Partitioning the urban carbon budget with carbonyl sulfide (COS) flux measurements conducted in a high-latitude city

Globally, cities correspond to most of the direct anthropogenic carbon emissions, and due to the global mega-trend of urbanization, their importance will only increase. The carbon budget of an area is often given as a net ecosystem exchange (NEE), which describes net CO₂ fluxes measured with eddy covariance (EC) method. However, these measurements alone cannot partition anthropogenic and biogenic carbon sources and sinks. Earlier studies on vegetated ecosystems have defined leaf-scale relative uptake (LRU) of carbonyl sulfide (COS) and CO₂ to partition the biogenic uptake, namely gross primary production (GPP).

In this research, the aim was to examine the suitability of using COS flux measurements to partition GPP from urban NEE, to better understand the effect of urban green areas on the carbon balance of cities. EC fluxes of CO₂ and COS were measured at ICOS Associated Ecosystem Station FI-Kmp station in Helsinki, Finland, during Winter 2020-2021 and July 2022. Urban LRU is estimated for a footprint dominated by urban parks. Then, GPP is estimated from the measured COS flux, using three methods with varying complexity, for more heterogeneous footprints with more pronounced anthropogenic influence. Estimates are compared with a more common carbon balance partitioning method where only ecosystem respiration is considered.

Preliminary results showed how LRU over urban park has similar behavior as forest LRU, and the values are same order of magnitude. COS flux can be used as a tracer for carbon uptake by photosynthesis also in urban areas. Two out of the three methods showed the daily dynamics of GPP qualitatively right, with more complex underestimating and simpler overestimating the GPP, respectively. When using COS as a proxy for GPP in a heterogeneous urban environment, errors arise due to anthropogenic emissions of COS, which are not expected in the original context of using the compound as a biogenic activity tracer. In the future, more focus will be put to adapt methods to determine anthropogenic influence.