Estimating regional methane emissions from agriculture using aircraft measurements of concentration profiles

This paper describes a “top-down” approach for estimating regional surface fluxes of methane, and its application to a pastoral farming region in New Zealand. The approach is based on air sampling from aircraft and interpretation by mesoscale dispersion modelling. The goal is an independent cross-check for an agricultural region of “bottom-up” emission estimation methods like those used for inventory reporting under the United Nations Framework Convention on Climate Change (UNFCCC). The “top-down” strategy infers emissions over an agricultural region from differences between methane concentrations measured upwind and downwind of the region. The approach was trialed over the Manawatu agricultural region of New Zealandʹs North Island. As expected, measured concentration differences were largest at low wind speeds (2–3 m s−1 ). However, during these low wind conditions the concentration differences could not be reliably inverted to provide emission estimates, because of the complex variability in air flow caused by topography and land–sea temperature contrasts. Useful emission estimates were obtained during days with higher wind speed (about 8 m s−1), as the strong synoptic-scale flow then suppressed the development of complex local flows. The upwind–downwind concentration differences were smaller during these conditions of stronger flow, so that precision limits to concentration measurements became significant. Methane fluxes (in the range of 20–100 mg m−2 d−1) calculated under the stronger wind speed conditions were consistent with “bottom-up” estimates scaled from per-animal emission factors, enhancing confidence in the inventory-reporting methodology.