Separating Methane Emissions from Agricultural Sources and Natural Gas: Direct Measurements of Excess Columns of CH4, C2H6, and NH3 in the Colorado Front Range

Methane (CH₄) is a major greenhouse gas emitted from biogenic, thermogenic, and pyrogenic sources. Here we demonstrate a novel approach to separate sources of CH₄ emissions based on a network of small portable sensors performing column measurements in the Northern Colorado Front Range (NCFR). In the study area CH₄ is emitted from biogenic sources such as concentrated animal feeding operations (CAFOs) and natural gas production and storage. In March 2015 we deployed a network of five Fourier Transform Spectrometers (FTS) to characterize the regional scale methane dome in Colorado’s Denver-Julesburg Basin based on excess vertical column measurements (the column enhancement inside the dome over background). Three EM27sun FTS measured CH₄, oxygen (O₂) and water vapor (H₂O) columns at Eaton, CO (inside the dome) and at two boundary sites; the CU mobile SOF (Solar Occultation Flux) measured ethane (C₂H₆), ammonia (NH₃), and H₂O at Eaton, CO. The column averaged dry air mole fractions XCH₄, XC₂H₆, and XNH₃ were determined using O₂ columns for air mass factor normalization, and background column was subtracted to derive excess vertical columns of DXCH4, DXC2H6, DXNH3 at Eaton, CO. Eaton is located both near CAFOs and at the northern edge of oil and natural gas production wells. Our approach for source apportioning methane employs a linear regression analysis that explains DXCH4 in terms of DXC2H6 as tracer for natural gas sources, and DXNH3 as tracer for CAFO emissions. The results of the source apportionment are compared with literature values of the NH₃/CH₄ and C₂H₆/CH₄ ratio to evaluate the method of excess columns, which is independent of boundary layer height.