Carbon and hydrogen isotopic ratios of atmospheric methane in the upper troposphere over the Western Pacific

We present the mixing ratio, δ ¹³C and δD of atmospheric CH ₄ 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 CH ₄ mixing ratio shows high values in the boreal summer-autumn, when the LT CH ₄ mixing ratio reaches a seasonal minimum. From tagged tracer experiments made using an atmospheric chemistry transport model, we found that such high CH ₄ values are due to rapid transport of air masses influenced by CH ₄ sources in South Asia and East Asia. The observed isotopic ratio data imply that these areas have CH ₄ sources with relatively low δ ¹³C and δD signatures such as biogenic sources. Latitudinal distributions of the annual average UT and LT CH ₄ 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 δ ¹³C, implying an additional contribution from reaction of CH ₄ with active chlorine in the marine boundary layer. δ ¹³C 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 CH ₄ mixing ratio and decreases in δ ¹³C and δD during 2007-2008 in the UT and LT over the Western Pacific, possibly due to enhanced biogenic emissions in the tropics and NH.