Barrier Wind Formation in the Upper Green River Basin of Sublette County, Wyoming, and Its Relationship to Elevated Ozone Distributions in Winter

Meteorological factors affect the concentrations and distributions of pollutants during episodes of degraded air quality. Over the last 10 years, the upper Green River basin (UGRB) of Sublette County, Wyoming, has experienced numerous wintertime ozone episodes stimulated by emissions from oil and natural gas development operations, resulting in the region being determined to be in marginal nonattainment of the National Ambient Air Quality Standards. Examination of surface wind field patterns in the UGRB using observations from a network of surface monitoring stations for 2011 and 2012, with an emphasis on ozone-episode days, confirms that increased ozone concentrations are most frequently measured on days on which winds are light and variable. Dispersion and dilution of ozone and its precursor pollutants on these days is therefore inefficient, and so these episodes invariably occur within and close by the gas fields. On days that instead experience afternoon southeasterly winds, episodes can often be observed at locations on the northwestern perimeter of the basin remote from pollutant source regions. Simulations using the Weather Research and Forecasting Model, conducted for the case study of 15 February 2011, identify these southeasterlies as barrier winds caused by southwesterly flow at 700 hPa impinging on the Wind River Mountains that flank the UGRB to the northeast. Characterization of the barrier wind and the overall airflow patterns facilitates more accurate future forecasting of the time-dependent geographical distribution of increased concentrations of ozone and other pollutants in the region.