In order to elucidate the phenomenon that the atmospheric methane (CH
4) concentration is higher in urban area, the influences of urban atmospheric structure on the temporal and geographical distributions of atmospheric CH
4 concentrations have been investigated in relation with the meteorological conditions by analyzing the continuous monitoring data in Nagoya City. When the CH
4 concentrations became higher in the downtown area during the daytime, the vertical profiles of potential temperature showed the stable atmospheric conditions above the urban area and the geographical distributions of wind speed and direction exhibited the stagnant atmospheric region around the urban surface area. These results suggested the stagnancy of a smog layer in the urban atmosphere. Statistical analysis of the concentrations of greenhouse gases (CO
2, CH
4) and air pollutants (CO, NMVOCs) as well as meteorological data through the year of 1995 also indicated that the concentration variation of CH
4 was deeply related to the smog layer formation under the stable atmospheric conditions. Thus, it is concluded that the higher concentration of CH
4 in urban area was observed even in the daytime when a smog layer was formed under fine weather and weak wind conditions.
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