Abstract
Abstract. The Uintah Basin in northeastern Utah, a region of intense oil and gas extraction, experienced ozone (O3) concentrations above levels harmful to human health for multiple days during the winters of 2009–2010 and 2010–2011. These wintertime O3 pollution episodes occur during cold, stable periods when the ground is snow-covered, and have been linked to emissions from the oil and gas extraction process. The Uintah Basin Winter Ozone Study (UBWOS) was a field intensive in early 2012, whose goal was to address current uncertainties in the chemical and physical processes that drive wintertime O3 production in regions of oil and gas development. Although elevated O3 concentrations were not observed during the winter of 2011–2012, the comprehensive set of observations tests our understanding of O3 photochemistry in this unusual emissions environment. A box model, constrained to the observations and using the near-explicit Master Chemical Mechanism (MCM) v3.2 chemistry scheme, has been used to investigate the sensitivities of O3 production during UBWOS 2012. Simulations identify the O3 production photochemistry to be highly radical limited (with a radical production rate significantly smaller than the NOx emission rate). Production of OH from O3 photolysis (through reaction of O(1D) with water vapor) contributed only 170 pptv day−1, 8% of the total primary radical source on average (primary radicals being those produced from non-radical precursors). Other radical sources, including the photolysis of formaldehyde (HCHO, 52%), nitrous acid (HONO, 26%), and nitryl chloride (ClNO2, 13%) were larger. O3 production was also found to be highly sensitive to aromatic volatile organic compound (VOC) concentrations, due to radical amplification reactions in the oxidation scheme of these species. Radical production was shown to be small in comparison to the emissions of nitrogen oxides (NOx), such that NOx acted as the primary radical sink. Consequently, the system was highly VOC sensitive, despite the much larger mixing ratio of total non-methane hydrocarbons (230 ppbv (2080 ppbC), 6 week average) relative to NOx (5.6 ppbv average). However, the importance of radical sources which are themselves derived from NOx emissions and chemistry, such as ClNO2 and HONO, make the response of the system to changes in NOx emissions uncertain. Model simulations attempting to reproduce conditions expected during snow-covered cold-pool conditions show a significant increase in O3 production, although calculated concentrations do not achieve the highest seen during the 2010–2011 O3 pollution events in the Uintah Basin. These box model simulations provide useful insight into the chemistry controlling winter O3 production in regions of oil and gas extraction.
Highlights
Tropospheric ozone (O3) is an air pollutant with severe respiratory health effects (Finlayson-Pitts and Pitts, 1997; Brown et al, 2008; Jerrett et al, 2009)
The motivation for the campaign was the occurrence of high O3 events during the winters of 2009–2010 and 2010–2011, which were characterized by cold-pool conditions in the Uintah basin, with snow on the ground
Other primary radical sources that were important included the photolysis of HCHO (52.3 %), HONO (25.8 %) and ClNO2 (12.8 %), as well as OH from the reaction of O3 with alkenes (1.5 %)
Summary
Tropospheric ozone (O3) is an air pollutant with severe respiratory health effects (Finlayson-Pitts and Pitts, 1997; Brown et al, 2008; Jerrett et al, 2009). In contrast to urban summertime O3 pollution events, these wintertime episodes occur during the darker months of the year in sparsely populated regions removed from the typical urban emissions of O3 precursor species Both the Upper Green River basin and the Uintah Basin are regions of extensive oil and gas production, the emissions from which are currently poorly constrained (Katzenstein et al, 2003; Petron et al, 2012; Gilman et al, 2013). Carter and Seinfeld (2012) performed box model calculations based on a lumped chemistry scheme investigating the sensitivities of O3 production to VOC and NOx concentrations during four separate case studies of O3 events in the Upper Green River basin In three of these studies, O3 production was found to be highly sensitive to VOC and HONO concentrations, whilst the fourth case was most sensitive to NOx. The difference in sensitivities between these case studies is attributed largely to differences in the observed VOC speciation at the different sites, and highlights the importance of increased observations of O3 precursors in regions of extensive oil and gas production. Work aims to improve our understanding of wintertime O3 production in general and in the Uintah Basin in particular
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