Inclusion of CO2 flux modelling in an urban canopy layer model and an evaluation over an old European city centre

In the context of climate change, the reduction of greenhouse gas emissions is a global concern. Recent publications estimate that 30–40% of total anthropogenic greenhouse gases are directly emitted by urban areas. This paper focuses on CO2, which is the main anthropogenic greenhouse gas, and presents an implementation of CO2 flux modelling in urban areas within the urban canopy model Town Energy Balance (TEB). Highly weather-dependent contributors to CO2 fluxes (buildings and vegetation) are explicitly modelled by TEB using the Building Energy Model (BEM) for buildings and Interactions between Soil, Biosphere and Atmosphere (ISBA) for urban vegetation. This approach allows the impacts of the urban microclimate on CO2 fluxes to be simulated. Non-weather-dependent contributors (traffic and human respiration) are simulated using simpler approaches. A sensitivity study applied to the centre of Toulouse, France, highlights the relevance of detailed input data related to traffic, building use and human behaviour to simulate accurate CO2 fluxes. The results show that traffic (48.5%) and buildings (42%) are the main contributors to the annual mean CO2 flux. A comparison of the model results with independent eddy-covariance flux data shows good agreement with a root mean square error of 15.3 μmol m−2 s−1 and demonstrates that the model is able to reproduce seasonally averaged daily cycles of CO2 fluxes. In future studies, this model can be used to quantify the impacts on CO2 fluxes of different urban development scenarios such as urban expansion, changes in urban form, changes in practices related to the heating of buildings or urban greening strategies. Keywords: CO2fluxes, Eddy covariance, Cities, Urban canopy layer model, Model evaluation