The NO_x dependence of bromine chemistry in the Arctic atmospheric boundary layer

Abstract. Arctic boundary layer nitrogen oxides (NO_x = NO₂ + NO) are naturally produced in and released from the sunlit snowpack and range between 10 to 100 pptv in the remote background surface layer air. These nitrogen oxides have significant effects on the partitioning and cycling of reactive radicals such as halogens and HO_x (OH + HO₂). However, little is known about the impacts of local anthropogenic NO_x emission sources on gas-phase halogen chemistry in the Arctic, and this is important because these emissions can induce large variability in ambient NO_x and thus local chemistry. In this study, a zero-dimensional photochemical kinetics model was used to investigate the influence of NO_x on the unique springtime halogen and HO_x chemistry in the Arctic. Trace gas measurements obtained during the 2009 OASIS (Ocean – Atmosphere – Sea Ice – Snowpack) field campaign at Barrow, AK were used to constrain many model inputs. We find that elevated NO_x significantly impedes gas-phase halogen radical-based depletion of ozone, through the production of a variety of reservoir species, including HNO₃, HO₂NO₂, peroxyacetyl nitrate (PAN), BrNO₂, ClNO₂ and reductions in BrO and HOBr. The effective removal of BrO by anthropogenic NO_x was directly observed from measurements conducted near Prudhoe Bay, AK during the 2012 Bromine, Ozone, and Mercury Experiment (BROMEX). Thus, while changes in snow-covered sea ice attributable to climate change may alter the availability of molecular halogens for ozone and Hg depletion, predicting the impact of climate change on polar atmospheric chemistry is complex and must take into account the simultaneous impact of changes in the distribution and intensity of anthropogenic combustion sources. This is especially true for the Arctic, where NO_x emissions are expected to increase because of increasing oil and gas extraction and shipping activities.