Intercomparison of biogenic CO₂ flux models in four urban parks in the city of Zurich

Quantifying the capacity and dynamics of urban carbon dioxide (CO₂) emissions and carbon sequestration is becoming increasingly relevant in the development of integrated monitoring systems for urban greenhouse gas (GHG) emissions. There are multiple challenges in achieving these goals, such as the partitioning of atmospheric measurements of CO₂ fluxes to anthropogenic and biospheric processes, the insufficient understanding of urban biospheric processes, and the applicability of existing biosphere models to urban systems. In this study, we applied four biosphere models of varying complexity - diFUME, JSBACH, SUEWS, VPRM - in four urban parks in the city of Zurich and evaluated their performance against in situ measurements collected over almost 2 years on park trees and lawns. In addition, we performed an uncertainty analysis of gross primary productivity (GPP), ecosystem respiration (Reco), and net ecosystem exchange (NEE) of CO₂ based on the differences between the estimates of the four models and compared the estimated uncertainties and biospheric fluxes with the monthly anthropogenic CO₂ emissions of a wide urban area surrounding the four parks. The results showed that, despite the large differences in model architecture, there was considerable agreement in the seasonal and diurnal GPP, Reco, and NEE estimates. Larger discrepancies between the four models were found for lawn GPP compared to tree GPP, while, for Reco, the differences between lawns and tree areas were similar. On an annual scale, all models agreed, on average, that lawns acted as CO₂ sources and tree-covered areas as CO₂ sinks during the simulation period, with the exception of diFUME, which simulated both tree and lawn areas as CO₂ sources. diFUME and VPRM were more accurate in capturing the onset of the tree leaf growth in spring compared to JSBACH and SUEWS. On the other hand, JSBACH and SUEWS simulated soil water availability more accurately than the satellite-derived water index used by VPRM. The in situ observations revealed a very high spatial variability in lawn Reco across the park areas. All models underestimated the lawn Reco during spring in mowed, sunny locations, whereas the model simulations were closer to the observed Reco in un-mowed, partially shaded locations. The mean monthly uncertainties in biogenic NEE reached 0.8 μmolm-2s-1, which is 10.2 % of the magnitude of the total CO₂ balance over the studied area during the month of June. This balance was composed of a mean anthropogenic flux of 8.7 μmolm-2s-1 and a mean biospheric flux of -0.5 μmolm-2s-1. Overall, this study highlights the importance of properly accounting for the biogenic CO₂ fluxes and their uncertainties in urban CO₂ balance studies, especially during the vegetation growing season, and shows that even simple models, such as VPRM, can adequately simulate the urban biospheric fluxes when appropriately parameterized.