The magnitude of the snow-sourced reactive nitrogen flux to the boundary layer in the Uintah Basin, Utah, USA

Abstract. Reactive nitrogen (N_r  =  NO, NO₂, HONO) and volatile organic carbon emissions from oil and gas extraction activities play a major role in wintertime ground-level ozone exceedance events of up to 140 ppb in the Uintah Basin in eastern Utah. Such events occur only when the ground is snow covered, due to the impacts of snow on the stability and depth of the boundary layer and ultraviolet actinic flux at the surface. Recycling of reactive nitrogen from the photolysis of snow nitrate has been observed in polar and mid-latitude snow, but snow-sourced reactive nitrogen fluxes in mid-latitude regions have not yet been quantified in the field. Here we present vertical profiles of snow nitrate concentration and nitrogen isotopes (δ¹⁵N) collected during the Uintah Basin Winter Ozone Study 2014 (UBWOS 2014), along with observations of insoluble light-absorbing impurities, radiation equivalent mean ice grain radii, and snow density that determine snow optical properties. We use the snow optical properties and nitrate concentrations to calculate ultraviolet actinic flux in snow and the production of N_r from the photolysis of snow nitrate. The observed δ¹⁵N(NO₃⁻) is used to constrain modeled fractional loss of snow nitrate in a snow chemistry column model, and thus the source of N_r to the overlying boundary layer. Snow-surface δ¹⁵N(NO₃⁻) measurements range from −5 to 10 ‰ and suggest that the local nitrate burden in the Uintah Basin is dominated by primary emissions from anthropogenic sources, except during fresh snowfall events, where remote NO_x sources from beyond the basin are dominant. Modeled daily averaged snow-sourced N_r fluxes range from 5.6 to 71  ×  10⁷ molec cm⁻² s⁻¹ over the course of the field campaign, with a maximum noontime value of 3.1  ×  10⁹ molec cm⁻² s⁻¹. The top-down emission estimate of primary, anthropogenic NO_x in Uintah and Duchesne counties is at least 300 times higher than the estimated snow NO_x emissions presented in this study. Our results suggest that snow-sourced reactive nitrogen fluxes are minor contributors to the N_r boundary layer budget in the highly polluted Uintah Basin boundary layer during winter 2014.