Influences of urban heat island phenomena on carbon dioxide and methane concentrations in urban atmosphere (Nagoya city [Japan])

"The concentrations of carbon dioxide and methane measured in Nagoya City were higher than the background concentration levels measured at Minami-Torishima Island. Then, the influences of urban atmospheric structures on those gas concentrations were analyzed, using a heat island intensity (DT) as an index. First, cluster analysis was carried out for temperature data sets obtained at 8 observatories in Nagoya City, and the observatories were classified into the urban- and suburban-area groups, according to the results from cluster analysis. The heat island intensity was defined as the difference in temperatures measured at two observatories, one of which was selected from an urban area group and the other from a suburban one. When the heat island intensity was above 0degC with wind velocity below 3 m/s, the atmospheric structure was stabilized. Under these stable atmospheric conditions, the concentrations of carbon dioxide and methane were 5-7% higher than those observed under the non-stable atmospheric conditions (DT<0degC with wind velocity above 3 m/s). These results indicate that the stable atmospheric structures in urban area contributed to the increase in the apparent concentrations of atmospheric trace gases. Moreover, it was found that the influences of the stable atmospheric structures on the concentrations of carbon dioxide and methane were almost the same in every year from 1992 to 1998. As the background concentration and the urban atmospheric concentration of carbon dioxide, their average concentrations during 1993 and 1998 were 362 ppmv and 382 ppmv at Minami-Torishima Island and Nagoya, respectively. The difference between background and urban atmospheric concentrations of carbon dioxide could be estimated as the contributions from stable atmospheric conditions (14 ppmv) and urban emission (6 ppmv). In the case of methane, the background and urban atmospheric concentrations during 1994 and 1998 were 1.78 ppmv and 1.95 ppmv, respectively, and the contributions from stable atmospheric structures and urban emission were estimated to be 0.06 ppmv and 0.11 ppmv, respectively."