Evaluation of sources of atmospheric methane using isotopic measurement and diffusion model analysis

对名古屋地区大气甲烷(CH4)通量进行了估算。名古屋市内13个站点大气甲烷的日变化与名古屋大学的大气氡(^&lt；222&gt；Rn)是同步的。甲烷和氡-222都是从地表释放到大气中，化学反应较少。从它们的观测和特征的相似之处来看，它们在大气中的行为预计也会相似。与氡-222类似，大气甲烷的日变化受当地气象条件和数十公里范围内当地甲烷排放的影响。根据大气甲烷浓度与氡-222之间的相关关系，估算了甲烷的面平均通量。名古屋地区是大气甲烷的净来源。城区的面源通量小于0.02gCH_4m~(-2)，且全年基本不变，而郊区的面源…通量基本不变。从7月到8月，更多的通量增加到0.04-0.06 GCH_4M^&lt；-2&gt；d^&lt；-1&gt；；组成了大气甲烷碳同位素测量特别是加速器质谱仪(AMS)测量的准备系统。根据大气甲烷浓度波动引起的碳同位素组成的变化，估算了当地甲烷源类型及其贡献。城市地区冬季排放的甲烷几乎都是化石甲烷，这些甲烷被消耗掉，来自于天然气泄漏等人为来源。而当夏季甲烷排放量增加时，郊区释放的化石甲烷对日甲烷的贡献率不到10%。估算的非化石甲烷的增量为-65@91，表明非化石甲烷是在厌氧环境中细菌产生的。因此，夏季郊区甲烷面通量的增加应归因于稻田。较少

The fluxes of atmospheric methane (CH_4) from Nagoya area were estimated. The diurnal fluctuations of atmospheric methane observed at 13 sites in Nagoya city and atmospheric radon (^<222>Rn) observed at Nagoya University were in phase. Both methane and radon-222 are released from the ground surface into the atmosphere, and are less chemically reactive. From their similarities of the observations and features, their behaviors in the atmosphere are expected to be similar. By analogy with radon-222, the diurnal fluctuation of atmospheric methane are affected by local meteorological conditions and local methane emission from the area within several tens of kilometers. From the correlations between the concentrations of atmospheric methane and radon-222, the areally averaged fluxes of methane were estimated. Nagoya area was net source of atmospheric methane. In urban area the areal fluxes were less than 0.02 gCH_4m^<-2>d^<-1> and almost constant through the year, whereas in suburb the areal … More flux increased to 0.04-0.06 gCH_4m^<-2>d^<-1> from July to August.The preparation system for carbon isotopic measurement, especially ^<14>C measurement by accelerator mass spectrometry (AMS), for atmospheric methane was assembled. The performance of the system is sufficient for AMS.The types of local methane sources and their contributions were estimated from the change of carbon isotopic compositions (delta^<13>C,^<14>C/^<12>C) in the consequence of the fluctuations of the concentrations of atmospheric methane. Almost all of methane released from urban area in winter was fossil methane which is depleted in ^<14>C and derived from anthropogenic sources such as leakage of natural gas. While, fossil methane released from suburban area contributes less than 10% of the diurnal methane in summer when methane emission increases. The estimated delta^<13>C value of non-fossil methane was -65@91 and suggested that non-fossil methane was bacterially produced in anaerobic environment. Thus, the increase of areal flux of methane in suburb in summer would be attributed to rice paddy fields. Less