17:15 〜 18:45
[AAS09-P02] 日本上空および極域における大気中メタン濃度と炭素・水素同位体比の時空間変動
Methane (CH4) is the second most important anthropogenic greenhouse gas, after carbon dioxide, contributing to climate change. Global average mole fraction of atmospheric CH4 increased rapidly through 1990s, stagnated in the former half of 2000s, and then increased again after 2007. The causes of the CH4 variations have been investigated by various methods, however, large uncertainties still remain. The carbon and hydrogen isotope ratios are effective constraints to estimate the CH4 source causing atmospheric CH4 variations, since each CH4 source category, biogenic, fossil fuel and biomass burning, has a characteristic range of the isotope ratios. In this study, we have analyzed air samples collected over Japan and at observation sites in the polar regions (Ny-Ålesund, Svalbard and Syowa Station, Antarctica) for the CH4 mole fraction and its carbon and hydrogen isotope ratios (δ13C and δD). Then, we simulated atmospheric CH4 mole fraction, δ13C, and δD using a three-dimensional atmospheric chemical transport model (NIES-TM) and compared them with observations to evaluate the variability of global CH4 emissions for 1995-2021.
The phase of the CH4 seasonal variations observed over Japan is different in the upper and lower troposphere, but the seasonal phase of the carbon/hydrogen isotope ratios are almost identical throughout the troposphere. The differences in the seasonal phases relationships between the CH4 mole fractions and isotopes may reflect differences in methane sink rates at different altitudes. The analysis of observed data at Ny-Ålesund and Syowa Station suggest that the seasonal changes at Ny-Ålesund are caused by the superposition of biogenic CH4 emissions and sinks by the reaction with OH, while, the seasonal changes at Syowa Station are mainly due to OH-induced sink. The secular increase of CH4 mole fraction and decrease of δ13C, and δD have accelerated after 2013 in particular.
Using the CH4 and isotope data observed at Syowa Station and forward simulations by NIES-TM, we estimated the fraction of each source category to the total CH4 emission. The result shows that biogenic, fossil fuel, and biomass burning sources accounted for 60.9%, 31.6%, and 7.5% of the total global CH4 emissions for 2008-2017, respectively. It is also suggested that the increase in biogenic CH4 emissions could be responsible for the global CH4 increase since 2006 and that the increase of the fossil fuel CH4 estimated by EDGARv6-7 could be overestimated since the late 2000s.
The phase of the CH4 seasonal variations observed over Japan is different in the upper and lower troposphere, but the seasonal phase of the carbon/hydrogen isotope ratios are almost identical throughout the troposphere. The differences in the seasonal phases relationships between the CH4 mole fractions and isotopes may reflect differences in methane sink rates at different altitudes. The analysis of observed data at Ny-Ålesund and Syowa Station suggest that the seasonal changes at Ny-Ålesund are caused by the superposition of biogenic CH4 emissions and sinks by the reaction with OH, while, the seasonal changes at Syowa Station are mainly due to OH-induced sink. The secular increase of CH4 mole fraction and decrease of δ13C, and δD have accelerated after 2013 in particular.
Using the CH4 and isotope data observed at Syowa Station and forward simulations by NIES-TM, we estimated the fraction of each source category to the total CH4 emission. The result shows that biogenic, fossil fuel, and biomass burning sources accounted for 60.9%, 31.6%, and 7.5% of the total global CH4 emissions for 2008-2017, respectively. It is also suggested that the increase in biogenic CH4 emissions could be responsible for the global CH4 increase since 2006 and that the increase of the fossil fuel CH4 estimated by EDGARv6-7 could be overestimated since the late 2000s.