## 2 Materials and Methods  
At the GIF monitoring site (48.7109 N, 2.1476 E), atmospheric COS was collected on an hourly basis from August 2014 to November 2021 using an Entech P7100 preconcentrator, and subsequently analyzed by gas chromatography (Varian Model 3800) with pulsed flame photometry detection (PFPD, [11; 18]). The comparability of our own calibration scale supplied by Air Products with that of NOAA-2004 [7] was assessed on a weekly basis using two long-term natural air standards consecutively, which were prepared in Aculife-treated cylinders and calibrated by NOAA-ESRL [11]. The same long-term air standard was used during the first six years of our approximately seven-year long time series. The latter was interrupted for only about two months in summer 2017, due to a failure of the Entech preconcentrator, and in spring 2020, because of the COVID-19 lockdown. Fortunately, data collected by a second instrument (quantum cascade laser spectrometer, mini QCLS COS/CO2/H2O, Aerodyne Res.) operated at the Trainou (TRN, FR, 47.9647 N-2.1126 E) monitoring site, about 80 km south-west of GIF, was available to fill the 2020 data gap [19]. The reader is referred to [11] for a description of COS measurements by QCLS. Post-processing of the COS measurements (selection of daytime (11-17 h UTC) data, calculation of the long-term trend and smoothing of the seasonal cycle, and calculation of the spring maximum and autumn minimum) used the CCGvu curve-fitting procedure developed by [20]. The procedure is fully described and available at [[http://www.esrl.noaa.gov/gmd/ccgg/mbl/crvfit/crvfit.html](http://www.esrl.noaa.gov/gmd/ccgg/mbl/crvfit/crvfit.html)]([http://www.esrl.noaa.gov/gmd/ccgg/mbl/crvfit/crvfit.html](http://www.esrl.noaa.gov/gmd/ccgg/mbl/crvfit/crvfit.html)) (accessed on 1 November 2021). The procedure first estimates a smoothing function by fitting the raw data to a first-order polynomial equation for the growth rate combined with a two-harmonics function for the annual cycle. In order to define short-term and long-term variations that are not captured by the function, the residuals (i.e., raw time series minus the full function) were fitted using a low-pass filter with 80 and 667 days as short-term and long-term cutoff values, respectively. The smooth curve combines the results of the function and of the filter using the short-term cutoff value. Detrended seasonal cycles were also calculated by subtracting the long-term trend (i.e., polynomial plus filtered residuals using the long-term cutoff value) to the smooth curve. An offline version of LMDz 6 [21], based on the general circulation model developed at the Laboratoire de Meteorologie Dynamique [22], was used to simulate atmospheric COS concentrations from fluxes described in other studies, discussed below. The LMDz grid features a 3.75\({}^{\circ}\)lon \(\times\) 1.9\({}^{\circ}\)lat horizontal resolution and a 200-300 m vertical resolution in the planetary boundary layer, with 39 sigma-pressure layers from the surface to about 75 km. European Center for Medium-Range Weather Forecasts (ECMWF) fields are prescribed and pre-computed mass fluxes from the full LMDz version are used for the offline version, which only solves the continuity equation for the tracers. LMDz was run for the years 2010 to 2019 at a 30-min time step to simulate 3-hourly average outputs. The GIF station generally samples air masses from Central France and countries at the northeastern edge of France, such as Belgium, Germany and Switzerland, as shown by the LMDz footprints (see Figure S14 of [17]). The ORCHIDEE LSM was used to simulate global vegetation and soil COS fluxes. ORCHIDEE is developed at the Institute Pierre Simon Laplace (IPSL) and computes the carbon, water, and energy balance between the land surfaces and the atmosphere. It was initially described in [23]. Plant species were grouped into 14 plant functional types (PFTs) based on similarities in morphology, phenology, climatic conditions, and photosynthetic pathways. A final PFT corresponded to bare soil.