Spatial variations in daytime methane and carbon dioxide emissions in two urban landscapes, Sakai, Japan

To obtain an accurate understanding of carbon dioxide (CO2) and methane (CH4) emissions from urban areas, it is important to estimate their spatial variations because the heterogeneous nature of urban land use results in different emissions patterns. We measured CH4 and CO2 fluxes from two urban landscapes in Japan with the eddy covariance method and evaluated the spatial distributions of fluxes by combining flux footprint analysis and mobile measurements of gas concentrations. CH4 hotspots were identified at sewage plants, oil refineries, and natural gas facilities in the urban center. The fluxes (60 +/- 65 nmol m(-2) s(-1)) affected by hotspots were higher than those in the suburban and residential areas (22 +/- 30 nmol m(-2) s(-1)). High CH4 concentrations of up to 5130 ppb from the hotspots were also observed in the mobile measurements. The measured fluxes showed that the study area generally acted as a CH4 source irrespective of the presence of hotspots. The mobile measurements suggested several CH4 sources: gas leaks from natural gas networks, sewage pipes, gas-powered air conditioners, and moats. The CO2 fluxes were larger in commercial and industrial areas than residential and suburb areas, and fluxes from vegetated areas were nearly CO2-neutral in the daytime.

Automated summary

The study found that sewage plants, oil refineries, and natural gas facilities in urban centers are hotspots for CH4 emissions, while CO2 fluxes are larger in commercial and industrial areas. Vegetated areas were nearly CO2-neutral during the daytime.