| dc.description.abstract | Emissions from unconventional oil and gas are poorly constrained in existing inventories and contribute to uncertainties in our understanding of air quality near oil and gas producing regions. Emissions from the Eagle Ford Shale in southern Texas, which is a top oil and gas producing region in the US, are particularly uncertain due to a lack of ambient air quality data and the extensive use of flaring. First, alkane emissions in the central Eagle Ford Shale were quantified using data collected by the state of Texas in a mass balance approach. The median emission rate from raw natural gas sources in the shale, calculated as a percentage of the total produced natural gas in the upwind region, was 0.7% with an interquartile range (IQR) of 0.5-1.3%, below the US Environmental Protection Agency’s (EPA) current estimates. However, storage tanks contributed 17% of methane emissions, 55% of ethane, 82% percent of propane, 90% of n-butane, and 83% of isobutane emissions. The inclusion of liquid storage tank emissions results in a median emission rate of 1.0% (IQR of 0.7-1.6%) relative to produced natural gas, overlapping the current EPA estimate of roughly 1.6%. However, a recently published study using aircraft data suggests that this estimate may be biased low due to the position of the downwind monitor. Nonetheless, we conclude that emissions from liquid storage tanks are likely a major source for the observed non-methane hydrocarbon enhancements in the Northern Hemisphere. Second, air quality measurements were performed at a field site in the western Eagle Ford Shale. Oil and gas sources dominated ambient VOC concentrations and plumes from nearby sources were identified. Trace gas ratios suggest that many plumes originated from low-temperature combustion sources, which are likely to be nearby flares based on knowledge of regional emissions sources. Modeling exercises with parameterized flaring emissions show that plumes are capable of reaching Shape Ranch, and the observed emission ratios are within a factor of two of the modeled emission ratios based on EPA emission factors. Flaring emissions should be studied further to understand the scope of air quality impacts associated with widespread flaring. | en |
