Modification and Comparison of Two Urban Vegetation Models Over Southern Ontario, Canada

Despite significant emissions of anthropogenic carbon dioxide (${\text{CO}}_2$) in cities, fluxes of ${\text{CO}}_2$ 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 ${\text{CO}}_2$ emissions estimated by two commonly used emission inventories. We find that during summer afternoons both the UrbanVPRM and SMUrF models predict ${\text{CO}}_2$ 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 ${\text{CO}}_2$, amounting to between a quarter and half of Toronto's human-caused ${\text{CO}}_2$ emissions during summer nights. This illustrates the significance of biogenic ${\text{CO}}_2$ fluxes on the urban ${\text{CO}}_2$ budget, especially on hourly timescales.