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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°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. Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

We present a comparison of atmospheric transport model (ATM) simulations for carbonyl sulfide (COS), within the framework of the atmospheric tracer transport model intercomparison project "TransCom‐COS." Seven ATMs participated in the experiment and provided simulations of COS mixing ratios over the years 2010–2018, using state‐of‐the‐art surface fluxes for various components of the COS budget: biospheric sink, oceanic source, sources from fire and industry. The main goal of TransCom‐COS is to investigate the impact of the transport uncertainty and emission distribution in simulating the spatio‐temporal variability of tropospheric COS mixing ratios. A control case with seasonal surface fluxes of COS was constructed. The results indicate that the COS mixing ratios are underestimated by at least 50 parts per trillion (ppt) in the tropics, pointing to a missing tropical source. In summer, the mixing ratios are overestimated by at least 50 ppt above 40°N, pointing to a likely missing sink in the high northern latitudes. Regarding the latitudinal profile, the model spread is greater than 60 ppt above 40°N in boreal summer. Regarding the seasonal amplitude, the model spread reaches 50 ppt at 6 out of 15 sites, compared to an observed seasonal amplitude of 100 ppt. All models simulated a too late minimum by at least 2–3 months at two high northern‐latitude sites, likely owing to errors in the seasonal cycle in the ocean emissions. This study highlighted the shortcomings in the COS global budget that need to be resolved before using COS as a photosynthesis tracer. Plain Language Summary: In this study, we evaluate the state‐of‐the‐art fluxes for various components of the carbonyl sulfide (COS) budget: biospheric sink, oceanic source, sources from fire and industry. A control case with seasonal surface fluxes of COS was constructed. Seven atmospheric transport models provided simulations of COS mixing ratios. Then, the simulated mixing ratios were evaluated against atmospheric measurements at several surface sites. Results show that all models fail to capture the observed latitudinal distribution and that the model spread is small compared to the model‐observation mismatch. In summer, the overestimated mixing ratios above 40°N point to a likely missing sink in the high northern latitudes. The underestimated mixing ratios in the tropics point to a missing tropical source. This study highlighted the shortcomings in the COS global budget that need to be resolved before using COS as a photosynthesis tracer. Key Points: The model‐observation mismatch suggests there is a missing source in the tropics and a missing sink in the high northern latitude in summerAt northern latitude sites, the model spread in seasonal amplitude reaches 50 ppt compared to a mean seasonal amplitude of about 100 pptThe diurnal rectifier effect is small, decreasing the seasonal amplitude by up to 20% at continental sites [ABSTRACT FROM AUTHOR]

The ability of global transport models to go up in resolution becomes discriminating for greenhouse gas atmospheric inversions. This paper describes the porting on Graphics Processing Units of the global transport model currently used in the European operational Copernicus Atmosphere Monitoring Service (CAMS) for CO2 and N2O inversions. It represents an important milestone to achieve sub‐degree resolution. The code includes not only the direct model but also its tangent‐linear and its adjoint versions which are needed in variational inversions. Tests were carried out for CO2 at a resolution of 2.50° in longitude, 1.27° in latitude and 79 layers in the vertical, corresponding to 1,626,768 3D cells, 4.5 times more than the current standard resolution of the model used in the CAMS reanalyzes. A month's worth of computation of the tangent‐linear and of the adjoint versions now takes 2.5 min, including 50 s for reading meteorological data. Plain Language Summary: Atmospheric transport models are intensively used to infer global greenhouse gas emissions and removals, from atmospheric measurements: a single global analysis involves repeated and long transport simulations. This intensive use has limited the spatial resolution of such analyses despite an increasing need for national greenhouse gas budgets, despite an increasing number of corresponding space‐based observations at kilometer resolution, and despite an increasing number of high‐quality surface measurements made in sites marked by strong local influences. For the global transport model currently used in the European operational atmosphere monitoring service, our objective within the next 5 years is to make it reach a resolution of about 50 km over the whole globe. This paper describes an important milestone in this direction with the porting of this transport model on hardware components initially developed for video display but now used for high performance computing. Tests were carried out at a resolution of 2.50° in longitude, 1.27° in latitude and 79 layers in the vertical, corresponding to 4.5 times more 3D cells than the current standard resolution of the model. The direct model itself now takes less time than reading the input meteorological data. Key Points: The workload in a Eulerian transport model on a longitude‐latitude grid can be embarrassingly parallel enough to run efficiently on a Graphics Processing UnitThe transport model of the Laboratoire de Météorologie Dynamique was run in a new grid of 1,626,768 3D cellsThe calculation time in the direct version of the model is now less than the time to read the input meteorological data [ABSTRACT FROM AUTHOR]

Carbonyl sulfide (COS) is an atmospheric trace gas of interest for C cycle research because COS uptake by continental vegetation is strongly related to terrestrial gross primary productivity (GPP), the largest and most uncertain flux in atmospheric CO 2 budgets. However, to use atmospheric COS as an additional tracer of GPP, an accurate quantification of COS exchange by soils is also needed. At present, the atmospheric COS budget is unbalanced globally, with total COS flux estimates from oxic and anoxic soils that vary between -409 and -89 GgS yr -1. This uncertainty hampers the use of atmospheric COS concentrations to constrain GPP estimates through atmospheric transport inversions. In this study we implemented a mechanistic soil COS model in the ORCHIDEE (Organising Carbon and Hydrology In Dynamic Ecosystems) land surface model to simulate COS fluxes in oxic and anoxic soils. Evaluation of the model against flux measurements at seven sites yields a mean root mean square deviation of 1.6 pmol m -2 s -1 , instead of 2 pmol m -2 s -1 when using a previous empirical approach that links soil COS uptake to soil heterotrophic respiration. However, soil COS model evaluation is still limited by the scarcity of observation sites and long-term measurement periods, with all sites located in a latitudinal band between 39 and 62 ∘ N and no observations during wintertime in this study. The new model predicts that, globally and over the 2009–2016 period, oxic soils act as a net uptake of -126 GgS yr -1 and anoxic soils are a source of + 96 GgS yr -1 , leading to a global net soil sink of only -30 GgS yr -1 , i.e. much smaller than previous estimates. The small magnitude of the soil fluxes suggests that the error in the COS budget is dominated by the much larger fluxes from plants, oceans, and industrial activities. The predicted spatial distribution of soil COS fluxes, with large emissions from oxic (up to 68.2 pmol COS m -2 s -1) and anoxic (up to 36.8 pmol COS m -2 s -1) soils in the tropics, especially in India and in the Sahel region, marginally improves the latitudinal gradient of atmospheric COS concentrations, after transport by the LMDZ (Laboratoire de Météorologie Dynamique) atmospheric transport model. The impact of different soil COS flux representations on the latitudinal gradient of the atmospheric COS concentrations is strongest in the Northern Hemisphere. We also implemented spatiotemporal variations in near-ground atmospheric COS concentrations in the modelling of biospheric COS fluxes, which helped reduce the imbalance of the atmospheric COS budget by lowering soil COS uptake by 10 % and plant COS uptake by 8 % globally (with a revised mean vegetation budget of -576 GgS yr -1 over 2009–2016). Sensitivity analyses highlighted the different parameters to which each soil COS flux model is the most responsive, selected in a parameter optimization framework. Having both vegetation and soil COS fluxes modelled within ORCHIDEE opens the way for using observed ecosystem COS fluxes and larger-scale atmospheric COS mixing ratios to improve the simulated GPP, through data assimilation techniques. [ABSTRACT FROM AUTHOR]

Carbonyl sulfide (COS), a trace gas showing striking similarity to CO 2 in terms of biochemical diffusion pathway into leaves, has been recognized as a promising indicator of the plant gross primary production (GPP), the amount of carbon dioxide that is absorbed through photosynthesis by terrestrial ecosystems. However, large uncertainties about the other components of its atmospheric budget prevent us from directly relating the atmospheric COS measurements to GPP. The largest uncertainty comes from the closure of its atmospheric budget, with a source component missing. Here, we explore the benefit of assimilating both COS and CO 2 measurements into the LMDz atmospheric transport model to obtain consistent information on GPP, plant respiration and COS budget. To this end, we develop an analytical inverse system that optimizes biospheric fluxes for the 15 plant functional types (PFTs) defined in the ORCHIDEE global land surface model. Plant uptake of COS is parameterized as a linear function of GPP and of the leaf relative uptake (LRU), which is the ratio of COS to CO 2 deposition velocities in plants. A possible scenario for the period 2008–2019 leads to a global biospheric sink of 800 GgS yr -1 , with higher absorption in the high latitudes and higher oceanic emissions between 400 and 600 GgS yr -1 most of which is located in the tropics. As for the CO 2 budget, the inverse system increases GPP in the high latitudes by a few GtC yr -1 without modifying the respiration compared to the ORCHIDEE fluxes used as a prior. In contrast, in the tropics the system tends to weaken both respiration and GPP. The optimized components of the COS and CO 2 budgets have been evaluated against independent measurements over North America, the Pacific Ocean, at three sites in Japan and at one site in France. Overall, the posterior COS concentrations are in better agreement with the COS retrievals at 250 hPa from the MIPAS satellite and with airborne measurements made over North America and the Pacific Ocean. The system seems to have rightly corrected the underestimated GPP over the high latitudes. However, the change in seasonality of GPP in the tropics disagrees with solar-induced fluorescence (SIF) data. The decline in biospheric sink in the Amazon driven by the inversion also disagrees with MIPAS COS retrievals at 250 hPa, highlighting the lack of observational constraints in this region. Moreover, the comparison with the surface measurements in Japan and France suggests misplaced sources in the prior anthropogenic inventory, emphasizing the need for an improved inventory to better partition oceanic and continental sources in Asia and Europe. [ABSTRACT FROM AUTHOR]

Land surface modelers need measurable proxies to constrain the quantity of carbon dioxide (CO2) assimilated by continental plants through photosynthesis, known as Gross Primary Production (GPP). Carbonyl sulfide (COS), which is taken up by leaves through their stomates and then hydrolysed by photosynthetic enzymes, is a candidate GPP proxy. A former study with the ORCHIDEE land surface model used a fixed ratio of COS uptake to CO2 uptake normalized to respective ambient concentrations for each vegetation type (Leaf Relative Uptake, LRU). COS leaf fluxes were then computed from GPP, and the resulting concentrations were transported with an atmospheric model which included all other known COS fluxes as inputs. Modelled COS concentrations could then be compared to COS measurements from the NOAA air sampling tower network. The LRU approach is known to have limited accuracy since the LRU ratio changes with variables such as Photosynthetically Active Radiation (PAR): while CO2 uptake slows under low light, COS uptake is not light limited. However, the LRU approach has been popular for COS-GPP proxy studies because of its ease of application and apparent low contribution to uncertainty for regional scale applications. In this study we refined the COS-GPP relationship and implemented in ORCHIDEE a mechanistic model that describes COS uptake by continental vegetation. We compared the simulated COS fluxes against measured hourly COS fluxes at two sites, and studied the model behaviour and links with environmental drivers. We performed simulations at global scale, and estimated the global COS uptake by vegetation to be -756 Gg S yr-1, in the middle range of former studies (-490 to -1335 Gg S yr-1 ). Based on the mechanistic approach in ORCHIDEE, we derived new LRU values for the different vegetation types, ranging between 0.92 and 1.72, close to recently published averages for observed values of 1.21 for C4 and 1.68 for C3 plants. We transported the COS using the monthly vegetation COS fluxes derived from both the mechanistic and the LRU approaches, and evaluated the simulated COS concentrations at NOAA sites. Although the mechanistic approach was more appropriate when comparing to high-temporal-resolution COS flux measurements, both approaches gave similar results when transporting with monthly COS fluxes and evaluating COS concentrations at stations. In our study, uncertainties between these two approaches are of second importance as compared to the uncertainties in the COS global budget, which are currently a limiting factor to the potential of COS concentrations to constrain GPP simulated by land surface models on the global scale. [ABSTRACT FROM AUTHOR]