Secondary formation of nitrophenols in an oil and gas production region: insights from observations during the Uintah Basin Winter Ozone Study (UBWOS) 2014

Nitrophenols play important roles in the atmosphere, for example as light-absorbing organic matter, phytotoxins, precursors of secondary organic aerosol (SOA) and HONO. Besides emissions from primary sources, nitrophenols are important secondary products from the oxidation of aromatics in the atmosphere. Nitrophenols were measured along with many other organic acids using an acetate ion (CH₃COO^-) time of flight chemical ionization mass spectrometer (TOF-CIMS) at a site in an oil and gas producing region during Uintah Basin Winter Ozone Study (UBWOS) in 2014. Nitrophenol and methyl-substituted nitrophenols exhibited distinct diurnal profiles, with high concentrations at night and a minimum observed during the daytime. This is consistent with photolysis being the dominant sink of these species. For this study, nitrophenols are most likely formed in situ, while primary emissions (e.g. biomass burning, oil and gas emissions) are negligible during this campaign. We will quantify the contributions of gas phase reactions of phenols with OH and NO₃ radicals to form nitrophenols. The importance of heterogeneous reactions resulting for nitrophenol formation, including N₂O₅ chemistry on surfaces and photolysis of nitrate in the snow will be also explored using vertical gradient measurements at two different heights (1.0 m and 18.5 m). In contrast with mono-nitrophenols, dinitrophenol showed a daytime maximum, indicating that the formation of dinitrophenol is driven by photochemistry in the atmosphere. The results of this study will be helpful to improving our understanding the budget of nitrophenols and oxidation mechanisms of aromatics in the atmosphere.