Abstract
Reactive nitrogen (Nr = 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 Nr 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
Nr 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 NOx sources from beyond the basin are dominant.
Modeled daily averaged snow-sourced Nr 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 NOx in Uintah and Duchesne
counties is at least 300 times higher than the estimated snow NOx
emissions presented in this study. Our results suggest that snow-sourced
reactive nitrogen fluxes are minor contributors to the Nr boundary
layer budget in the highly polluted Uintah Basin boundary layer during
winter 2014.
Citation
ID:
208381
Ref Key:
zatko2016atmosphericthe