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Otherwise, the analysis is derived from first-principles (cell dimensions, pressure, temperature, and methane’s per-molar absorptivity) ().Together, these concentration measures yield methane values that are accurate to better than 60 ppb (Robert Provencal, LGR, personal communication, 04/16/2019), which is relatively small considering that our methane data vary from ~2.0 ppm (ambient level) to ~30 ppm (peak mole fraction).To date, estimation of greenhouse gas (GHG) emissions from the natural gas (NG) value chain have focused on upstream (production) and midstream (gathering, transmission, and storage) operations. A study proposed a tipping point of 3.2% (2.7% using revised GWPs () along the NG value chain.In this study, we estimate methane emissions from an important downstream consumer of NG, the ammonia fertilizer industry, which commonly uses NG as a feedstock and a fuel for the production of ammonia and other upgraded products. Some studies sought to estimate methane emissions (e.g.An example is shown in Figure , a well-developed methane plume was observed downwind of the ammonia fertilizer plant, while ambient background methane mixing ratios were found when sampling upwind of the plant.After identifying the facility-introduced plume, the GSV car would pass through the plume multiple times to make repeated samplings.The point-source assumption is based on the observation that the NG-related equipment is usually clustered within a radius of ~100 m from the center region (as visually inspected from Google Earth), while the source-to-sensor distances are 500 to 2,000 m (Table parametrization (Supplemental materials Text S2).
The filter was later used to mask and adjust the raw mixing ratio data to remove the adverse effects caused by this issue.Local emissions, such as small pipeline leaks, can be identified as small spikes in the data (Figure ).In the data post-processing, elevated mixing ratios due to local emissions were excluded from the analysis, and we focused on the broad plumes.Map of NG-based ammonia fertilizer plants in the U. A Google Earth representation of NG-based ammonia fertilizer plants in the U. DOI: https://doi.org/10.1525/elementa.358.f125% of the total of 23 NG-based ammonia fertilizer plants in the U. The data quality control protocol ruled out some data from the six plants surveyed (Supplemental materials Text S1).Key characteristics and sampling conditions related to the six plants that were successfully surveyed are summarized in Table , in ppm) along public roads downwind of the ammonia fertilizer plants.