional analyses.