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To evaluate CO2 emissions in urban areas and their temporal and spatialvariability, continuous measurements of CO2 fluxes were conducted usingthe eddy covariance method at three locations in Sakai, Osaka, Japan. Basedon the flux footprint at the measurement sites, CO2 fluxes from thethree sites were partitioned into five datasets representing a dense urbancenter, a moderately urban area, a suburb, an urban park, and a rural area.A distinct biological uptake of CO2 was observed in the suburb, urbanpark, and rural areas in the daytime, whereas high emissions were observedin the dense and moderate urban areas in the daytime. Weekday CO2emissions in the dense urban center and suburban area were approximately50 % greater than emissions during weekends and holidays, but the otherlandscapes did not exhibit a clear weekly cycle. Seasonal variations in theurban park, rural area, and suburban area were influenced by photosyntheticuptake, exhibiting the lowest daily emissions or even uptake during thesummer months. In contrast, the dense and moderately urban areas emittedCO2 in all seasons. CO2 emissions in the urban areas were high inthe winter and summer months, and they significantly increased with theincrease in air temperature in the summer and the decrease in airtemperature in the winter. Irrespective of the land cover type, all urbanlandscapes measured in this study acted as net annual CO2 sources, withemissions ranging from 0.5 to 4.9 kg C m−2 yr−1. The magnitude ofthe annual CO2 emissions was negatively correlated with the greenfraction; areas with a smaller green fraction had higher annual CO2emissions. Upscaled flux estimated based on the green fraction indicatedthat 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 eddycovariance measurements is useful for characterizing the spatial andtemporal variations in net CO2 fluxes from urban areas. Multiplemethods would be required to evaluate the rationale behind the fluxes andovercome the limitations in the future

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