Source apportionment of non-methane hydrocarbons, NOx and H2S data from a central monitoring station in the Eagle Ford shale, Texas

Abstract

Unconventional oil and gas exploration in the US has become a significant new source of atmospheric hydrocarbons. Field measurements and monitoring have been initiated to determine integral effects from this geographically dispersed source in and downwind of shale areas, driven mostly by concerns related to photochemical ozone production. The Texas Commission on Environmental Quality (TCEQ) deployed its first air quality monitor near the Eagle Ford shale in south Texas in summer 2013, followed by a more centrally located monitor in winter 2014/15. Here, we report on the latter monitor’s 2015 data, showing at times extraordinarily high levels of saturated hydrocarbons, similar to earlier findings in this area. Using hydrocarbon ratios, we establish that the dominant sources at this site appear to be oil and gas exploration. A non-negative matrix factorization (NMF) analysis revealed six consistent source factors, of which two were associated with pre-existing local sources from car traffic and industry, three with regional oil and gas exploration, and one with diesel emissions. The dominant source factors were associated with evaporative and fugitive emissions, and with flaring and (diesel-powered) compressor engine emissions. The former is a major source of saturated hydrocarbons while the latter is a major source of NOx and unsaturated hydrocarbons, confirming earlier findings. Due to the rural nature of the site, road traffic is a minor NOx source in this area, and the NMF results support inventory estimates showing oil and gas exploration to be the dominant regional source of NOx emissions. The NMF based source apportionment results also suggests that benzene levels in this rural area in 2015, while comparable to levels in Houston now, were probably three to five times lower before the shale boom.