Quantifying the Boundary-Layer Dynamics of Carbon Dioxide from A Built Environment Using A Coupled Urban Land-Atmospheric Model

Abstract:

Urban areas are significant carbon contributors in the global carbon cycle due to intense carbon emissions from traffic and other human activities and lack of vegetation for carbon absorption. A better understanding on urban carbon variations is important to quantify urban contributions to regional and global carbon budgets under the challenge of climate changes. In this study, we applied a coupled urban land-atmospheric model to simulate the diurnal and seasonal variations of carbon dioxide (CO₂) fluxes in the urban boundary layer (UBL) for the Phoenix Metropolitan area, Arizona. The lower boundary conditions of this model are provided by the CO₂ fluxes measured using an eddy covariance tower. By analyzing the tower measurements in the urban canopy layer, the highest concentrations of CO₂ in a typical weekday coincide with the busiest traffic. Besides, there are more CO₂ emissions in winter than in summer possibly due to additional natural gas consumptions for heating. Based on the coupled model, we simulated the diurnal and seasonal evolutions of the mean CO₂ concentration as well as the vertical profiles of CO₂ concentration in the UBL. It was found that the anthropogenic CO₂ sources in a built terrain effectively altered the carbon dynamics in the overlying atmosphere in contrast to its rural surroundings. We also changed the urban landscape characteristics including vegetation fraction, surface roughness, and building density to study their impacts on the CO₂ dynamics in the UBL. Overall, the coupled urban land-atmospheric model provides a useful stand-alone tool for quantifying the urban carbon cycle, and can be extended to more general applications such as urban air quality problems.