A novel approach to infer regional emissions from airborne methane measurements

Airborne trace gas concentration measurements can be used to infer regional emissions. Foregoing full 3D transport inversion methods, emissions are generally estimated using bulk methods. Measuring concentrations across the upwind and downwind wall of the area of interest, at different altitudes in the boundary layer, allows to specify trace gas inflow and outflow and from the difference to estimate the surface flux.

Here we propose a novel alternative method. We fly parallel tracks over the region of interest at low altitude (200ft), aligned with the wind. The resulting trace gas signal typically can be viewed as a series of concentration peaks superimposed on a linear gradient. We interpret this as that the positive (or negative) gradient is the result of diffuse sources (or sinks) in the landscape. The concentration peaks we interpret as the resulting from point sources, i.e. gaussian plumes intersected by the flight track.

We demonstrate promising results using methane concentrations obtained over a rural landscape in the Netherlands dominated by dairy farms on (drained) organic soils. In this setting we interpret diffuse sources of methane to originate  from the very wet parts of the landscape, i.e. ditches and (near) inundated parcels. The concentration peaks we trace back to cattle herds of (small clusters of) individual farms. We will show more details of methodology and results. Finally, we  discuss uncertainties and compare obtained emissions  with bottom-up estimates in National Inventory Reports and other statistics, as well as recent inversion studies. With the latter we concur that the Netherlands maybe under-reporting its rural methane emissions.