Carbon dioxide exchange in urban areas: modeling, uncertainties and sensitivity analysis

Abstract:

Carbon dioxide is the primary greenhouse gas (GHG) source that drives the global climate change in past centuries. Much effort of GHG mitigation as the countermeasure to global changes has focused on the urban areas – the hotspots of fossil fuel and concentrated emission and pollutants. Despite their critical role in CO₂ exchange in urban ecosystem, emission sources from vegetation and soil surfaces are largely overlooked in existing urban land surface models. In this study, we parameterized the biogenic CO₂ exchange in cities using an advanced single-layer urban canopy model, by incorporating a plant physiological model in the built environment. In addition, the proposed model also includes the anthropogenic CO₂ fluxes especially that from traffic emissions, based on gridded dataset. We evaluate the proposed model using CO₂ measurements from an eddy covariance flux tower located at west Phoenix, Arizona, USA. The model results are in good agreement with the observed carbon flux over the built terrain, with a RMSE of 0.21 mg m^-2s^-1.

In addition, we characterized the uncertainty of key model parameters and conducted a series of numerical simulations based on advanced Markov chain Monte Carlo algorithm. The results of simulation show that the urban morphology (the canyon aspect ratio), urban irrigation, and the physiological properties of urban vegetation predominate the plant CO₂ exchange. In contrast, the CO₂ budget is relatively insensitive to material properties of urban facets in the built environment. In addition, we decomposed the CO₂ budget to identify the underlying mechanisms of urban vegetation, from tall trees to low vegetation, on regulating the urban CO₂ processes. The findings of the stochastic simulations help to unravel the interplay of urban CO₂ flux and local microclimate, improve model calibration, and aid the urban design towards a low carbon city.