Quantification of Methane and Ethane Emissions from the San Juan Basin

Abstract:

Methane (CH₄), a potent greenhouse gas, and the primary component of natural gas, is emitted from areas of high fossil fuel production and processing. Recently, persistent and large methane emissions (~0.59 Tg yr^-1) from the four corners area of the United States have been identified using satellite (SCIAMACHY) observations taken over the years 2003 to 2009. These emissions appear to be the largest CH₄ anomaly (positive deviation above background values) in the contiguous U.S., and exceed bottom-up inventory estimates for the area by 1.8 to 3.5 times. The majority of emissions sources expected to contribute to this anomalous CH₄ signal are located in the San Juan basin of New Mexico, and include harvesting and processing of natural gas, coal, and coalbed CH₄. The magnitude of CH₄ emissions from the San Juan basin have not yet been directly quantified using airborne measurements. Additionally, changing fossil fuel-related activities in the basin may have altered the magnitude of CH₄ emissions compared to estimates derived from 2003-2009 satellite measurements. Here, we present in-situ airborne observations of CH₄ over the San Juan basin, which allow tight quantification of CH₄ fluxes using the mass balance method. Observations over the basin were taken for multiple wind directions on multiple days in April, 2015 to obtain a robust estimate of CH₄ emissions. The flux of ethane (C₂H₆), the second most abundant component of natural gas and a tracer species indicative of fossil-derived CH₄, was also quantified. Substantial C₂H₆ emissions may affect regional air quality and chemistry through its influence on tropospheric ozone production.