Using open-path dual-comb spectroscopy to monitor methane emissions from simulated grazing cattle

<p>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 (CH<span class="inline-formula">₄</span>) release experiment was performed to determine if dual-comb spectroscopy (DCS) can detect CH<span class="inline-formula">₄</span> 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 CH<span class="inline-formula">₄</span> concentrations from 10 point sources of methane simulating cattle emissions. The CH<span class="inline-formula">₄</span> mole fractions and wind velocity data were used to calculate CH<span class="inline-formula">₄</span> flux using an inverse dispersion model, and the simulated fluxes were then compared to the actual CH<span class="inline-formula">₄</span> release rate. For a source located 60 m from the downwind path, the DCS system detected 10 nmol mol<span class="inline-formula">⁻¹</span> CH<span class="inline-formula">₄</span> horizontal concentration gradient above the atmospheric background concentration with a precision of 6 nmol mol<span class="inline-formula">⁻¹</span> in 15 min interval. A CH<span class="inline-formula">₄</span> release of 3970 g d<span class="inline-formula">⁻¹</span> was performed, resulting in an average concentration enhancement of 24 nmol mol<span class="inline-formula">⁻¹</span> of CH<span class="inline-formula">₄</span>. The calculated CH<span class="inline-formula">₄</span> flux was 4002 g d<span class="inline-formula">⁻¹</span>, showing good agreement with the actual CH<span class="inline-formula">₄</span> release rate. Periodically altering the downwind path, which may be needed to track moving cattle, did not adversely affect the ability of the instruments to determine the CH<span class="inline-formula">₄</span> flux. These results give us confidence that CH<span class="inline-formula">₄</span> flux can be determined by grazing cattle with low disturbance and direct field-scale measurements.</p>