Air quality measurements in the western Eagle Ford Shale
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abstract
The Eagle Ford Shale in southern Texas remains one of the most productive oil and gas regions in the US. Like the Permian Basin and Bakken Shale, ubiquitous natural gas flaring serves as an uncertain source of trace gas emissions within the Eagle Ford. A lack of ambient air quality data, especially in the western shale, impedes a thorough understanding of trace gas emissions within the shale and the subsequent local/regional air quality impacts. Meteorological and trace gas instrumentation was deployed to Shape Ranch in southwestern Dimmit County, near the US/Mexico border, from April to November of 2015. Mixing ratios of CO, NOx, O3, and VOCs did not exceed ambient air quality standards and were generally lower than mixing ratios measured in US cities, with the exception of alkanes. A non-negative matrix factorization demonstrated the dominance of oil and gas-sector emission sources in local trace gas variability, with combustion processes and transport of continental air also present. An analysis of NOx/CO and NOx/CO2 ratios during periods of O3 titration, identified by the ambient NOx/O3 ratio, suggested that combustion and biospheric sources contributed to emissions of NOx, CO, and CO2. In-plume NOx/CO2 ratios indicated relatively low-temperature combustion sources, with median NOx/CO2 ratios close to that expected for natural gas flaring (0.54 ppb/ppm), and much lower than emission ratios for internal combustion engines (<10 ppb/ppm). However, the NOx/CO2 ratio within these plumes exhibited a large variability, spanning more than an order of magnitude. Future research should focus on improving flaring emission factors and flaring volume estimates such that their air quality impacts can be better understood.
