Ozone photochemistry in an oil and natural gas extraction region during winter: Simulations of a snow-free season in the Uintah Basin, Utah
The Uintah Basin in northeastern Utah, a region of intense oil and gas extraction, experienced ozone (O 3) con-centrations above levels harmful to human health for multi-ple days during the winters of 2009–2010 and 2010–2011. These wintertime O 3 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 pro-cess. 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 O 3 production in regions of oil and gas development. Although elevated O 3 concentrations were not observed during the winter of 2011–2012, the comprehen-sive set of observations tests our understanding of O 3 pho-tochemistry in this unusual emissions environment. A box model, constrained to the observations and using the near-explicit Master Chemical Mechanism (MCM) v3.2 chem-istry scheme, has been used to investigate the sensitivities of O 3 production during UBWOS 2012. Simulations iden-tify the O 3 production photochemistry to be highly radical limited (with a radical production rate significantly smaller than the NO x emission rate). Production of OH from O 3 pho-tolysis (through reaction of O(1 D) with water vapor) con-tributed only 170 pptv day −1 , 8 % of the total primary rad-ical source on average (primary radicals being those pro-duced from non-radical precursors). Other radical sources, including the photolysis of formaldehyde (HCHO, 52 %), ni-trous acid (HONO, 26 %), and nitryl chloride (ClNO 2 , 13 %) were larger. O 3 production was also found to be highly sen-sitive to aromatic volatile organic compound (VOC) concen-trations, due to radical amplification reactions in the oxida-tion scheme of these species. Radical production was shown to be small in comparison to the emissions of nitrogen ox-ides (NO x), such that NO x acted as the primary radical sink. Consequently, the system was highly VOC sensitive, despite the much larger mixing ratio of total non-methane hydro-carbons (230 ppbv (2080 ppbC), 6 week average) relative to NO x (5.6 ppbv average). However, the importance of radi-cal sources which are themselves derived from NO x emis-sions and chemistry, such as ClNO 2 and HONO, make the response of the system to changes in NO x emissions uncer-tain. Model simulations attempting to reproduce conditions Published by Copernicus Publications on behalf of the European Geosciences Union. 8956 P. M. Edwards et al.: Simulations of a snow-free season in the Uintah Basin, Utah expected during snow-covered cold-pool conditions show a significant increase in O 3 production, although calculated concentrations do not achieve the highest seen during the 2010–2011 O 3 pollution events in the Uintah Basin. These box model simulations provide useful insight into the chem-istry controlling winter O 3 production in regions of oil and gas extraction.