Using Open-Path Dual-Comb Spectroscopy to Monitor Methane Emissions from Simulated Grazing Cattle

Abstract. Accurate whole-farm or herd-level measurements of livestock methane emissions are necessary for anthropogenic greenhouse gas inventories and to evaluate mitigation strategies. A controlled methane (CH4) release experiment was performed to determine if dual comb spectroscopy (DCS) can detect CH4 concentration enhancements produced by a typical herd of beef cattle in an extensive grazing system. Open-path DCS was used to measure downwind and upwind CH4 concentrations from ten point-sources of methane simulating cattle emissions. The CH4 mixing ratio along with wind velocity data were used to calculate CH4 flux using an inverse dispersion model, and the simulated fluxes were then compared to the actual CH4 release rate. For a source located 60 m from the downwind path, the DCS system detected 10 nmol mol-1 CH4 horizontal concentration gradient above the atmospheric background concentration with a precision of 6 nmol mol-1 in 15-min interval. A CH4 release of 3970 g day-1 was performed resulting in an average concentration enhancement of 24 nmol mol-1 of CH4. The calculated CH4 flux was (4002±1498) g day‑1 in agreement with the actual release rate. Periodically altering the downwind path, which may be needed to track moving cattle, did not adversely affect the ability to determine the CH4 flux. The measurement was only limited by maintaining sufficient reflected power from the remote retroreflectors over the open path to achieve a sufficient signal-to-noise ratio. These results give us confidence that CH4 flux can be determined by grazing cattle with low disturbance and direct field-scale measurements.