Spatial distribution of atmospheric oil and natural gas volatile organic compounds in the Northern Colorado Front Range

Abstract

Methane and nonmethane volatile organic compounds (VOCs) were monitored near Boulder in the Northern Colorado Front Range to investigate their spatial distribution and sources as a part of the Front Range Air Pollution and Photochemistry Experiment (FRAPPE) and the Deriving Information on Surface conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) campaign, in summer 2014. A particular emphasis was the study of the contribution of emissions from oil and natural gas (O&NG) operations on the regional air quality. One network extended along an elevation gradient from the City of Boulder (elevation ≈1,600 m) to the University of Colorado Mountain Research Station (≈2900 m) on the eastern slopes of the Rocky Mountains. Light alkane petroleum hydrocarbons had the highest mole fraction of the VOCs that could be analyzed with the applied techniques. The longer lived VOCs ethane and propane decreased with increasing elevation, suggesting that Boulder and the surrounding plains were a source of these anthropogenic compounds. VOC diurnal time series showed a few events with elevated mole fractions at the mountain sites, which were likely the result of the upslope transport of plumes with elevated VOCs from the plains. Within the other site network, which extended into suburban East Boulder County (EBC), VOCs were monitored at 5 sites increasingly close to O&NG development in the Denver Julesburg Basin. Mean mole fractions and variability of primarily O&NG-associated VOCs (ethane, propane, butane isomers) increased by a factor of 2.4–5.2 with closer proximity to the O&NG producing region. Median mole fractions of C2–C5 n-alkanes and of imuch-butane at the EBC sites were higher than those previously reported from 28 larger urban areas in the United States. Among the VOCs that could be quantified with the gas chromatography methods, VOCs most clearly associated to O&NG-related emissions (C2–C5 alkanes) accounted for 52%–79% of the VOC hydroxyl radical reactivity (OHR). The horizontal gradient in OHR of the considered VOCs, with ≈3 times higher values at the furthest eastern sites, points toward higher chemical reactivity and ozone production potential from these ozone precursors in the eastern area of the county than within the City of Boulder.