High Chlorine Concentrations in an Unconventional Oil and Gas Development Region and Impacts on Atmospheric Chemistry

Growth in unconventional oil and gas development (UOGD) in the United States has increased airborne emissions, raising environmental and human health concerns. To assess the potential impacts on air quality, we deployed instrumentation in Karnes City, Texas, a rural area in the middle of the Eagle Ford Shale. We measured several episodes of elevated Cl₂ levels, reaching maximum hourly averages of 800 ppt, the highest inland Cl₂ concentration reported to date. Concentrations peak during the day, suggesting a strong local source (given the short photolysis lifetime of Cl₂) and/or a photoinitiated production mechanism. Well preproduction activity near the measurement site is a plausible source of these high Cl₂ levels via direct emission and photoactive chemistry. ClNO₂ is also observed, but it peaks overnight, consistent with well-known nocturnal formation processes. Observations of organochlorines in the gas and particle phases reflect the contribution of chlorine chemistry to the formation of secondary pollutants in the area. Box modeling results suggest that the formation of ozone at this location is influenced by chlorine chemistry. These results suggest that UOGD can be an important source of reactive chlorine in the atmosphere, impacting radical budgets and the formation of secondary pollutants in these regions.