Abstract
Point-source methane emission flux quantification is required to help constrain the global methane budget. Facility-scale fluxes can be derived using in situ methane mole fraction sampling, near-to-source, which may be acquired from an unmanned aerial vehicle (UAV) platform. We test a new non-dispersive infrared methane sensor by mounting it onto a small UAV, which flew downwind of a controlled methane release. Nine UAV flight surveys were conducted on a downwind vertical sampling plane, perpendicular to mean wind direction. The sensor was first packaged in an enclosure prior to sampling which contained a pump and a recording computer, with a total mass of 1.0 kg. The packaged sensor was then characterised to derive a gain factor of 0.92 ± 0.07, independent of water mole fraction, and an Allan deviation precision (at 1 Hz) of ±1.16 ppm. This poor instrumental precision and possible short-term drifts made it non-trivial to define a background mole fraction during UAV surveys, which may be important where any measured signal is small compared to sources of instrumental uncertainty and drift. This rendered the sensor incapable of deriving a meaningful flux from UAV sampling for emissions of the order of 1 g s−1. Nevertheless, the sensor may indeed be useful when sampling mole fraction enhancements of the order of at least 10 ppm (an order of magnitude above the 1 Hz Allan deviation), either from stationary ground-based sampling (in baseline studies) or from mobile sampling downwind of sources with greater source flux than those observed in this study. While many methods utilising low-cost sensors to determine methane flux are being developed, this study highlights the importance of adequately characterising and testing all new sensors before they are used in scientific research.
Highlights
The global methane budget is poorly constrained [1], due in part to the lack of accurate quantification of emissions from anthropogenic facility-scale sources [2], such as landfill sites [3,4], oil and gas infrastructure facilities [5,6] or herds of cattle [7,8]
In order to prepare the High Performance Platform (HPP) methane sensor for unmanned aerial vehicle (UAV) sampling, it was first packaged inside an enclosure
Mapped [X] measurements acquired during each UAV flight survey were plotted on a plane perpendicular to mean wind direction, to test their suitability in flux quantification
Summary
The global methane budget is poorly constrained [1], due in part to the lack of accurate quantification of emissions from anthropogenic facility-scale sources [2], such as landfill sites [3,4], oil and gas infrastructure facilities [5,6] or herds of cattle [7,8]. With average annualised atmospheric global methane mole fraction ([X]) on the increase [9,10], it is essential that emission fluxes from facility-scale point sources are accurately quantified, using top-down (atmospheric measurement-based) methods, in order to validate bottom-up (component-based) flux estimates [11,12]. A range of flux quantification techniques can be used to derive facility-scale top-down flux estimates using either remote sensing or in situ sampling [13,14]. In situ [X] measurements on a vertical downwind flux plane, near to source, can be used to derive a flux by employing a variety of techniques [14] such as mass balance box modelling [27,28], the tracer dispersion method [29,30] or a Gaussian plume inversion [31,32]. The near-field Gaussian plume inversion (NGI) method, described by Shah et al [33], is an example of a traditional flux quantification technique, adapted for near-field sampling
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