| 24 Aug 2026
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.
Dear authors,
Thank you for investigating the potential use of COS as a proxy for GPP in urban areas, where anthropogenic sources can play a significant role. This is a highly important topic, as only a limited number of measurements have been conducted to date that cover and attempt to disentangle the different sources and sinks present in urban environments.
The manuscript is very well written and detailed. The advantages and disadvantages of using COS as a proxy in urban areas are well discussed. However, in my view, the implications and potential applications of using COS as a proxy for GPP estimation could be highlighted more clearly.
Furthermore, I only have a few minor comments, which are listed below.
Title: “tall tower”, how important is this? would Eddy covariance be enough. now the fact that you used a tall tower looks crucial since it is in the title and at the front.
L23-24: please add a reference
L37: “The potential of COS to track CO2 uptake is based on the similar pathway to the leaf through the leaf stomata.” This is a difficult sentence, could you rephrase it?
L40: “destroyed”, hydrolysed would be more precise I believe
L98: were there no condensation problems using an unheated line?
L100: if the measurements are not used, no need to write it down unless you want to specify they are available for downloading
L113: also here please focus on what was used for the specific experiment, could become unnecessary complicated otherwise.
L175: a natural question comes up when reading this section, which flag was used in the analysis? This will be answered later on but maybe also stating here could be helpful.
L312: Fcos_meas is discussed after the model results but in the figure legend and description it is labelled first. Please keep same orders.
Fig 6. Could significance be added?
L533: this connection to GPP is very nice, if possible, more of those implications connections between your findings and GPP could be made.