Intercomparison of Atmospheric Carbonyl Sulfide (TransCom-COS): 2. Evaluation of Optimized Fluxes Using Ground-Based and Aircraft Observations

We present a comparison of atmospheric transport models that simulate carbonyl sulfide (COS). This is part II of the ongoing Atmospheric Transport Model Inter-comparison Project (TransCom-COS). Differently from part I, we focus on seven model intercomparison by transporting two recent COS inversions of NOAA surface data within TM5-4DVAR and LMDz models. The main goals of TransCom-COS part II are (a) to compare the COS simulations using the two sets of optimized fluxes with simulations that use a control scenario (part I) and (b) to evaluate the simulated tropospheric COS abundance with aircraft-based observations from various sources. The output of the seven transport models are grouped in terms of their vertical mixing strength: strong and weak mixing. The results indicate that all transport models capture the meridional distribution of COS at the surface well. Model simulations generally match the aircraft campaigns HIAPER Pole-To-Pole Observations (HIPPO) and Atmospheric Tomography Mission (ATom). Comparisons to HIPPO and ATom demonstrate a gap between observed and modeled COS over the Pacific Ocean at 0-40 & DEG;N, indicating a potential missing source in the free troposphere. The effects of seasonal continental COS uptake by the biosphere, observed on HIPPO and ATom over oceans, is well reproduced by the simulations. We found that the strength of the vertical mixing within the column as represented in the various atmospheric transport models explains much of the model to model differences. We also found that weak-mixing models transporting the optimized flux derived from the strong-mixing TM5 model show a too strong seasonal cycle at high latitudes. Carbonyl sulfide (COS) is a significant sulfur-containing trace gas in the atmosphere, which makes it important for studying climate change. One of the reasons it is worth investigating is because plants take up COS in a similar way as CO2 during photosynthesis. However, the atmospheric sources and sinks of COS are not well understood. To address this knowledge gap, we evaluated the state-of-the-art optimized surface COS fluxes from the inverse models TM5-4DVAR and LMDz, and then seven atmospheric transport models were used to simulate COS mole fractions by transporting the optimized fluxes under the TransCom-COS protocol. The results showed good agreement between the simulated COS and COS observations on independent platforms. The study also revealed that COS drawdown due to plant uptake can be observed over Pacific and Atlantic Oceans. However, discrepancies between the model simulations and observations were mainly found in free troposphere, emphasizing the need for further investigation into COS chemistry and model transport differences. These findings provide important reference for further investigation of COS global distribution and budget analysis. Simulations in seven models propagating optimized carbonyl sulfide (COS) fluxes derived from two inversions agree with independent observationsSimulated and observed COS drawdowns are captured in boundary layer over the Pacific and Atlantic Oceans due to plant uptake over landsWeak vertical mixing models using fluxes optimized from the fast-mixing TM5 model overestimate the COS seasonal amplitude at high latitudes