Constraints on Asian and European Sources of Methane from CH4 - C2H6 - CO Correlations in Asian Outflow

Aircraft observations in Asian outflow from the TRACE-P aircraft mission (March-April 2001) show persistently strong CH₄/C₂H₆ correlations with uniform slope (r2=0.8-0.95, slope=45-59 mol/mol for individual flights) and more variable CH₄/CO correlations (r2=0.6-0.88, slope=0.4-1.2 mol/mol). We apply a global 3-D simulation of the CH₄-C₂H₆-CO system, including consistent representations of sources and sinks for all three species, to interpret these correlations and the CH₄ concentration enhancements observed in TRACE-P toward improved estimates of Asian and European CH₄ sources. We use as a priori a state-of-the-science global CH₄ inventory constrained with NOAA/CMDL observations [J.S. Wang et al., J. Geophys. Res. 2003]. This inventory is found to overestimate CH₄ concentration enhancements in the Asian outflow while giving an unbiased simulation of CH₄-C₂H₆-CO correlations. Matching the TRACE-P observations in subtropical outflow requires a 70% decrease in seasonal Southeast Asian biomass burning emissions for CH₄, C₂H₆ and CO, implying a general overestimate of biomass burned. Matching the observations in the Asian outflow north of 30° N can be achieved by using the Streets et al. [J. Geophys. Res., 2003] inventory for anthropogenic Asian emissions and by reducing the European and Russian sources by 25%. The former change increases the CH₄/C₂H₆ and CH₄/CO slopes, while the latter decreases the slopes, and the combination of the two changes maintains the unbiased simulation of the observed slopes. The resulting simulation also provides a good match to the observed CH₄ background and to the NOAA/CMDL data.