Simulation of atmospheric COS mixing ratio : Evaluating the impact of transport and emission distribution on COS tropospheric variability using ground-based, aircraft, and FTIR data

For the first time, we present a comparison of atmospheric transport models for Carbonyl Sulfide (COS), a promising photosynthesis tracer, within the framework of the ongoing Atmospheric Tracer Transport Model Intercomparison Project (TransCom). Seven atmospheric transport models participated in the inter-comparison experiment and provided simulations of COS mixing ratios in the troposphere over a 10-year period (2010–2019), using prescribed state of the art surface fluxes for each component of the atmospheric COS budget (i.e., linked to vegetation, soil, ocean, fire and industry). The main goals of TransCom-COS are (a) to investigate the roles of the transport uncertainty and emission distribution in simulating the spatio-temporal variability of COS mixing ratio in the troposphere and (b) to assess the sensitivity of simulated tropospheric COS mixing ratio to the seasonal variability of the COS terrestrial fluxes. Models were run with the same prior emissions and without chemistry to isolate differences due to transport. Two COS flux scenarios were compared: one using a biospheric flux with a monthly time resolution and the other one using a biospheric flux with a tri-hourly time resolution. In addition, we investigated the sensitivity of the simulated concentrations to different biospheric fluxes and to indirect oceanic emissions through DMS. The modelled COS mixing ratios were assessed against observations from in situ surface stations, aircraft and ground based FTIR stations. 

Using the state of the art surface fluxes for each component of the COS budget, preliminary results indicate that all transport models fail to capture the surface latitudinal distribution of COS. The COS mixing ratios are underestimated by at least 50 ppt in the tropics, pointing to a missing tropical source. In summer, the mixing ratios are overestimated by at least 50 ppt above 40N, pointing to a likely missing sink in the high northern latitudes during this period. The surface variability of COS mixing ratios is more sensitive to transport models than to a change in biospheric fluxes (two estimates based on different global Land Surface Models). Regarding the seasonal mean latitudinal profiles, the model spread is greater than 60 ppt above 40N in boreal summer and in the vicinity of anthropogenic sources. Regarding the seasonal amplitude, the model spread reaches 50 ppt at 6 sites out of 15, compared to an observed seasonal amplitude of 100 ppt. All models simulated a too late minimum by 2 to 3 months at northern sites ALT, BRW owing to likely errors in the seasonal cycle in the ocean emissions. Finally, the temporal resolution of the biospheric fluxes (monthly versus tri-hourly) has a small impact (less than 20 ppt) on the mean seasonal cycle at stations from the NOAA network.