Development of a low-maintenance measurement approach to continuously estimate methane emissions: A case study

The chemical breakdown of organic matter in landfills represents a significant source of methane gas (CH₄). Current estimates suggest that landfills are responsible for between 3% and 19% of global anthropogenic emissions. The net CH₄ emissions resulting from biogeochemical processes and their modulation by microbes in landfills are poorly constrained by imprecise knowledge of environmental constraints. The uncertainty in absolute CH₄ emissions from landfills is therefore considerable. This study investigates a new method to estimate the temporal variability of CH₄ emissions using meteorological and CH₄ concentration measurements downwind of a landfill site in Suffolk, UK from July to September 2014, taking advantage of the statistics that such a measurement approach offers versus shorter-term, but more complex and instantaneously accurate, flux snapshots. Methane emissions were calculated from CH₄ concentrations measured 700m from the perimeter of the landfill with observed concentrations ranging from background to 46.4ppm. Using an atmospheric dispersion model, we estimate a mean emission flux of 709μgm^-2s^-1 over this period, with a maximum value of 6.21mgm^-2s^-1, reflecting the wide natural variability in biogeochemical and other environmental controls on net site emission. The emissions calculated suggest that meteorological conditions have an influence on the magnitude of CH₄ emissions. We also investigate the factors responsible for the large variability observed in the estimated CH₄ emissions, and suggest that the largest component arises from uncertainty in the spatial distribution of CH₄ emissions within the landfill area. The results determined using the low-maintenance approach discussed in this paper suggest that a network of cheaper, less precise CH₄ sensors could be used to measure a continuous CH₄ emission time series from a landfill site, something that is not practical using far-field approaches such as tracer release methods. Even though there are limitations to the approach described here, this easy, low-maintenance, low-cost method could be used by landfill operators to estimate time-averaged CH₄ emissions and their impact downwind by simultaneously monitoring plume advection and CH₄ concentrations.