Tall tower eddy covariance campaign reveals complex emission patterns for carbonyl sulfide

Abstract. Carbonyl sulfide (COS) is a useful tracer of gross primary productivity (GPP), but the magnitude and variability of anthropogenic COS emissions in urban environments remain poorly constrained.

COS fluxes were investigated in Zurich, Switzerland, using tall-tower eddy covariance (EC) measurements as part of the ICOS Cities project from August 2022 to April 2023. Source-area variability was analysed using the Flux Footprint Prediction (FFP) model, and compared with biogenic COS fluxes simulated by the Simple Biosphere 4 (SiB4) model.

Zurich acted as a net sink of COS throughout most of the campaign (median −4.4 pmol m^-2 s^-1). The most vegetated south-western sector exhibited approximately three times the COS uptake of the south-eastern sector (−6.7 and −1.9 pmol m^-2 s^-1, respectively), while the south-eastern sector exhibited the weakest uptake, consistent with stronger anthropogenic fluxes. Footprint-weighted land-cover analysis showed that spatial variability in COS exchange reflected the balance between vegetation uptake and anthropogenic fluxes, with the latter masking the biospheric signal in densely built sectors. SiB4 reproduced the seasonal evolution of COS exchange but underestimated the sink following the preceding compound drought event and during an exceptionally warm winter, highlighting limitations in representing stomatal conductance and urban vegetation.

These results demonstrate both the potential and the challenges of using COS as a tracer of urban GPP. Improved anthropogenic COS emission inventories, ecosystem models, and urban flux observations are needed to improve COS-based estimates of urban carbon uptake.