## 3 Results and Discussion
Because the spring maximum and SCA both follow a clear decreasing trend over 6 years, it can be hypothesized that a causal relationship exists between the decrease in COS in the atmosphere and the reduction in the COS SCA through the role played by the biogenic sinks in the global budget of this gas. The rationale behind this hypothesis was tested with our model simulations. Mechanistic soil and vegetation COS models have recently been implemented in the ORCHIDEE LSM [27; 28]. The implementation of spatio-temporal variations in the near-surface atmospheric COS concentrations in the modeling of biogenic COS fluxes (Figure 3) helped reduce the imbalance of the atmospheric budget of this gas by lowering its uptake by soils and vegetation globally [28]. A reduction of about 8% in the vegetation uptake in the NH was simulated by the ORCHIDEE LSM between 2016 and 2019 (Figure 3). The atmospheric COS at GIF followed a similar decreasing trend from 2019 onward (Figure 2A), leading to an apparent reduction in the simulated vegetation uptake of about 10% during the period of 2016-2021. As the dominant influence on the seasonal variation in the atmospheric COS is terrestrial vegetation uptake in the Northern Hemisphere, e.g., [36],  
Figure 3: Evolution between 2010 and 2019 of NH mean annual atmospheric COS concentration and NH mean annual plant COS uptake computed with a monthly variable atmospheric COS concentration. Adapted from [17; 27]. Note that land uptake is expressed negatively, following the atmospheric convention. the trends in the COS SCA and vegetation uptake were expected to evolve in parallel, as the observations and simulations at the GIF site suggest. A more quantitative appraisal of the sensitivity of the simulated tropospheric COS mixing ratio to the seasonal variability in the COS terrestrial fluxes is shown in the following. Using surface fluxes described in previous studies for each component of the COS budget (Table 1) and the LMDz transport model, the results indicate that the COS seasonality at GIF is characterized by two offsetting components from plant uptake and ocean emissions, regardless of the selected time period (Figure 4A,C). Consequently, the transport of anthropogenic emissions imposes the overall seasonality of the COS concentrations modeled at the GIF site, even though the observations lag the net signal by 2 to 4 months. The Irish site of Mace Head (MHD), which faces the Atlantic Ocean, is the sole site belonging to NOAA's network of COS monitoring stations in Europe. It is a marine regional reference, and MHD can be considered upwind of GIF. At MHD, the simulated net signal appears to be essentially driven by ocean fluxes, with the simulated seasonal maximum and minimum delayed by 2 to 3 months compared to the observat  
Figure 4: Simulations of the mean seasonal cycle of tropospheric COS mixing ratios over the years 2011–2015 (left column, NH mean annual atmospheric COS concentration in the range 483.7–491.5 ppt) and 2016–2019 (right column, decreasing NH mean annual atmospheric COS concentration in the range 486.9–467.8 ppt, see Figure 3) at the GIF (**A**,**C**) and MHD (**B**,**D**) monitoring stations. Detrended seasonal cycles were calculated by subtracting the long-term trend from the smooth curve, and then averaged. These simulations only account for the first-order relationship between the COS plant/soil fluxes and NH mean annual atmospheric COS concentrations. The net signal (black line) was obtained from a global LMDz simulation using all the dominant sources and sinks, while the signal components (colored lines) were obtained by running the global atmospheric simulation with one component flux at a time. Observations are shown in grey (dashed line). The time series of COS mixing ratios were detrended and filtered to remove the synoptic variability (see Section 2). An identical plot using optimized marine COS emissions is shown in Figure S1 for comparison. selected time period (Figure 4B,D).