Air quality measurements in the western Eagle Ford Shale

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.

In Situ Sampling of NOx Emissions from United States Natural Gas Flares Reveals Heavy-Tail Emission Characteristic

“…In the Eagle Ford, our observed NOx:CO 2 slopes are consistent with combustion plumes attributed to flares in Roest and Schade (mean values of 0.33–1.47 ppb/ppm depending on the correlation and uncertainty thresholds). 10 Both recent field studies found variability in observed flaring NOx emission ratios, with a general skewness toward higher values, consistent with the findings presented here.…”

“…This difference in average value is possibly due to the high fractional methane content of the gas in their study region (median of 0.845) compared to our U.S. regions (Table ), as well as the higher expected temperatures of these large offshore flares. In the Eagle Ford, our observed NOx:CO 2 slopes are consistent with combustion plumes attributed to flares in Roest and Schade (mean values of 0.33–1.47 ppb/ppm depending on the correlation and uncertainty thresholds) . Both recent field studies found variability in observed flaring NOx emission ratios, with a general skewness toward higher values, consistent with the findings presented here.…”

“…In the U.S., studies have investigated NOx from the oil and gas sector, deploying bottom-up accounting methods as well as airborne, ground, and satellite measurements and found regionally elevated NOx attributable to oil and gas (O&G) activities. − Dix et al estimated that 7% and 11% of NOx emissions in the Permian and Bakken were from flaring in 2015; however, the relative role was found to change depending on the stage of operation (i.e., drilling versus production) . While flaring is not the only source of NOx in O&G regions, recent works aim to investigate the direct impact of flaring on local air quality and human health. − On larger scales, temporal trends in flaring have been explored using various space-based data products.…”

Tracking NO2 Pollution Changes Over Texas: Synthesis of In Situ and Satellite Observations

“…Recognizing air quality problems in Texas, several studies have been conducted utilizing existing routine measurements as well as measurements made during some dedicated field campaigns, such as Texas Air Quality Study (TexAQS) (Berkowitz et al., 2005; Daum et al., 2003) in 2000 and TexAQS II (Parrish et al., 2009) in 2006, Deriving Information on Surface conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER‐AQ) (Anderson et al., 2014) in 2014, and TRacking Aerosol Convection interactions Experiment—Air Quality (TRACER‐AQ) (Jensen et al., 2021) in 2021. Most prior studies were concerned with emissions of NO x and hydrocarbons, from industrial complexes and powerplant changes of NO 2 levels over cities (Demetillo et al., 2020; Duncan et al., 2016; Lamsal et al., 2015) and oil and natural gas extraction regions (Majid et al., 2017; Roest & Schade, 2020; Schade & Roest, 2016, 2018) in Texas. Continuous hourly air quality observations, including NO 2 , from the air quality system (AQS) of US Environmental Protection Agency (EPA) and Texas Commission on Environmental Quality (TCEQ) monitoring networks (Schade & Roest, 2016, 2018; Zhang et al., 2018) have been a valuable resource for studying tropospheric composition, air quality, and their linkage to public health.…”

Standardized Reporting Needed to Improve Accuracy of Flaring Data

“…The majority of such flares is highly visible, especially at night. They represent an enormous waste of a natural resource [8,11,[18][19][20], while, at the same time, acting as a local source of air pollution due to incomplete combustion and the generation of NO x [21][22][23]. Routine flaring, therefore, has become a target of environmental groups, which demand its reduction and eventual elimination [24].…”