The mixing ratio of atmospheric methane (CH₄) over the two large cities of Osaka and Kyoto achieves its maximum in the central part of the city and decreases toward the suburbs, with the same distribution patterns for nonmethane hydrocarbons (NMHC) and carbon momocide(CO).
It is shown at CH₄ is emitted from the same sources as NMHC and CO, whose contribution amounts to about 4% of the yearly mean of observed methane. The emission ratio of CH₄ to total hydrocarbons is estimated to be 8.3±1.0%, calculated in terms of carbon number. The emission ratio of CH₄ to CO is estimated to be 0.034±0.006 in volume.