5:15 PM - 6:30 PM
[AAS05-P23] Temporal and spatial variations of the mole fraction, carbon and hydrogen isotope ratios of atmospheric methane in the Western Pacific region
Keywords:Methane, Isotopic ratio, Western Pacific
To understand atmospheric CH4 variations in the Western Pacific region, we have conducted systematic observations of the CH4 mole fractions and its carbon and hydrogen isotope ratios (d13C and dD) using air samples collected on board container ships that regularly sail between Japan and Australia/New Zealand. In this study, we analyzed the observation data from 2006 to 2019.
In the temperate zones of both hemispheres, clear seasonal cycles in the CH4 mole fractions, d13C and dD were observed. Using an atmospheric four-box model with prescribed CH4 sink, we estimated the contribution of CH4 sources to the seasonal CH4 change. The results suggest that the seasonal change in the northern hemisphere is caused by the balance between CH4 release from biogenic CH4 sources such as wetlands in summer and CH4 destruction by the reaction with OH radicals that become maximum also in summer. On the other hand, the seasonal CH4 change in the southern hemisphere is almost determined by the seasonal changes in the amount of the CH4 destruction by OH radicals.
In the Western Pacific, the CH4 mole fractions, d13C and dD showed clear latitudinal gradients, with CH4 mole fraction increasing and d13C and dD decreasing from south to north throughout a year. These observational facts suggest that there are isotopically lighter CH4 sources (microbial origin) in the northern hemisphere. The latitudinal distributions of the CH4 mole fractions and d13C obtained in this study were compared with those observed by the National Oceanic and Atmospheric Administration (NOAA/GMD) at background sites far from CH4 source region (hereafter called as NOAA sites). During the boreal summer, the CH4 and d13C in the Western Pacific showed lower and higher values, respectively, compared to those at the NOAA sites, suggesting that the Western Pacific region could be more strongly affected by the destruction by OH radicals. On the other hand, in the boreal winter, the CH4 mole fractions were higher in the Western Pacific than the background NOAA sites. This would be due to the prevailing westerly winds in the winter season, which transported the air-mass strongly influenced by land CH4 sources into the Western Pacific region.
The CH4 mole fractions observed in the Western Pacific, at Syowa Station(69.0° S, 39.4° E) and Ny-Alesund(78.6° N, 11.6° E) showed increasing trends after 2006. In contrast, d13C and dD decreased after 2006.We employed the four-box model to investigate the cause of the long-term CH4 variability. When the CH4 destruction rate by reaction with OH radical is fixed after 2006, an increase in CH4 emission after 2006 and decreases in d13C and dD of aggregated CH4 sources in all latitudinal bands after 2007 are required to reproduce the observed CH4, d13C and dD values. This suggests that biogenic CH4 emission has increased since 2006.
In the temperate zones of both hemispheres, clear seasonal cycles in the CH4 mole fractions, d13C and dD were observed. Using an atmospheric four-box model with prescribed CH4 sink, we estimated the contribution of CH4 sources to the seasonal CH4 change. The results suggest that the seasonal change in the northern hemisphere is caused by the balance between CH4 release from biogenic CH4 sources such as wetlands in summer and CH4 destruction by the reaction with OH radicals that become maximum also in summer. On the other hand, the seasonal CH4 change in the southern hemisphere is almost determined by the seasonal changes in the amount of the CH4 destruction by OH radicals.
In the Western Pacific, the CH4 mole fractions, d13C and dD showed clear latitudinal gradients, with CH4 mole fraction increasing and d13C and dD decreasing from south to north throughout a year. These observational facts suggest that there are isotopically lighter CH4 sources (microbial origin) in the northern hemisphere. The latitudinal distributions of the CH4 mole fractions and d13C obtained in this study were compared with those observed by the National Oceanic and Atmospheric Administration (NOAA/GMD) at background sites far from CH4 source region (hereafter called as NOAA sites). During the boreal summer, the CH4 and d13C in the Western Pacific showed lower and higher values, respectively, compared to those at the NOAA sites, suggesting that the Western Pacific region could be more strongly affected by the destruction by OH radicals. On the other hand, in the boreal winter, the CH4 mole fractions were higher in the Western Pacific than the background NOAA sites. This would be due to the prevailing westerly winds in the winter season, which transported the air-mass strongly influenced by land CH4 sources into the Western Pacific region.
The CH4 mole fractions observed in the Western Pacific, at Syowa Station(69.0° S, 39.4° E) and Ny-Alesund(78.6° N, 11.6° E) showed increasing trends after 2006. In contrast, d13C and dD decreased after 2006.We employed the four-box model to investigate the cause of the long-term CH4 variability. When the CH4 destruction rate by reaction with OH radical is fixed after 2006, an increase in CH4 emission after 2006 and decreases in d13C and dD of aggregated CH4 sources in all latitudinal bands after 2007 are required to reproduce the observed CH4, d13C and dD values. This suggests that biogenic CH4 emission has increased since 2006.