Seasonal Variations of CO₂, CH₄, N₂O and CO in the Mid-Troposphere over the Western North Pacific Observed Using a C-130H Cargo Aircraft

Abstract

Seasonal variations of carbon dioxide (CO₂), methane (CH₄), carbon monoxide (CO), and nitrous oxide (N₂O), in the mid-troposphere over the western North Pacific, are investigated using air samples collected onboard a C-130H aircraft. These samples were obtained between Atsugi Base (35.45°N, 139.45°E) and Minamitorishima (MNM; 24.28°N, 153.98°E), once a month, from September 2010 to September 2012. Increasing trends of CO₂ and N₂O and large variability of CH₄ and CO (at approximately 6 km) have been found. During summer, concentrations of CH₄ and CO were found to increase with height over MNM. High concentrations of CH₄ were persistently observed in the mid-troposphere throughout the observation period. The average enhancement ratios of CH₄ to CO above background levels (ΔCH₄/ΔCO) were 0.47 and 1.2 ppb/ppb for winter-spring and summer-fall, respectively. The results suggested that the high CH₄ concentrations originated primarily from fossil fuel combustions in winter-spring, while there could be an additional contribution from increased biogenic sources during summer-fall. Because a surface station in MNM rarely observed the summer-fall high CH₄ concentration values in the mid-troposphere, the aircraft measurements could provide a powerful constraint on the CH₄ emission estimates for Asia, in addition to that provided by the surface measurements. This aircraft measurement program is regularly conducted for the long-term monitoring of the greenhouse gases in the mid-troposphere, and it has a significant role for filling the data gap of the existing measurement network.