10 Aug 2022
10 Aug 2022
Status: this preprint is currently under review for the journal ACP.

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

Shreta Ghimire1, Zachary J. Lebo1, Shane Murphy1, Stefan Rahimi2, and Trang Tran3 Shreta Ghimire et al.
  • 1Department of Atmospheric Science, University of Wyoming
  • 2Institute of Environment and Sustainability, University of California Los Angeles
  • 3Desert Research Institute

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 (NOX) 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 NOX 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 NOX 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.

Shreta Ghimire et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-456', Anonymous Referee #2, 30 Aug 2022
  • CC1: 'Comment on acp-2022-456', Seth Lyman, 15 Sep 2022
  • RC2: 'Comments on acp-2022-456', Anonymous Referee #3, 05 Nov 2022

Shreta Ghimire et al.

Shreta Ghimire et al.


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Short summary
High wintertime ozone levels have occurred often in recent years in mountain basins with oil and gas production facilities. Photochemical modeling of ozone production serves as a basis for understanding the mechanism by which it occurs and for predictive capability. We present photochemical model simulations of ozone formation and accumulation in the Upper Green River Basin, Wyoming, demonstrating the model's ability to accurately simulate wintertime ozone.