Emissions of CH₄ and N₂O over the United States and Canada based on a receptor-oriented modeling framework and COBRA-NA atmospheric observations

We present top-down emission constraints for two non-CO₂ greenhouse gases in large areas of the U.S. and southern Canada during early summer. Collocated airborne measurements of methane and nitrous oxide acquired during the COBRA-NA campaign in May - June 2003, analyzed using a receptor-oriented Lagrangian particle dispersion model, provide robust validation of independent bottom-up emission estimates from the EDGAR and GEIA inventories. We find that the EDGAR CH₄ emission rates are slightly low by a factor of 1.08 ± 0.15 (2σ), while both EDGAR and GEIA N₂O emissions are significantly too low, by factors of 2.62 ± 0.50 and 3.05 ± 0.61, respectively, for this region. Potential footprint bias may expand the statistically retrieved uncertainties. Seasonality of agricultural N₂O emissions may help explain the discrepancy. Total anthropogenic U.S. and Canadian emissions would be 49 Tg CH₄ and 4.3 Tg N₂O annually, if these inventory scaling factors applied to all of North America.