Fine Scale Spatial and Temporal Allocation of NOx Emissions from Unconventional Oil and Gas Development Can Result in Increased Predicted Regional Ozone Formation

Abstract

The impact of detailed spatial and temporal allocation of unconventional oil and gas development (UOGD) NOx emissions on predicted ozone formation was examined using hydraulic fracturing emissions in the Eagle Ford Shale region of Texas as a case study. Hydraulic fracturing occurs at specific well sites, lasting only 1–2 weeks prior to production. Four scenarios for spatial and temporal allocation of hydraulic fracturing NOx emissions were developed. In one scenario, NOx emissions were evenly distributed to all active wells in the Eagle Ford region, with continuous emissions throughout the year. In other scenarios, NOx emissions from hydraulic fracturing engines in Karnes County were allocated only to fractured wells, with durations ranging from 2 days to 2 weeks. In the month of August, predicted daily maximum of 8 h average (MDA8) O₃ concentrations were consistently 6, 8, and 10 ppb higher over wide regions for the two-week, one-week, and two-day emission periods, respectively, compared to the annual county level distribution, demonstrating that detailed spatial and temporal allocation of NOx emissions in regions like the Eagle Ford Shale, with abundant biogenic VOCs, impacts predicted ozone formation.

Allocating certain emissions from unconventional oil and gas development to specific point sources at the times that the emissions are active can significantly impact predicted ozone formation in some regions.