Complexity of carbon dioxide flux in urban areas: A comparison with natural surfaces

The characteristics of turbulent CO₂ transport and its dissimilarity with heat and water vapor are investigated over both natural and urban areas. A novel index TS is proposed to effectively quantify the transport similarity between two scalars. By comparison, it is found that the transport of CO₂ shows great complexity in urban areas. It is ideal in natural areas that heat, water vapor, and CO₂ are efficiently transported by thermal plumes (i.e., the dominant coherent structures under unstable conditions), and that the transport similarity among them becomes increasingly evident with the increase of atmospheric instability. However, in urban areas, the transport of CO₂ shows significant dissimilarity from that of heat and water vapor, and it is hard to detect the role of thermal plumes. Furthermore, it is observed that the sector-average CO₂ flux in urban areas changes largely with the wind blowing from different urban functional areas. Specially, for a given direction, there might be contrasting characteristics in CO₂ transport under different unstable conditions. These features can be explained by the flux footprint. Since the CO₂ sources and sinks are distributed heterogeneously in urban areas, the variation of footprint areas with wind direction or atmospheric instability, causes the alternation between source-dominated (i.e., upward) and sink-dominated (i.e., downward) CO₂ transport. Therefore, the role of coherent structures in CO₂ transport is substantially confused by spatially-confined sources/sinks in urban areas, leading to significant transport dissimilarity between CO₂ and heat or water vapor and thus the great complexity in CO₂ transport. The findings in this study are helpful to promote the understanding of the global carbon cycle in depth.