Eddy covariance measurements of Carbonyl sulfide (COS) to partition the urban carbon flux

Cities are global hot spots of anthropogenic CO2 emissions, contributing over 70% of the global emissions. The heterogeneous urban ecosystems generate net CO2 fluxes (NEE) consist of many different anthropogenic and biogenic components, which cannot be distinguished from one another measuring only CO2 with eddy covariance technique. Carbonyl sulfide (COS) is a trace gas taken up through the same pathway as CO2 but not emitted back to the atmosphere. Hence, it can be used as a proxy for gross primary production (GPP).

The aim of this work is to examine the suitability of COS flux measurements to partition GPP and other components from urban NEE. For this, intensive measurement campaigns were conducted at ICOS Associated Ecosystem Station FI-Kmp in Helsinki between summer 2023 and spring 2024, and in Zürich over winter 2022–2023.

During the growing period, significant COS uptake associated with photosynthesis was observed in wind directions with high fraction of vegetated surfaces. In summer, in a highly vegetated urban area (Helsinki), we were able to successfully estimate GPP using COS flux and partition it from NEE. In wintertime, however, we saw an influence of anthropogenic emissions on COS fluxes, which complicates GPP estimates and sheds light on the details of global budget of COS, which has not been closed to the
date. We show how COS based GPP estimates can be made in urban regions, but limitations on scale and accuracy need to be considered based on the flux source area.