Abstract. Methane emission fluxes from many facility-scale sources
may be poorly quantified, potentially leading to uncertainties in the global
methane budget. Accurate atmospheric measurement-based flux quantification
is 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 a
controlled release of methane gas. Two unmanned-aerial-vehicle (UAV)
platforms were used to perform 22 flight surveys downwind of a point-source
methane gas release from a regulated cylinder with a flowmeter. One UAV was
tethered to an instrument on the ground, while the other UAV carried an
on-board prototype instrument (both of which used the same near-infrared
laser technology). Both instruments were calibrated using certified
standards to account for variability in the instrumental gain factor,
assuming fixed temperature and pressure. Furthermore, a water vapour
correction factor, specifically calculated for the instrument, was applied
and is described here in detail. We also provide guidance on potential
systematic uncertainties associated with temperature and pressure, which may
require further characterisation for improved measurement accuracy. The NGI
technique was then used to derive emission fluxes for each UAV flight
survey. We found good agreement of most NGI fluxes with the known controlled
emission flux, within uncertainty, verifying the flux quantification
methodology. The lower and upper NGI flux uncertainty bounds were, on
average, 17 %±10(1σ) % and 227 %±98(1σ) % of the controlled emission flux, respectively. This range of
conservative uncertainty bounds incorporate factors including the
variability in the position of the time-invariant plume and potential for
under-sampling. While these average uncertainties are large compared to
methods such as tracer dispersion, we suggest that UAV sampling can be
highly complementary to a toolkit of flux quantification approaches and may
be a valuable alternative in situations where site access for tracer release
is problematic. We see tracer release combined with UAV sampling as an
effective approach in future flux quantification studies. Successful flux
quantification using the UAV sampling methodology described here
demonstrates its future utility in identifying and quantifying emissions
from methane sources such as oil and gas extraction infrastructure
facilities, livestock agriculture, and landfill sites, where site access may
be difficult.