Nitrate chemistry in the northeast US - Part 1: Nitrogen isotope seasonalitytracks nitrate formation chemistry

Despite significant precursor emission reductions in the US over recent decades, atmospheric nitrate deposition remains an important terrestrial stressor. Here, we utilized statistical air mass back trajectory analysis and nitrogen stable isotope deltas (d(15N)) to investigate atmospheric nitrate spatiotemporal trends in the northeastern US from samples collected at three US EPA Clean Air Status and Trends Network (CASTNET) sites from December 2016-2018. For the considered sites, similar seasonal patterns in nitric acid (HNO3) and particulate nitrate (pNO(3)) concentrations were observed with spatial differences attributed to nitrogen oxide (NOx) emission densities in source contributing regions that were typically = 1000 km. Significant spatiotemporal d(N-15) variabilities in HNO3 and pNO(3 )were observed with higher values during winter relative to summer, like previous reports from CASTNET samples collected in the early 2000s for our study region. In the early 2000s, d(N-15) of atmospheric nitrate in the northeast US had been suggested to be driven by NOx emissions; however, we did not find significant spatiotemporal changes in the modeled NOx emissions by sector and fuel type or d(N-15, NOx) for the source regions of the CASTNET sites. Instead, the seasonal and spatial differences in the observed d(N-15) of atmospheric nitrate were driven by nitrate formation pathways (i.e., homogeneous reactions of NO2 oxidation via hydroxyl radical or heterogeneous reactions of dinitrogen pentoxide on wetted aerosol surfaces) and their associated d(N-15) fractionation. Under the field conditions of low NOx relative to O-3 concentrations and when d(N-15, NOx) emission sources do not have significant variability, we demonstrate that d(15N) of atmospheric nitrate can be a robust tracer for diagnosing nitrate formation.