Keywords:methane, isotopic ratio, UT/LMS region, backward trajectory analyses
Aircraft observation campaigns over northern high latitudes have been conducted several times to elucidate spatial and temporal variations of CH4 concentration and their sources on the surface. However, simultaneous observations of CH4 concentration and its isotopic ratios (δ13C and δD) in the upper troposphere/lowermost stratosphere (UT/LMS) are quite limited, although such observations provide crucial information for quantifying contributions of sources/sinks of CH4 to its atmospheric variations. In this study, we present spatiotemporal variations of CH4, δ13C and δD using monthly on-board commercial airliners in UT/LMS over the Eurasian continent from April 2012. In the LMS, CH4 and δ13C, δD showed clear aniti-phase seasonal variations; seasonal maximum (minimum) of the CH4 concentration (δ13C, δD) was found in November to January and seasonal minimum (maximum) was in March to May. The observed seasonal variations can be explained by effective flushing of the LMS air with the tropospheric air (high CH4 and low δ13C and δD) from summer to autumn, and by subsidence of the deeper stratospheric air (low CH4 and high δ13C and δD) from winter to spring. Backward trajectory analyses with ERA-Interim reanalysis data were conducted for all air samples. By classifying the results into four seasons, it was found in each season that the correlation of δ13C or δD with potential velocity (PV) at each sampling point is improved by employing the PV values at locations where each air mass is suited 2-3 weeks before. Such an improvement is probably made, reflecting that isotopically heavier CH4 generally originates in higher altitudes and/or latitudes, and CH4 with lighter isotopes in lower altitudes and/or latitudes. We also examined the chemical pathways of CH4 destruction in the extratropical UT/LMS based on correlations between CH4 and δ13C. The enriched δ13C values with the lower CH4 concentrations indicate occurrence of reactions of CH4 with Cl and O(1D), in addition to the major destruction pathway via OH.