Estimation of the origin of atmospheric methane by the use of atmospheric diffusion analysis and measurements of atmospheric radioactivity and isotope ratio.

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A Method was examined for evaluating methane flux by comparing measured value of methane concentration in air with calculated one from numerical analysis of a diffusion equation. The numerical values for the variation of radon concentration obtained by using one-dimensional non-steady state diffusion equation explain well the temporal variation of radon concentration : Thus the reliability of the numerical solution of the diffusion equation was revealed, and we can determine the methane flux by using the numerical analysis method.Taking notice of the similarity of the diurnal variation pattern of methane and radon concentrations in air, a method was developed for evaluating methane flux based on the knowledge of the behavior of radon in air. The averaged methane flux at the suburbs of Nagoya city area was obtained. At the south-west part of Nagoya city, regional methane flux increased in July and August and it amounted to 0.068 CH_4・m^<-2>・d^<-1>. According to the analysis of carbon isotope ratio of methane sampled at a site of Yatomi, it revealed that almost all of the methane are supplied from non-fossil origin ; It is considered to originate from paddy field.A method for evaluating vast aerial non-uniform flux of methane was proposed by using the similarity of the diurnal variations of radon and methane. The amount of annually flux was 15.12 GgCH_4 in Aichi prefecture and was 1.52 GgCH_4 in Nagoya city.