High winter ozone pollution from carbonyl photolysis in an oil and gas basin

Abstract

The United States is now experiencing the most rapid expansion in oil and gas production in four decades, owing in large part to implementation of new extraction technologies such as horizontal drilling combined with hydraulic fracturing. The environmental impacts of this development, from its effect on water quality to the influence of increased methane leakage on climate, have been a matter of intense debate. Air quality impacts are associated with emissions of nitrogen oxides (NOx = NO + NO2) and volatile organic compounds (VOCs), whose photochemistry leads to production of ozone, a secondary pollutant with negative health effects. Recent observations in oil- and gas-producing basins in the western United States have identified ozone mixing ratios well in excess of present air quality standards, but only during winter. Understanding winter ozone production in these regions is scientifically challenging. It occurs during cold periods of snow cover when meteorological inversions concentrate air pollutants from oil and gas activities, but when solar irradiance and absolute humidity, which are both required to initiate conventional photochemistry essential for ozone production, are at a minimum. Here, using data from a remote location in the oil and gas basin of northeastern Utah and a box model, we provide a quantitative assessment of the photochemistry that leads to these extreme winter ozone pollution events, and identify key factors that control ozone production in this unique environment. We find that ozone production occurs at lower NOx and much larger VOC concentrations than does its summertime urban counterpart, leading to carbonyl (oxygenated VOCs with a C = O moiety) photolysis as a dominant oxidant source. Extreme VOC concentrations optimize the ozone production efficiency of NOx. There is considerable potential for global growth in oil and gas extraction from shale. This analysis could help inform strategies to monitor and mitigate air quality impacts and provide broader insight into the response of winter ozone to primary pollutants.

Understanding high wintertime ozone pollution events in an oil- and natural gas-producing region of the western US

“…Similar to the finding of Edwards et al (2014), our sensitivity analysis shows that it is important to quantify the primary and secondary sources of CH 2 O in the UB during wintertime. Despite the underprediction of CH 2 O, its photolysis is a dominant source of ozone-producing radicals within the model.…”

“…According to their box model study, the photochemical regime during that period was characterized as radical limited. Edwards et al (2014) used a similar chemical box model for the conditions observed in the winter of 2013 in the UB. The study concluded the importance of the build-up and photolysis of carbonyls in deriving high O 3 in the UB during the winter of 2013.…”

Nitrous acid formation in a snow-free wintertime polluted rural area

“…The 2012 Uintah Basin Wintertime Ozone Study (UBWOS) took place between January and early March 2012 in the Uintah Basin, Utah, USA, to study elevated wintertime O 3 formation (e.g., Edwards et al, 2014;NOAA, 2012). The UB is located in the northeast corner of Utah and is part of the Colorado Plateau.…”

“…O 3 levels measured in the UB in 2012 were lower than those observed in the subsequent two winters. The absence of snow likely led to more convective mixing, which prevented pollutants from building up near the surface and reacting with sunlight to form O 3 (Lyman and Shorthill, 2013;Edwards et al, 2014). Furthermore, without snow cover, photochemical formation of O 3 was less efficient, as the actinic flux was 1.6 to 2 times smaller (Lyman and Shorthill, 2013;Edwards et al, 2014).…”

Tropospheric ozone and its precursors from the urban to the global scale from air quality to short-lived climate forcer

“…With the increase in unconventional oil and natural gas has come an increase in fugitive emissions of hydrocarbons to the atmosphere in quantities great enough to influence local-and regional-scale ozone production (Katzenstein et al, 2003;Kemball-Cook et al, 2010;Edwards et al, 2014 (Schnell et al, 2009). In situ measurements revealed that the high wintertime ozone levels were due to a strong temperature inversion that trapped NO x and hydrocarbon emissions from the natural gas industry.…”