Atmos. Chem. Phys., 12, 8095-8113, 2012
www.atmos-chem-phys.net/12/8095/2012/
doi:10.5194/acp-12-8095-2012
© Author(s) 2012. This work is distributed
under the Creative Commons Attribution 3.0 License.
Carbon and hydrogen isotopic ratios of atmospheric methane in the upper troposphere over the Western Pacific
T. Umezawa1,*, T. Machida2, K. Ishijima3, H. Matsueda4, Y. Sawa4, P. K. Patra3, S. Aoki1, and T. Nakazawa1
1Center for Atmospheric and Oceanic Studies, Graduate School of Science, Tohoku University, Sendai, Japan
2National Institute for Environmental Studies, Tsukuba, Japan
3Research Institute for Global Change, JAMSTEC, Yokohama, Japan
4Meteorological Research Institute, Tsukuba, Japan
*now at: Max Planck Institute for Chemistry, Mainz, Germany

Abstract. We present the mixing ratio, δ13C and δD of atmospheric CH4 using commercial aircraft in the upper troposphere (UT) over the Western Pacific for the period December 2005–September 2010. The observed results were compared with those obtained using commercial container ships in the lower troposphere (LT) over the same region. In the Northern Hemisphere (NH), the UT CH4 mixing ratio shows high values in the boreal summer–autumn, when the LT CH4 mixing ratio reaches a seasonal minimum. From tagged tracer experiments made using an atmospheric chemistry transport model, we found that such high CH4 values are due to rapid transport of air masses influenced by CH4 sources in South Asia and East Asia. The observed isotopic ratio data imply that these areas have CH4 sources with relatively low δ13C and δD signatures such as biogenic sources. Latitudinal distributions of the annual average UT and LT CH4 mixing ratio intersect each other in the tropics; the mixing ratio value is lower in the UT than in the LT in the NH and the situation is reversed in the Southern Hemisphere (SH), due mainly to the NH air intrusion into the SH through the UT. Such intersection of the latitudinal distributions is observable in δD but not in δ13C, implying an additional contribution from reaction of CH4 with active chlorine in the marine boundary layer. δ13C and δD show low values in the NH and high values in the SH both in the UT and in the LT. We also observed an increase in the CH4 mixing ratio and decreases in δ13C and δ

Citation: Umezawa, T., Machida, T., Ishijima, K., Matsueda, H., Sawa, Y., Patra, P. K., Aoki, S., and Nakazawa, T.: Carbon and hydrogen isotopic ratios of atmospheric methane in the upper troposphere over the Western Pacific, Atmos. Chem. Phys., 12, 8095-8113, doi:10.5194/acp-12-8095-2012, 2012.
 
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