Abstract
Tropospheric air samples were collected aboard an aircraft up to an altitude of 7km. Vertical and horizontal distributions of tropospheric methane (CH4) over Japan were measured using a gas-chromatograph equipped with a flame ionization detector.
CH4 mixing ratios in vertical and latitudinal profiles varied widely. The CH4 mixing ratio was high when the air from the mid-to-high latitudes of the Asian continent source region of CH4 was sampled. The CH4 mixing ratio was highest in air from Siberia and rather low in air from China. When this air mixed with air from the stratosphere—very clear from the high O3 mixing ratio—the effect on the CH4 mixing ratio was not clear because CH4 is photochemically stable and does not apparently decrease from the upper troposphere to the lower stratosphere. When the aircraft went directly across tropopause folding, however, very high O3 and low CH4 mixing ratios were observed. The distance of transport from the intrusion region would be an important factor for both O3 and CH4 mixing ratios. Low CH4 mixing ratio was observed when the air was sampled in maritime air mass because they were not affected by Asian continental sources.
Between February 1986 and February 1992, mean CH4 mixing ratios over Japan (25-44°N, 124-144°E) steadily increased. Using data between November and February, we obtained an average annual increase of 12.0±7.0 (confidence interval: 70%) ppbv/year using a linear least square fit. From February 1992 to December 1993, the mean volume mixing ratio decreased slightly. The overall annual increase from February 1986 to February 1996 was 9.6±4.9 ppbv/year. These results indicate that the increase was larger in the mid-1980s than in the 1990s and has been declining.
