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. Methane emission fluxes from many facility-scale sourcesmay be poorly quantified, potentially leading to uncertainties in the globalmethane budget. Accurate atmospheric measurement-based flux quantificationis urgently required to address this. This paper describes the first test(using unbiased sampling) of a near-field Gaussian plume inversion (NGI)technique, suitable for facility-scale flux quantification, using acontrolled release of methane gas. Two unmanned-aerial-vehicle (UAV)platforms were used to perform 22 flight surveys downwind of a point-sourcemethane gas release from a regulated cylinder with a flowmeter. One UAV wastethered to an instrument on the ground, while the other UAV carried anon-board prototype instrument (both of which used the same near-infraredlaser technology). Both instruments were calibrated using certifiedstandards to account for variability in the instrumental gain factor,assuming fixed temperature and pressure. Furthermore, a water vapourcorrection factor, specifically calculated for the instrument, was appliedand is described here in detail. We also provide guidance on potentialsystematic uncertainties associated with temperature and pressure, which mayrequire further characterisation for improved measurement accuracy. The NGItechnique was then used to derive emission fluxes for each UAV flightsurvey. We found good agreement of most NGI fluxes with the known controlledemission flux, within uncertainty, verifying the flux quantificationmethodology. The lower and upper NGI flux uncertainty bounds were, onaverage, 17 %±10(1σ) % and 227 %±98(1σ) % of the controlled emission flux, respectively. This range ofconservative uncertainty bounds incorporate factors including thevariability in the position of the time-invariant plume and potential forunder-sampling. While these average uncertainties are large compared tomethods such as tracer dispersion, we suggest that UAV sampling can behighly complementary to a toolkit of flux quantification approaches and maybe a valuable alternative in situations where site access for tracer releaseis problematic. We see tracer release combined with UAV sampling as aneffective approach in future flux quantification studies. Successful fluxquantification using the UAV sampling methodology described heredemonstrates its future utility in identifying and quantifying emissionsfrom methane sources such as oil and gas extraction infrastructurefacilities, livestock agriculture, and landfill sites, where site access maybe difficult.

Testing the near-field Gaussian plume inversion flux quantification technique using unmanned aerial vehicle sampling