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

估算了名古屋地区大气甲烷（CH_4）通量。在名古屋市13个观测点观测到的大气甲烷和在名古屋大学观测到的大气氡（^ Rn）的日变化<222>具有同步性。甲烷和氡-222都是从地表释放到大气中，化学反应性较低。从它们的观测和特征的相似性来看，它们在大气中的行为预计是相似的。与氡-222类似，大气甲烷日变化受当地气象条件和几十公里范围内当地甲烷排放量的影响。从大气甲烷和氡-222浓度之间的相关性，甲烷的面积平均通量估计。名古屋地区是大气甲烷的净来源。在城区，地表通量小于0.02gCH_4m ^<-2>d^，<-1>且几乎全年不变，而在郊区，地表通量则小于0.02gCH_4m ^ d^，且在郊区，地表通量大于0.02gCH_4m ^d 关于我们 7 ~ 8月，甲烷通量增加到0.04- 0.06gCH_4m·<-2>d ~<-1>（-1）。组装了用于大气甲烷碳同位素测量，特别<14>是加速器质谱（AMS）测量~（12）C的制备系统。根据大气甲烷浓度波动引起的碳同位素组成变化（Δ ^ C，Δ C/Δ C），估算了本地甲烷源的类型及其贡献<13><14><12>。城市冬季释放的甲烷几乎全部是化石甲烷，其^ C贫化<14>，来源于天然气泄漏等人为源。而在夏季，当甲烷排放增加时，郊区释放的化石甲烷对甲烷日变化的贡献不足10%。非<13>化石甲烷的δ ^ C值为-65@91，表明非化石甲烷是在厌氧环境中由细菌产生的。因此，夏季城郊甲烷面通量的增加主要是由于稻田的贡献。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