Diurnal, weekly, seasonal, and spatial variabilities in carbon dioxide flux in different urban landscapes in Sakai, Japan

Abstract. To evaluate CO₂ emissions in urban areas and their temporal and spatial variability, continuous measurements of CO₂ fluxes were conducted using the eddy covariance method at three locations in Sakai, Osaka, Japan. Based on the flux footprint at the measurement sites, CO₂ fluxes from the three sites were partitioned into five datasets representing a dense urban center, a moderately urban area, a suburb, an urban park, and a rural area. A distinct biological uptake of CO₂ was observed in the suburb, urban park, and rural areas in the daytime, whereas high emissions were observed in the dense and moderate urban areas in the daytime. Weekday CO₂ emissions in the dense urban center and suburban area were approximately 50 % greater than emissions during weekends and holidays, but the other landscapes did not exhibit a clear weekly cycle. Seasonal variations in the urban park, rural area, and suburban area were influenced by photosynthetic uptake, exhibiting the lowest daily emissions or even uptake during the summer months. In contrast, the dense and moderately urban areas emitted CO₂ in all seasons. CO₂ emissions in the urban areas were high in the winter and summer months, and they significantly increased with the increase in air temperature in the summer and the decrease in air temperature in the winter. Irrespective of the land cover type, all urban landscapes measured in this study acted as net annual CO₂ sources, with emissions ranging from 0.5 to 4.9 kg C m⁻² yr⁻¹. The magnitude of the annual CO₂ emissions was negatively correlated with the green fraction; areas with a smaller green fraction had higher annual CO₂ emissions. Upscaled flux estimated based on the green fraction indicated that the emissions for the entire city were 3.3 kg C m⁻² yr⁻¹, which is equivalent to 0.5 Tg C yr⁻¹ or 1.8 Mt CO₂ yr⁻¹, based on the area of the city (149.81 km²). A network of eddy covariance measurements is useful for characterizing the spatial and temporal variations in net CO₂ fluxes from urban areas. Multiple methods would be required to evaluate the rationale behind the fluxes and overcome the limitations in the future.