Estimating Methane Emissions in the Surat Basin, Australia, including turbulent vertical Fluxes

Last year we described the campaign and the first results (Kelly et al., 2019; Neininger et al., 2019).

This year we will give an update on methods applied for estimating the regional methane emissions on a scale of about 10'000 km², and sub-regions of about 2'500 km².

Two approaches were applied:

1. The classical mass balance, where the inflow and the outflow of an imaginary box was calculated, based on almost perfect Lagrangian cross-sections (following the air mass).
2. A mass balance for the part of the boundary layer, where flight tracks were available (below 300 m above ground), supplemented by vertical turbulent fluxes to above this height.

In the best case, the two methods are leading to similar emission rates. The advantage of method (2) is, that the long flight legs can be limited to the lower boundary layer, which is especially useful when a convective boundary layer is reaching up to typically 2 km or higher above the surface.

The method worked quite well for water vapour, CO₂ and sensible heat, where fully resolved turbulent fluxes could be calculated based on 10 Hz measurements along the flight legs. Since CH₄ could only be measured with a temporal resolution of about two seconds (0.5 Hz), these a-priori results of the turbulent vertical fluxes are less consistent. However, by applying factors of turbulent versus advective fluxes from the other species, the agreement between the two methods was improved. The turbulent transport to above the 300-metre-layer during the convective conditions was about equal to the accumulation in this layer.

Since estimating the height of the convective boundary layer and the assumption that the mixing is perfect for approach (1) has many limitations, using method (2) has the advantage that less assumptions on homogeneity of the atmosphere above the densely observed layer has to be made. Even when the concentration profiles and the wind are known from vertical soundings (excursions to above the convective boundary layer), the horizontal inhomogeneity remains unknown. When using the vertical turbulent fluxes into this unknown volume above the lower layer, inhomogeneous mixing is not a problem.

The challenge of method (2) is to measure fast and precise enough for the quantification of the vertical fluxes. When concentrating on this, one could save time by omitting high soundings, improving the horizontal coverage, and therefore the statistics for the vertical fluxes.

References

Kelly et al.: Direct Measurement of Coal Seam Gas and Agricultural Methane Emissions in the Surat Basin, Australia. EGU 2019.

Neininger, B., J. M. Hacker and W. Lieff: Airborne Measurements for estimating Methane Emissions in the Surat Basin, Australia. EGU 2019.