Constraints on Asian and European sources of methane from CH₄‐C₂H₆‐CO correlations in Asian outflow

Aircraft observations of Asian outflow from the Transport and Chemical Evolution Over the Pacific (TRACE‐P) aircraft mission over the NW Pacific (March and April 2001) show large CH 4 enhancements relative to background, as well as strong CH 4 ‐C 2 H 6 ‐CO correlations that provide signatures of regional sources. We apply a global chemical transport model simulation of the CH 4 ‐C 2 H 6 ‐CO system for the TRACE‐P period to interpret these observations in terms of CH 4 sources and to explore in particular the unique constraints from the CH 4 ‐C 2 H 6 ‐CO correlations. We use as a priori a global CH 4 source inventory constrained with National Oceanic and Atmospheric Administration (NOAA) Climate Monitoring and Diagnostics Laboratory (CMDL) surface observations [ Wang et al. , 2004 ]. We find that the observed CH 4 concentration enhancements and CH 4 ‐C 2 H 6 ‐CO correlations in Asian outflow in TRACE‐P are determined mainly by anthropogenic emissions from China and Eurasia (defined here as Europe and eastern Russia), with only little contribution from tropical sources (wetlands and biomass burning). The a priori inventory overestimates the observed CH 4 enhancements and shows regionally variable biases for the CH 4 /C 2 H 6 slope. The CH 4 /CO slopes are simulated without significant bias. Matching both the observed CH 4 enhancements and the CH 4 ‐C 2 H 6 ‐CO slopes in Asian outflow requires increasing the east Asian anthropogenic source of CH 4 , and decreasing the Eurasian anthropogenic source, by at least 30% for both. The need to increase the east Asian source is driven by the underestimate of the CH 4 /C 2 H 6 slope in boundary layer Chinese outflow. The Streets et al. [2003] anthropogenic emission inventory for east Asia fits this constraint by increasing CH 4 emissions from that region by 40% relative to the a priori, largely because of higher livestock and landfill source estimates. Eurasian sources (mostly European) then need to be reduced by 30–50% from the a priori value of 68 Tg yr −1 . The decrease of European sources could result in part from recent mitigation of emissions from coal mining and landfills.