Abstract Despite significant emissions of anthropogenic carbon dioxide () in cities, fluxes of to and from urban ecosystems can significantly impact local carbon budgets. In this work, we use the city of Toronto, Canada, as a testbed to compare two urban vegetation models: the Solar‐induced chlorophyll fluorescence for Modeling Urban biogenic Fluxes (SMUrF) model and the Urban Vegetation Photosynthesis and Respiration Model (UrbanVPRM). We make several adjustments to both models to improve their agreement with three eddy‐covariance flux towers in the region surrounding the city, enhance the spatial resolution, and better represent biogenic fluxes in urban areas. Compared to flux tower observations, the net ecosystem exchange estimates improved substantially during the spring and autumn for the updated UrbanVPRM and during spring and summer for the updated SMUrF model. These adjustments also result in significantly better agreement between the two models in Toronto during 2018–2021. While discrepancies remain between the updated models, likely due to the use of different driving variables, they are substantially smaller than differences between anthropogenic emissions estimated by two commonly used emission inventories. We find that during summer afternoons both the UrbanVPRM and SMUrF models predict uptake of between half and all of Toronto's mean anthropogenic summer afternoon emissions, depending on the inventory used. During nights and the non‐growing season, vegetation emits , amounting to between a quarter and half of Toronto's human‐caused emissions during summer nights. This illustrates the significance of biogenic fluxes on the urban budget, especially on hourly timescales.
Plain Language Summary Despite large human‐caused emissions of carbon dioxide (), exchanges of to and from urban plants and soils can significantly impact the total absorbed or emitted in cities. We use the city of Toronto, Canada, to compare two urban vegetation models that estimate the amount of absorbed and emitted by plants and soils. We make several adjustments to both models to improve how well they compare to tower‐based measurements in the region surrounding the city, enhance the spatial resolution, and better represent exchanges of between vegetation and the atmosphere in urban areas. This also results in better agreement between the two model estimates in the city of Toronto. Differences between the updated models, likely caused by different types of input data, are significantly smaller than the range of estimates of human‐caused emissions. During summer afternoons both updated vegetation models predict significant absorption of : between half and all of Toronto's human‐caused summer afternoon emissions, depending on the emissions data set. During nights and the winter, vegetation and soils return a significant portion of this absorbed to the atmosphere. This shows the importance of vegetation in urban exchanges especially on an hourly basis.
Key Points Modifications to the Solar‐induced chlorophyll fluorescence for Modeling Urban biogenic Fluxes (SMUrF) and Urban Vegetation Photosynthesis and Respiration Model (UrbanVPRM) vegetation models improve agreement with flux towers and each other over the city of Toronto Discrepancies between the updated SMUrF and UrbanVPRM outputs are likely caused by differences in the driving variables of the two models Vegetation models estimate large CO2 uptake during summer afternoons compared to anthropogenic emissions but return a fraction at night