Measuring acetylene with a cavity ring-down spectroscopy gas analyser and its use as a tracer to quantify methane emissions

Abstract. Facility-scale methane emission fluxes can be derived by comparing tracer and methane mole fraction measurements downwind of a methane emission source where a co-located tracer gas is released at a known flux rate. Acetylene is a commonly used tracer for methane due to its availability, low cost and low atmospheric background. Acetylene mole fraction can be measured using infrared gas analysers such as the Picarro G2203, using cavity ring-down spectroscopy. However, failure to calibrate tracer gas analysers may influence methane flux estimation, if raw mole fraction measurements diverge from their true levels. We conducted extensive Picarro G2203 laboratory characterisation testing. Picarro G2203 acetylene measurements were calibrated by diluting a high concentration of acetylene with ambient air. In order to determine the precise level of acetylene in each calibration gas mixture, a high concentration methane source was diluted in an identical way, with reliable methane mole fraction measurements used to quantify the true level of dilution. It was found that raw Picarro G2203 acetylene mole fraction measurements could be corrected through direct multiplication with a calibration gain factor of 0.94, derived by applying a linear fit between raw measured and reference acetylene mole fraction. However, this calibration is only valid from an acetylene mole fraction of 1.16 ppb, below which unstable measurements were observed by the Picarro G2203 tested in this study. A field study was then conducted by performing fourteen successful transects downwind of an active landfill site, where acetylene was released from a single point location at a fixed flow rate. Methane fluxes were derived by integrating the methane and acetylene mole fraction plumes, as a function of distance along the sampling road. This resulted in a flux variability of 56 % between methane flux estimates from different transects which was principally due to flux errors associated with the tracer release location and downwind sampling positioning. Methane fluxes were also derived using raw uncalibrated Picarro G2203 acetylene mole fraction measurements instead of calibrated measurements, which resulted an average methane emission flux underestimation of 7.6 %, compared to fluxes derived using calibrated measurements. Unlike a random uncertainty, this 7.6 % bias represents a consistent flux underestimation that cannot be reduced with improvements to the field sampling methodology. This study therefore emphasises the equal importance of calibrating both target as well as tracer gas measurements, regardless of the instrument being used to obtain these measurements. Otherwise, biases can be induced within target gas flux estimates. For the example of methane, this can influence our understanding of the role of certain facility scale emissions within the global methane budget.