## 1 Introduction  
The uptake of atmospheric carbonyl sulfide (COS) by terrestrial vegetation is the major sink in the global budget of this gas [1]. Moreover, the plant uptake of COS has been recognized as a promising tracer of carbon dioxide (CO\({}_{2}\)) exchange during photosynthesis [2], also known as gross primary production (GPP). Thus, long-term atmospheric COS records, derived from ice-core, firn, and ambient air samples, as well as ground-based observations by solar-viewing Fourier transform interferometry (FTIR), were used by [PERSON] et al. (2018) to estimate the historical growth of global GPP [3]. Illustrations of the high potential of atmospheric COS measurements, carried out in the framework of the NOAA-ESRL network, were also provided by [PERSON] et al. (2021) and [PERSON] et al. (2021) to help understand the seasonal and spatial distribution of plant carbon uptake across the Eastern United States, and the atmospheric CO\({}_{2}\) seasonal cycle amplification observed in the Arctic and boreal North America, respectively [4, 5]. However, the Siberian and temperate ecosystems (particularly those of the Central and Eastern United States, and Southeastern Europe) shape the Northern Hemisphere's CO\({}_{2}\) seasonal amplification rather than the Arctic-boreal North American ecosystems [6]. Unfortunately, atmospheric COS mole fraction surface observations in the middle and high latitudes of Europe (i.e., Mace Head station (MHD,IE, 53.3\({}^{\circ}\) N [7]); Jungfraujoch station (CH, 46.5\({}^{\circ}\) N [8]); Lutjewad station (NL, 53.4\({}^{\circ}\) N [9]); Hyytiala station (FI, 61.8\({}^{\circ}\) N [10]); Gif-sur-Yvette station (GIF, FR, 48.7\({}^{\circ}\) N [11]) are scarcer than over North America, comparability of data from different laboratories are not ensured, and airborne measurements are still in the early stages [12; 13], which prevents the application of the approach of [5] to Europe. Note that nine out of thirteen FTIR, operated as part of the Network for Detection of Atmospheric Composition Change (NDACC) in the Northern Hemisphere north of 35\({}^{\circ}\)N, are located in Europe and Japan, and yet none are found in Siberia [14]. Although NDACC monitoring stations have proven their utility in assessing tropospheric long-term trends and the seasonality in COS [8; 14; 15], it is unlikely that trends in COS and CO\({}_{2}\) seasonal cycle amplitude (SCA) could be assessed simultaneously with sufficient precision from NDACC retrievals, as the tropospheric CO\({}_{2}\) signal (e.g., the SCA) is damped in comparison to Total Carbon Column Observing Network (TCCON) retrievals [16].  
In their study, [PERSON] et al. (2021) used atmospheric COS measurements collected essentially over North America between 2009 and 2013, a period during which a slight rise in COS was observed in the Northern Hemisphere (NH) by NDACC [14] and by NOAA ground stations [7], essentially because the year 2009 was anomalously low in this latter case (Figure 1). CO\({}_{2}\) mole fractions exhibited a significant increase during this period (Figure 1).  
Hence, during the 2009-2013 period, the change in the NH COS-to-CO\({}_{2}\) ratio was very small (1.23 \(\pm\) 0.01). An inflexion point was reached in 2013 (Figure 1). The trend in the NH COS-to-CO\({}_{2}\) ratio is now driven by decreasing COS and continuously rising CO\({}_{2}\).  
In this paper, we address the implications for the COS surface fluxes of the trends observed in COS concentrations and SCA, in light of the recently updated COS time series at GIF (2014-2021). Moreover, simulations were performed using the ORCHIDEE land-surface model (LSM) and the LMDz global atmospheric transport model to evaluate our current understanding of the biogeochemistry of COS in Western Europe against field observations.  
Figure 1: Time-evolving NH means of COS and CO2 mixing ratios and of the COS-to-CO2 ratio calculated from NOAA data (2008–2019) using the method of [17], following [7].