5:15 PM - 6:30 PM
[ACG37-P02] Continuous measurement of atmospheric methane mixing ratio at a cool-temperate deciduous forest site, Takayama, Japan
Keywords:Methane, Forest ecosystems
Methane (CH4) is a greenhouse gas that contributes to global warming next to carbon dioxide (CO2). Forest ecosystems other than wetlands are considered to be CH4 sinks due to the oxidative activities by soil microbes while it has also been pointed out that CH4 may be released from trees. However, the factors governing CH4 budgets in the ecosystem are not fully understood. To examine the variations in the atmospheric mixing ration and the budgets of CH4 in the forest ecosystem and the factors governing them, continuous measurement of the atmospheric CH4 mixing ratio has initiated at a cool-temperate deciduous forest site in the central Japan, Takayama (TKY: 36o08'N, 137o25'E, 1420 m) since July 2020. TKY is the longest monitoring site in the AsiaFlux network.
The continuous CH4 measurement was made using a laser spectroscopy analyzer (Ultra-Portable GGA, Los Gatos Research). Meteorological instruments and air intakes for the CH4 mixing ratio measurement were mounted on a 27-m tower. Sample air was collected from six different heights (27, 18, 8.8, 5.8, 2.0 and 1.3 m); the top level was above the canopy, the second just above the canopy and the other four below the canopy. The air was drawn by a diaphragm pump and introduced into the analyzer after removing aerosol particles and water vapor. The sample flow rate and the pressure in the sample cell of the analyzer were stabilized, and the analyzer was placed in a temperature-controlled box to reduce output drift. The analyzer was calibrated using three air-based CO2-CH4 standards with different mixing ratios every three hours. The overall precision was estimated to be less than 0.8 ppb, and the measured CH4 values were almost in agreement with those measured by a flask sampling method using an FID-GC.
The atmospheric CH4 observed at all heights did not show clear diurnal variation but fluctuations with a period of several days while the atmospheric CO2 showed clear diurnal variation from summer to early fall reflecting photosynthesis and respiratory activities. The variation patterns of the atmospheric CH4 observed at all heights were very similar, but the mixing ratios were lower at 1.3 m than at 27 m during most of the observation period. The vertical CH4 difference between 27 m and 1.3 m above the ground clearly showed positive correlation with soil temperature near the ground surface, which is consistent with the seasonal variation in intensity of soil CH4 sink observed at TKY using a soil chamber. The vertical difference also showed negative correlation with soil moisture, which suggests that the oxidation of CH4 is enhanced under dry conditions. Significant sources of CH4 from the above-ground parts of trees were not detected from the observed vertical profiles.
On the other hand, it was found from backward trajectory analyses that the observed high and low CH4 on the several-day scale described above were related to long-range atmospheric transport originated from the Asian Continent and the North Pacific Ocean, respectively. The results suggest that the high CH4 reflects the CH4 emissions originated from wetlands and anthropogenic activities on the continent.
We plan to further investigate the mechanism governing variations in the CH4 mixing ratio and the CH4 budgets in the forest ecosystem at TKY through data accumulation, genetic analyses of soil microbes, etc.
Acknowledgements
We would like to thank members of River Basin Research Center of Gifu University and AIST for their support for our observation. A. Kudo and C. Abe for their technical support for our measurements. This research was supported by JSPS KAKENHI Grant Numbers JP18H03365, JP19H03301 and JP19H01975.
The continuous CH4 measurement was made using a laser spectroscopy analyzer (Ultra-Portable GGA, Los Gatos Research). Meteorological instruments and air intakes for the CH4 mixing ratio measurement were mounted on a 27-m tower. Sample air was collected from six different heights (27, 18, 8.8, 5.8, 2.0 and 1.3 m); the top level was above the canopy, the second just above the canopy and the other four below the canopy. The air was drawn by a diaphragm pump and introduced into the analyzer after removing aerosol particles and water vapor. The sample flow rate and the pressure in the sample cell of the analyzer were stabilized, and the analyzer was placed in a temperature-controlled box to reduce output drift. The analyzer was calibrated using three air-based CO2-CH4 standards with different mixing ratios every three hours. The overall precision was estimated to be less than 0.8 ppb, and the measured CH4 values were almost in agreement with those measured by a flask sampling method using an FID-GC.
The atmospheric CH4 observed at all heights did not show clear diurnal variation but fluctuations with a period of several days while the atmospheric CO2 showed clear diurnal variation from summer to early fall reflecting photosynthesis and respiratory activities. The variation patterns of the atmospheric CH4 observed at all heights were very similar, but the mixing ratios were lower at 1.3 m than at 27 m during most of the observation period. The vertical CH4 difference between 27 m and 1.3 m above the ground clearly showed positive correlation with soil temperature near the ground surface, which is consistent with the seasonal variation in intensity of soil CH4 sink observed at TKY using a soil chamber. The vertical difference also showed negative correlation with soil moisture, which suggests that the oxidation of CH4 is enhanced under dry conditions. Significant sources of CH4 from the above-ground parts of trees were not detected from the observed vertical profiles.
On the other hand, it was found from backward trajectory analyses that the observed high and low CH4 on the several-day scale described above were related to long-range atmospheric transport originated from the Asian Continent and the North Pacific Ocean, respectively. The results suggest that the high CH4 reflects the CH4 emissions originated from wetlands and anthropogenic activities on the continent.
We plan to further investigate the mechanism governing variations in the CH4 mixing ratio and the CH4 budgets in the forest ecosystem at TKY through data accumulation, genetic analyses of soil microbes, etc.
Acknowledgements
We would like to thank members of River Basin Research Center of Gifu University and AIST for their support for our observation. A. Kudo and C. Abe for their technical support for our measurements. This research was supported by JSPS KAKENHI Grant Numbers JP18H03365, JP19H03301 and JP19H01975.