Variations of tropospheric methane over Japan during 1988-2010

• Published in: Tellus. Series B, Chemical and physical meteorology Vol. 66; no. 1; pp. 23837 - 18
• Main Authors: Umezawa, Taku, Goto, Daisuke, Aoki, Shuji, Ishijima, Kentaro, Patra, Prabir K., Sugawara, Satoshi, Morimoto, Shinji, Nakazawa, Takakiyo
• Format: Journal Article
• Language: English
• Published: Stockholm Taylor & Francis 01.01.2014; Ubiquity Press; Stockholm University Press
• Subjects: Air masses; Aircraft; aircraft observation; Altitude; Asian outflow; atmospheric chemistry transport model; Carbon; East Asia; Emission; Emissions; Full text; Gases; Growth rate; long-term variation; Meteorology; Methane; seasonality; Simulation; Summer; Transport; vertical profile; Wetlands; East Asia; Japan; Japan, Honshu, Miyagi Prefect., Tohoku
• ISSN: 0280-6509; 1600-0889
• DOI: 10.3402/tellusb.v66.23837

We present observations of CH 4 concentrations from the lower to upper troposphere (LT and UT) over Japan during 1988-2010 based on aircraft measurements from the Tohoku University (TU). The analysis is aided by simulation results using an atmospheric chemistry transport model (i.e. ACTM). Tropospheric CH 4 over Japan shows interannual and seasonal variations that are dependent on altitudes, primarily reflecting differences in air mass origins at different altitudes. The long-term trend and interannual variation of CH 4 in the LT are consistent with previous reports of measurements at surface baseline stations in the northern hemisphere. However, those in the UT show slightly different features from those in the LT. In the UT, CH 4 concentrations show a seasonal maximum in August due to efficient transport of air masses influenced by continental CH 4 sources, while LT CH 4 reaches its seasonal minimum during summer due to enhanced chemical loss. Vertical profiles of the CH 4 concentrations also vary with season, reflecting the seasonal cycles at the respective altitudes. In summer, transport of CH 4 -rich air from Asian regions elevates UT CH 4 levels, forming a uniform vertical profile above the mid-troposphere. On the other hand, CH 4 decreases nearly monotonically with altitude in winter-spring. The ACTM simulations with different emission scenarios reproduce general features of the tropospheric CH 4 variations over Japan. Tagged tracer simulations using the ACTM indicate substantial contributions of CH 4 sources in South Asia and East Asia to the summertime high CH 4 values observed in the UT. This suggests that our observations over Japan are highly sensitive to CH 4 emission signals particularly from Asia.