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Carbonyl sulfide (OCS) is used to quantify the carbon capture potential of the biosphere because of its direct correlation with CO2uptake during photosynthesis. However, to constrain the urban biosphere signal, it is necessary to evaluate potential anthropogenic sources. We conducted two sampling campaigns in the Metropolitan Area of Barcelona (AMB), Spain, during May (full COVID lockdown) and October 2020 to measure the spatial distribution and variability of OCS in four urban land uses as follows: built, urban forest, urban park, and peri‐urban agriculture. The OCS background levels determined at Tibidabo (442 m asl) were approximately 484 ± 20 ppt and 407 ± 8 ppt for May and October 2020, respectively, and agreed with other seasonal surveys conducted in Europe during that same period. Averaged emissions were in the range of +Δ12 ± 40 ppt for the city and were +Δ9.4 ± 40.9 ppt for urban +Δ22.1 ± 48 ppt for urban green +Δ20.7 ± 42.9 ppt for agricultural and −Δ4.8 ± 19.6 ppt for forest. The urban values ranged from neutral to above background, suggesting nearby anthropogenic and marine emissions such as +Δ150 ppt in Montjuic, which is downwind of Barcelona's harbor. During the crop‐growing season in May, the agricultural areas consistently showed values below the background (up to −Δ76 ppt in Gavà, uptake) at 7:00 UTC when the land breezes were dominant, while later in the morning, when the sea breeze are developed, the plant sink is masked by the transport of marine emissions. Urban forests located north of Tibidabo showed OCS values up to −Δ70 ppt, suggesting significant uptake by urban forests. We conclude that determining the urban biosphere signal using OCS as a tracer is more complex than expected because the marine and anthropogenic emissions from the port strongly impact the spatial‐temporal distribution of OCS. Carbonyl sulfide (OCS) is a gas used to quantify the carbon capture potential of the biosphere. However, cities have potential anthropogenic OCS sources that can impede a clear constraint of the urban biosphere signal. We conducted two sampling campaigns in the Metropolitan Area of Barcelona (AMB), Spain, during May (full COVID lockdown) and October 2020 to measure the variability of OCS in four urban land uses: built, urban forest, urban park, and peri‐urban agriculture. The OCS background levels determined at Tibidabo (were approximately 484 ± 20 ppt and 407 ± 8 ppt for May and October 2020, respectively). The urban built sites were typically above background, suggesting nearby anthropogenic and marine emissions. During the crop‐growing season, the agricultural areas consistently showed values below the background suggesting plant OCS uptake when the land breezes were dominant, while later in the morning, when the sea breezes developed, the influence of marine emission resulted in values above the background. Urban forests showed significant OCS uptake. We conclude that determining the urban biosphere signal using OCS as a tracer is more complex than expected because the marine and anthropogenic emissions from the port strongly impact the spatial‐temporal distribution of OCS. The urban built sites were typically above background, suggesting nearby anthropogenic and marine emissionsAgricultural areas consistently showed values below the background suggesting plant Carbonyl sulfide (OCS) uptake when the land breezes were dominantDetermining urban biosphere signal using OCS is complex because marine and anthropogenic emissions impact its spatial‐temporal distribution The urban built sites were typically above background, suggesting nearby anthropogenic and marine emissions Agricultural areas consistently showed values below the background suggesting plant Carbonyl sulfide (OCS) uptake when the land breezes were dominant Determining urban biosphere signal using OCS is complex because marine and anthropogenic emissions impact its spatial‐temporal distribution

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. 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 CO2during 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 Simulations in seven models propagating optimized carbonyl sulfide (COS) fluxes derived from two inversions agree with independent observations Simulated and observed COS drawdowns are captured in boundary layer over the Pacific and Atlantic Oceans due to plant uptake over lands Weak vertical mixing models using fluxes optimized from the fast‐mixing TM5 model overestimate the COS seasonal amplitude at high latitudes

Gross primary production (GPP) by boreal forests is highly sensitive to environmental changes. However, GPP simulated by land surface models (LSMs) remains highly uncertain due to the lack of direct photosynthesis observations at large scales. Carbonyl sulfide (COS) has emerged as a promising proxy to improve the representation of GPP in LSMs. Because COS is absorbed by vegetation following the same diffusion pathway as CO2during photosynthesis and not emitted back to the atmosphere, incorporating a mechanistic representation of vegetation COS uptake in LSMs allows using COS observations to refine GPP representation. Here, we perform ecosystem COS flux and GPP data assimilations to constrain the COS‐ and GPP‐related parameters in the ORCHIDEE LSM for boreal evergreen needleleaf forests (BorENF). Assimilating ecosystem COS fluxes at Hyytiälä forest increases the simulated net ecosystem COS uptake by 14%. This increase largely results from changes in the internal conductance to COS, highlighting the need to improve the representation of COS internal diffusion and consumption. Moreover, joint assimilation of ecosystem COS flux and GPP at Hyytiälä improves the simulated latent heat flux, contrary to the GPP‐only data assimilation, which fails to do so. Finally, we scaled this assimilation framework up to the boreal region and find that the joint assimilation of COS at Hyytiälä and GPP fluxes at 10 BorENF sites increases the modeled vegetation COS uptake up to 18%, but not GPP. Therefore, this study encourages the use of COS flux observations to inform GPP and latent heat flux representations in LSMs. Carbon uptake by boreal forests is highly sensitive to environmental changes. There is large uncertainty about how much carbon dioxide (CO2) boreal forests absorb through photosynthesis, as represented by land surface models. Carbonyl sulfide (COS), a trace gas that tracks photosynthesis, can help improve the representation of simulated plant CO2uptake because COS and CO2share a common pathway during leaf uptake. Using a mechanistic model of biospheric COS processes implemented in the ORCHIDEE land surface model, we assimilated ecosystem COS flux and plant CO2uptake measured at Hyytiälä boreal forest. We find that this joint assimilation improves the simulated plant CO2uptake, as well as transpiration because of the strong link between COS, CO2and H2O fluxes through stomatal diffusion. Scaling up this assimilation framework to evergreen needleleaf boreal forests, we find that assimilating ecosystem COS flux and plant CO2uptake data increases the vegetation COS uptake for this biome, but not plant CO2uptake. Our results imply that COS has the potential to constrain both plant carbon uptake and transpiration in land surface models, which should be further investigated, especially during drought events. Jointly assimilating ecosystem carbonyl sulfide (COS) flux and gross primary production data improves both simulated plant carbon uptake and transpirationAssimilating ecosystem COS flux does not increase gross primary production (GPP) over boreal evergreen needleleaf forests, in contrast with previous inversion studiesCOS flux observations help to identify misrepresentation of the sensitivity of GPP to droughts in models Jointly assimilating ecosystem carbonyl sulfide (COS) flux and gross primary production data improves both simulated plant carbon uptake and transpiration Assimilating ecosystem COS flux does not increase gross primary production (GPP) over boreal evergreen needleleaf forests, in contrast with previous inversion studies COS flux observations help to identify misrepresentation of the sensitivity of GPP to droughts in models

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. 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. 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 The model‐observation mismatch suggests there is a missing source in the tropics and a missing sink in the high northern latitude in summer At northern latitude sites, the model spread in seasonal amplitude reaches 50 ppt compared to a mean seasonal amplitude of about 100 ppt The diurnal rectifier effect is small, decreasing the seasonal amplitude by up to 20% at continental sites

Environmental context Models are needed to predict the importance of the changes in marine emissions of dimethylsulfide (DMS) in response to ocean warming, increased stratification and acidification, and to evaluate the potential effects on the Earth’s climate. We use complementary simulations to further our understanding of the marine cycle of DMS in subtropical waters, and show that a lack of phosphorus may exert a more important control on surface DMS concentrations than an excess of light. Abstract The occurrence of a summer DMS paradox in the vast subtropical gyres is a strong matter of debate because approaches using discrete measurements, climatological data and model simulations yielded contradictory results. The major conclusion of the first appraisal of prognostic ocean DMS models was that such models need to give more weight to the direct effect of environmental forcings (e.g. irradiance) on DMS dynamics to decouple them from ecological processes. Here, the relative role of light and phosphorus on summer DMS dynamics in subtropical waters is assessed using the ocean general circulation and biogeochemistry model NEMO-PISCES in which macronutrient concentrations were restored to monthly climatological data values to improve the representation of phosphate concentrations. Results show that the vertical and temporal decoupling between chlorophyll and DMS concentrations observed in the Sargasso Sea during the summer months is captured by the model. Additional sensitivity tests show that the simulated control of phosphorus on surface DMS concentrations in the Sargasso Sea is much more important than that of light. By extending the analysis to the whole North Atlantic Ocean, we show that the longitudinal distribution of DMS during summer is asymmetrical and that a correlation between the solar radiation dose and DMS concentrations only occurs in the Sargasso Sea. The lack of a widespread summer DMS paradox in our model simulation as well as in the comparison of discrete and climatological data could be due to the limited occurrence of phosphorus limitation in the global ocean.