Simulations of winter ozone in the Upper Green River Basin, Wyoming, using WRF-Chem

Abstract. In both the Upper Green River Basin (UGRB) of Wyoming and the Uintah Basin of Utah, strong wintertime ozone (O₃) formation episodes leading to O₃ concentrations exceeding the 8-hour O₃ NAAQS (70 ppb) have been observed over the last two decades. Wintertime O₃ events in the UGRB were first observed in 2005 and since then have continued to be observed intermittently when meteorological conditions are favorable, despite significant efforts to reduce emissions. While O₃ formation has been successfully simulated using observed volatile organic compound (VOC) and nitrogen oxide (NO_X) concentrations, successful simulation of these wintertime episodes using emission inventories in a 3-D photochemical model has remained elusive. An accurate 3-D photochemical model driven by an emission inventory is critical to understand which emission sources have the most impact on O₃ formation. In the winter of 2016–2017 (December 2016–March 2017) several high O₃ events were recorded with concentrations exceeding 70 ppb. This study uses the Weather Research Forecasting model with chemistry (WRF-Chem) to simulate one of the high O₃ events observed in the UGRB during March of 2017. The WRF-Chem simulations were carried out using the 2014 edition of the Environmental Protection Agency National Emissions Inventory (EPA-NEI 2014v2), which includes estimates of emissions from non-point oil and gas production sources. Simulations were carried out with two different chemical mechanisms: the Model for Ozone and Related Chemical Tracers (MOZART) and the Regional Atmospheric Chemistry Mechanism (RACM), and the results were compared with the observed data from 7 weather and air quality monitoring stations in the UGRB operated by Wyoming Department of Environmental Quality (WYDEQ). The simulated meteorology compared favorably to observations in terms of predicting temperature inversions and surface temperature and wind speeds. Notably, because of snow cover present in the basin, the photolysis surface albedo was modified in all simulations. Without this modification, none of the simulations formed O₃ exceeding 70 ppb, though the models were relatively insensitive to the exact photolysis albedo if it was over 0.65. The MOZART simulation produced more O₃ in the basin than the RACM simulation and compares better with the observations. However, while O₃ precursors NO_X and NMHC are predicted similarly in simulations with both chemistry mechanisms, simulated NMHC mixing ratios are a factor of six lower than the observations, while NO_X mixing ratios are also underpredicted but are much closer to the observations within the region of oil and gas production. The results show that both the RACM and MOZART chemical mechanisms were able to produce O₃ even though the NMHC mixing ratios in the model were a factor of six too low, an intriguing result for future studies.