Quantifying Methane Emissions from Point Sources Using Dual-Comb Spectroscopy

Despite the importance of atmospheric methane (CH₄) for the global climate system, there are still considerable uncertainties in the global CH₄ budget. These uncertainties arise from the scarcity of field observations and limited understanding of underlying mechanisms controlling CH₄ global sources and sinks. The livestock sector is considered to be the largest anthropogenic source of methane emissions in the U.S. with most of the CH₄ emissions originated from dairy and beef production. It has been estimated that the grazing phase of beef production accounts for 70 to 80% of the total CH₄ emissions from the beef industry. However, most of CH₄ emission data from cattle are derived from respiration chambers that interfere with animal behavior and forage intake. Dual comb spectroscopy (DCS) is an emerging technique that offers the unique ability to measure trace gas concentrations over long paths, ranging from hundreds of meters to kilometers, and reliably measures very small horizontal gradients of concentration. The DCS measurements will be combined with an inverse dispersion model (IDM) to estimate CH_4­ emission from cattle at high temporal (< 1 h) resolution. The objective of this study was to evaluate the performance of DCS system to estimate methane emissions from point sources in a large grazing system. The DCS system was used to measure line-averaged CH₄ mixing ratios from two paths with a total length of ~202 m. A gas manifold was used to release CH₄ from 10 outlets located approximately 50 m from the downwind path at a constant rate of ~380 g outlet^-1 day^-1. A proportional valve, pressure transducer and critical orifices were used to ensure a constant flow rate from all outlets. The concentration data and turbulence statistics were combined with an inverse dispersion model to estimate the total CH₄ emissions from the point sources. The simulated fluxes were slightly higher (400 g outlet^- day^-1) than the release rate (~380 g outlet^-1 day^-1). The CH₄ concentration difference between downwind and upwind paths ranged from 10 to 35 nanomol mol^-1, indicating that the DCS system is capable of resolving small horizontal concentration gradients expected in extensive grazing systems.