Characterization of Natural Gas and Agricultural Methane Emission Variability with Open-Path Dual-Comb Spectroscopy

Methane is an important chemical feedstock and a common atmospheric pollutant emitted from many sectors of the modern economy. Methane is also a potent greenhouse gas - atmospheric concentrations have risen steadily since the beginning of the Industrial Revolution and are now consistent with unfavorable Representative Concentration Pathway models. An accurate understanding of the distribution of methane emissions throughout the economy is required to efficiently develop and apply effective mitigation technologies. Within the United States, the natural gas and agriculture sectors are believed to be major emissions contributors. Field observations can help validate inventory estimates through characterization of the magnitude, frequency, and variability of emissions.

We report economic sector apportionment results from a multi-month (Oct. 2021 - Jan. 2022) continuous observation campaign in the Colorado northern Front Range at the Platteville Atmospheric Observatory (PAO). Simultaneous observations of methane and two tracer gases, ethane and ammonia, were made using open-path, mid-infrared and near-infrared dual-comb spectrometers and a commercial gas analyzer. Ethane and ammonia are assumed to reflect natural gas and agricultural methane emissions respectively. From the time series data, a multi-variate linear regression model related observed methane to scaled contributions from ethane and ammonia, yielding a series of time varying tracer gas ratios. Variability in tracer gas ratios suggests a wide range of sources and processes underlying methane emissions around PAO. We examine the diurnal, spatial, and temperature dependences of these tracer gas ratios. Further, we compare our observations to both EPA and EDGAR oil and gas emissions inventories.