High Resolution Inverse Modeling of Emissions in the Uinta Basin of Utah

The HARC 3D micro-scale forward and adjoint chemical transport model and the 4D variational data assimilation technique were applied to the estimation of oil and gas facility emissions based on observations collected at the Horsepool site during the 2013 Uinta Basin Ozone Study (UBOS). Inversely modeled species included nitrogen oxides; the radical precursors, nitrous acid and formaldehyde; and the hydrocarbons, ethene, propene, toluene, and xylenes. The model domain covered a 20 km by 20 km region of the Uinta Basin, with a horizontal resolution of 400 m, and 12 variable resolution layers in the vertical up to 307 m AGL. The kinematic effects of topography were simulated with the QUIC wind model using a background wind based on available meteorological measurements. Parameterized photolysis rates were doubled to account for the effects of snow cover. The background reactivity of alkanes and other longer-lived VOCs not explicitly included in the truncated chemical mechanism was set at a plausible value based on UBOS observations coinciding with high concentrations of peak ozone. A three-hour time period from 9:00 am – 12:00 pm MST on January 26, 2013 was selected as the data assimilation window. The model results suggest that there are important primary emissions of formaldehyde in the Uinta Basin, whose total magnitude in the simulated domain may be comparable to the formaldehyde emissions of large refineries. In addition, primary emissions of nitrous acid and fluxes of secondary nitrous acid formed on snow surfaces may both contribute significantly to the cold ozone phenomenon in the Uinta Basin.